NORDIC OPTICAL TELESCOPE
Nordic Optical Telescope Scientific Association
Priorities for Design
Dimensions and Weight
Cell for Primary Mirror
Cassegrain Focus Station
Encoders and Transducers
Sensitivity to Wind
Sensitivity to Humidity
Sensitivity to Dust
A short description is given of the Nordic Optical Telescope and some of its
most important features. Heaviest emphasis is given to properties being of high
priority. These properties are intended to define a low cost
compact telescope in a small enclosure. With a moderately large aperture,
highest weight has been given to design features providing optimum image
quality. Further, the telescope has only a Cassegrain focus station, which
should minimize instrumental polarization and favour image quality and speed.
High interest in observations in the infrared wavelength range, also in day
time, emphasizes the need for high pointing and tracking accuracy. The
telescope has an altazimuth mounting, a fast, f/2.0, primary mirror of Ritchey-Chrétien
type, made of Zerodur, with a diameter of 2.56 metres and an aspect ratio of
13.5. The effective f ratio of the telescope is 11.0 and the unvignetted field
of view 25 arc minutes. The image quality
specification is for the combined optical system 80 per cent geometrical
energy within 0.3
arc seconds in passive mode. For the integrated optomechanical system, the
corresponding specification is 80 per cent geometrical energy within 0.4 arc
seconds for zenith distances smaller than 60 degrees. A focal reducer (to be
installed) gives an effective telescope f ratio of 5.0 over a field of ten arc
minutes. An integrated servo system under complete computer control manages
rotation of the telescope with respect to azimuth and altitude axes, rotation
of instrument adapter, pneumatic support system for the primary mirror,
focusing movements and lateral displacements plus tilting (if necessary) of
secondary mirror, autoguider corrections, rotation of building and data
handling for implementation of refraction correction. In order to improve
optical quality, wobbling has not been included for the secondary mirror.
chopping has been preferred. For ancillary instrumentation mechanically
attached to the Cassegrain focus, the maximum free distance behind the adapter
is 1500 millimetres and thecorresponding maximum weight is 250 kg. More and
heavier instruments can be mounted elsewhere and have optical fibre feeding
from the instrument adapter. Over the available sky, pointing and tracking
accuracies have been set at two arc seconds and one tenth of an arc second,
RMS, respectively. The accuracy of both pointing and tracking should be
unaffected by wind with speeds up to around 22 m/s.
However, safety reasons dictate closing of dome already for lower wind speeds,
depending on circumstances. Instructions to the observer are communicated via
the control system. A field rotator is integrated in the instrument adapter.
The zenith singularity has a size of 30 arc minutes times 20 arc minutes. For
an object on the optical axis, dome vignetting affects a field of 1.3degrees
extension around the zenith singularity. The lowest resonance frequencies for
the azimuth and altitude axes are 7.4 and 14 Hz, respectively. The sky
available, limited by safety software in altitude, extends to -56 degrees
in declination. Decided measures have been taken to
minimize effects of telescope turbulence. This includes greatest restrictivity
with heat sources but also cooling of ambient air. Limitations with respect to
telescope operation at high wind speed and in high relative humidity are
indicated by the control system, advising the observer in these respects.
Key words: Telescope -
In 1983 and 1984 agreement was reached in the four Nordic countries Denmark,
Finland, Norway and Sweden concerningconstruction of a major observing facility
for Nordic astronomers, the Nordic Optical Telescope (NOT). It was intendedto
cover observing needs in the Northern Hemisphere for astronomers from the four
participating countries and be of high quality for both the optical and the
infrared wavelength ranges. The telescope was to be erected in a place
offeringoptimum conditions. Already before funding was decided, Nordic
astronomers had agreed that the obvious choice for telescope site was at La
Palma, Canary Islands.
Priorities for Design
The definition of major design features for the NOT was not an entirely easy
matter. The main reason for this was that it should, as well as possible,
satisfy the needs of research groups, working on programmes and with
observational methods of a very diversified nature.
The wish to reach faint limiting magnitudes and high spectral resolution called
for a large collecting surface. This was not controversial among astronomers
but had to be matched against existing budgets. It was unanimously decided to
push for high observing power through a design, featuring compactness.
The imaging capability of the telescope was given special attention. It was
quickly decided to choose image quality as first scientific priority.
Obviously, image quality is of prime importance not only for imaging in itself
but also improves very effectively the resulting observing power of the
telescope for most purposes. Evidently, giving first priority to image quality
implied high emphasis on site, optics and mechanical structure as well as on
design of enclosure.
Scientific interests in high accuracy
polarimetric work meant that effects of instrumental polarization had to be
kept as small as possible. This, in turn, calledfor an optical solution with a
minimum of elements and a maximum of optical symmetry (Andersen, 1981, 1982).
Already early, it was agreed that the telescope should be of high quality
notonly for observations in the optical wavelength region but also for work in
the infrared spectral range. This implied that it had to be infrared-clean
and have high pointing and tracking accuracy.
In many ways, image quality of a telescope is, from a scientific point of view,
much more interesting than the size of the collecting surface. It is often
pointed out that decrease of prevailing image size by a certain factor gives a
result for observing power equivalent to that of an increase of the size of the
collecting surface by the same factor. In practice, this is only part of the
For observations of stellar objects free from effects of crowding there is
equivalence between decrease of image size and increase of collecting surface
regarding limiting magnitude and/or observing speed. However, as soon aseffects
of crowding have to be taken into account, decrease of image size is much more
advantageous than increase of collecting surface. In many cases, an increase of
the collecting surface does not improve conditions at all, whereas decrease of
image size may turn out crucial. Examples are crowded stellar clusters,
aggregates and multiple stars and also galaxies for which detailed structure is
the target. Stating this in a slightly different way, it may be said that for
many observational problems, a somewhat smaller telescope can, with increased
observing time, obtain the same results as a large telescope, provided that
observing conditions are stable. On the other side,it is easy to find many
examples of observing problems for which increase of collecting surfaces does
not imply any improvement.
In conclusion, it can be stated, that high image quality is always a great
asset and often a necessary prerequisite for our possibility to solve
observing problems. Moreover, concerning increase of observing power, decrease
of image size is an imcomparably more economic solution than increase of
collecting surface. As a result, for the NOT, image quality was entitled to the
For all types of optical observations, the choice of telescope site is a matter
of utmost consequences. For far too many potentially powerful telescopes,
output flow and quality are, in practice, limited by doubtful quality of sites
rather than by limiting instrumental factors. Obviously, the more sophisticated
the telescope, the more critical becomes the choice of site.
In the case of the NOT, the site problem was given ample attention. Taking into
account all major parameters defining site quality for observations in the
optical and infrared wavelength regions and, at thesame time, assigning high
weight to image quality, it was decided to locate the telescope on the premises
of the Observatorio del Roque de los Muchachos. The observatory grounds cover a
generous area (Sanchez, 1985), coinciding with the highest part of the Caldera
de Taburiente (a crater of very large dimensions and with an origin still open
to dispute), which, in turn, completely dominates the island of La Palma.
Within the observatory area, differential site testing (Ardeberg and Andersen,
1990) was made of places selected on account of local topographic
features. As a result of this testing and negotiations with other groups
belonging to the Comité Cientifico Internacional (CCI), it was decided
to erect the NOT at Cruz del Fraile, a mountain ridge defining approximately a
line.At its South end linked to the summit of the Caldera de Taburiente, Roque
de los Muchachos, the North end of Cruz del Fraile is at a distance of around
350 metres from the summit, nearly exactly in the prevailing upwind
Located at 2382 metres above sea level, the NOT site is the highest of the
observatory. Whereas the other telescopes define a loose cluster close to
Fuente Nueva, the NOT is several hundred metres away, accompanied only
by its own service building and infrastructure.
More details on the site of the NOT can be found in a separate description.
The complete installation for the NOT comprises the telescope in its building,
a service building and infrastructural elements (Ardeberg, 1985, 1987). Items
belonging to NOT infrastructure are a lifting device for the primary
mirror, a non-break-power
installation, a transformer and a heat exchanger. Finally, an access road is
included with space permitting proper access to the telescope building for
vehicles, including heavy trucks. In order to secure optimum image quality, the
telescope is installed close to the North extreme of the mountain ridge Cruz
del Fraile. With this position, prevailing winds should result in a maximum of
undisturbed laminar flow. At the same time, with the wind from theleast
favourable direction, from the Roque de los Muchachos summit, maximum
turbulence relaxation should be due, as a result both of distance from the
summit and of local topography. Both effects were verified during site testing.
The service building has been erected about 85 metres South of the telescope
building, close to prevailing downwind
direction. It is located somewhat lower than the telescope building, in a
natural cavity in the Cruz del Fraile ridge.
Normally, and always during observation, the mirror lift is kept in collapsed
position close to the service building. Also the non-break-power
device, thetransformer and the heat exchanger are placed close to the service
building. The access road links telescope and service buildings to the main
observatory road close to the Roque de los Muchachos summit.For more details
regarding telescope building and infrastructure, reference is made to special
descriptions of these items.
The telescope has an altazimuth mounting. Clearly, part of the reason for this
choice is of economical nature, as this type of mounting, with its compactness,
reduces both weight and cost of the telescope proper, at the same time as it
allows for a rather small and, thereby, low cost
enclosure. However, altazimuth mounting also has some further, rather important
consequences regarding image quality, pointing and tracking. Finally, as
compared to the more conventional equatorial type of mounting, altazimuth
mounting implies some restrictions concerning field rotation and zenith
singularity.Regarding the consequences of telescope compactness, reference is
made to sections describing mechanical structure, optics, mirror cells and wind
sensitivity. Further reference is made to sections dealing with enclosure,
image quality, pointing, tracking, field rotation and zenith singularity.
Dimensions and Weight
The NOT has a welded structure of steel with a total height in zenith position
of around 9.4 metres. The corresponding height above the upper fork base, or
above the observing floor, is approximately 7.2 metres. The total weight of the
telescope structure is about 44 tons. The moving part of the telescope has a
total weight close to 35 tons.
The largest and heaviest single structure of the telescope is the fork. It is,
in total and including the cover for the altitude gear, around 5.6 metres high
with 4.1 metres above the upper fork base. The total width is 6.1 metres in the
direction of the altitude axis and 4.6 metres in the direction perpendicular to
the altitude axis. The centre of the altitude axis is 2.9 metres above the
upper fork base. The total weight of the fork, excluding bearings, gears and
drives, is somewhat above 17 tons.
The second most prominent telescope structure is the tube. Excluding
theinstrument adapter, but including the primary mirror cell and the top unit,
it has a total length of around 5.5 metres. The distance between the surfaces
of the primary and secondary mirrors is 4.2 metres. The centre section has a
width of 3.4 metres and the top ring a diameter of 3.0 metres. The total weight
of the telescope tube, excluding optical elements and instrument adapter, is
somewhat below seven tons.
Mirror blanks are made of Zerodur by Schott Glaswerke in Mainz, Federal
Republic of Germany. Zerodur is a glass ceramics material of highest
homogeneity and longterm
dimensional stability. It has an exceedingly low thermal expansion coefficient,
being around 1/10000000 1/K, or even lower.
For practical purposes, this is fully negligible as long as reasonable
precautions are taken concerning temperature conservation of and around
For the mirror blanks, strict requirements were defined concerning
birefringence, striae and presence of bubbles and inclusions. These
requirements were met with ample safety margins. For the primary mirror blank,
the stress birefringence resulting from permanent stresses was found to be
lower than or equal to 10 nanometres per centimetre and the stress
birefringence caused by streaks of striae was below detection level. The
numbers and sizes of bubbles and inclusions were far belowrequirements inside
the critical zone and well below requirements outside it. The average linear
coefficient of thermal expansion was found to be below detection limit, or less
than 1/100000000 1/K, for temperatures
between 0 and 50 degrees C. For the secondary mirror blank,
stress birefringence resulting from permanent stress was smaller than 4
nanometres per centimetre and stress birefringence caused by streaks of striae
was, as for the primary mirror blank, below detection level. Bubbles and
inclusions were not significant and the average linear coefficient of thermal
expansion was found to be -6/100000000 1/K.
Optical figuring was made in the Optics Laboratory at Tuorla, outside Turku in
Finland, an institute of the Turku University. The laboratory is blasted into
the rock, about ten metres below ground. This ensures thermal and mechanical
stability. A vertical shaft above the laboratory, ending in its upper part in a
1 m telescope, provides a tower of 25metres height and excellent possibilities
for test measurements of the mirror under figuring, maintaining its working
position. Thus, the programme mirror can remain on its supports throughout the
figuring procedure. This increases accuracy and decreases figuring time. The
shaft has been thermally insulated. The thermal stability of the laboratory is
better than 0.5 degree C, also over very longperiods, and the
integrated turbulence over the optical test path is of the order of 0.1 arc
seconds. For further details regarding the Optics Laboratory at Tuorla,
reference is made to Korhonen et al. (1985). Manufacturing of optics for the
Nordic Optical Telescope is described in Korhonen et al. (1985) and in Haarala
et al. (1988).
The primary mirror has a diameter of 2.56 metres and an aspect ratio of 13.5.
This means that it is a mirror of the new generation, of low weight and
necessitating a sophisticated support system in order not to deform
significantly. However, it is still rigid enough not to need active optics
control to maintain its correct surface figure. The weight of the primary
mirror is 1925 kg.
The surface of the primary mirror is of hyperbolic Ritchey-Chrétien
type, and its f ratio is 2.0. The choice of a surface of Ritchey-Chrétien
type was made to ensure a large field of highest optical quality, free from
spherical aberration and coma. The central hole in the primary mirror has a
diameter of 340 millimetres. The primary mirror is faster and thinner than
those of all other telescopes of similar size so far constructed (Korhonen et
al., 1985; Korhonen, 1987).
The secondary mirror has a diameter of 0.51 metres and an aspect ratio of 6.3.
The axial radius of curvature is -2287
millimetres and the surface is hyperbolic. The weight of the secondary mirror
is 35kg. These parameters define a somewhat conventional type of secondary
mirror. The solution is perfect for the NOT, especially taking into account the
decision to replace, for observations in the infrared wavelength region, the
function of a wobbling secondary mirror with that of a focal plane
chopper. Further comments regarding wobbling secondary mirror versus focal plane
chopping can be found below under "Top End". The equivalent focal length of the
telescope is 28160 millimetres. The combined optical system gives, at the
Cassegrain focus station, an effective f ratio of 11.0. This corresponds to an
image scale of 0.13 millimetres per arc second. The unvignetted field of view
is 25 arc minutes in diameter. The total field of view is 30 arc minutes in
diameter with a maximum relative vignetting effect of 2.5%.
Specifications on the optics are severe. The combined optical system should, in
passive mode, give 80 per cent geometrical energy within 0.3 arc seconds. In
practice, the result was that the primary mirror alone had to deliver 80 per
cent geometrical energy within lessthan 0.25 arc seconds in passive mode. This,
in turn, meant that it had to be figured over the complete surface to an
accuracy better than about 30 nanometres RMS.
optical system gives freedom from spherical aberration and coma. The
aberrations remaining off the optical axis are field curvature and marginal
astigmatism. The radius of the field curvature is -963
millimetres. For this reason, without correction, optimal image quality will be
obtained over a limited field only. In practice, image spread due to field
curvature will be below a tenth of an arc second out to a (radial) distance
from the optical axis of somewhat more than 2.5 arc minutes. This corresponds
to a field of more than 40 millimetres in diameter, which is larger than the
surface covered by all linear detectors presently available. Out to a (radial)
distance from the optical axis close to 5 arc minutes, image spread due to
field curvature is still not more than three tenths of an arc second. As a
result, effects of field curvature may, for most purposes, be regarded as
insignificant forfields up to 10 arc minutes or around 80 millimetres in
diameter. For larger fields, image spread increases rapidly as a function of
distance from the optical axis and field correction becomes a necessity. For
the zone limited by 25 and 30 arc minutes diameter, respectively, to be used
for autoguiding, proper focusing gives an image quality corresponding to 80 %
geometrical energy inside from 0.5 to 0.8 arc seconds. This is fully acceptable
also for guiding of highest precision or with an rms error smaller than one
tenth of an arc second.
For use in imaging of extended objects and other types of observation for which
a telescope f ratio lower than the standard value 11.0 may be favourable, a
focal reducer will be made available. It converts the effective telescope f
ratio to 5.0 and covers a field of ten arc minutes.
Cell for Primary Mirror
With a 2.56 m primary mirror with an aspect ratio of 13.5, good image quality
requires a primary mirror cell of considerable sophistication. In practice, the
mirror cell has to ensure that deviations of the mirror surface always, also in
difficult positions, are smaller than optical tolerances. This means that all
deviations larger than around 30 nanometres have to be avoided (Andersen, 1986;
Andersen and Jessen, 1985).
The solution chosen is a circular mirror cell with a box section and a
pneumatic support system. The primary mirror is floated axially on air bellows.
These bellows are symmetrically distributed along three concentric rings. Three
supporting sections are included. Each section has 15 metal bellows and one
fixed point with a load cell. The pressure of the supporting air bellows is
adjusted to guarantee that they accurately take the weight of the mirror. The
air pressure to be applied to the totally 45 bellows isdetermined from the
forces measured for the fixed supports (Ardeberg and Andersen, 1988, 1990).
For each of the three supporting sections, the force on the corresponding load
cell determines the air pressure to be applied to the 15 supporting bellows.
This air pressure is adjusted to provide a resulting force on the load cell
close to zero. For this, a closed loop control system is used. The supporting
bellows are provided with compressed air from a compressor in the ground floor
of the telescope building, connected to the bellows via a system of pipes and
hoses. All pressure adjustment is done completely automatically and under full
electronic control (Andersen, 1986).
Further, the primary mirror cell has to support the mirror in the transverse
direction. The transverse supports have to take load only in a plane
perpendicular to that of the altitude axis. The mirror cell has 20 transverse
mirror supports,working in parallel. They are of push-pull
type with lever arms and counterweights. The lever arms transmit forces to the
edge of the mirror.
As compared to a more conventional type of mirror support system, based on use
of counterweights, the choice of a pneumatic mirror support system offers a
number of important advantages. The weight is considerably lower. In itself and
also through its implications on the weight of other structural details, this
translates into a lower cost. In addition, residual errors in the floating
supports do not result in a cumulative force on the fixed supports.
The support system described has a basic bandwidth of between two and three Hz.
At the same time, the resonance frequency of the fixed point suspension of the
primary mirror is around eleven Hz. For the corresponding oscillation mode, an
electromechanical active damping device has been installed.
Apart from the advantages described, all duly used to improve performance
anddecrease cost of the telescope, there is an advantage still pending. A
pneumatic support system of the type chosen, acting on a primary mirror with an
aspect ratio as high as 13.5, is, potentially, a good basis for installation of
an active optical system. In this context, it should be clearly stated, that
the design of the telescope, including the cell for the primary mirror, has, in
itself, been made with purely passive optics in mind. Calculations performed
demonstrate that the NOT, in a passive mode, should reach the specifications
adopted regarding image quality. At the same time, we have hope that we will
reach an image quality regime on the extreme side for earthbased telescopes. In
itself rather exciting, this calls for a certain preparation for a possible
At the same time as the mirror cell has to prevent undue deformation of the
primary mirror, it has to carry the weight of the instrument adapter and the
ancillary instrumentation attached to the Cassegrain focus station. With a
total maximum distance of 1500 millimetresbehind the adapter allowed for such
instrumentation, an adapter weight of 900 kg and a maximum allowed weight of
the ancillary instrumentation in Cassegrain focus of 250 kg, forces on the
mirror cell can be considerable. This implies that high rigidity is mandatory.
Reference is also made to details under "Instrumental Adapter".The cover unit
for the primary mirror consists of six equal sections, roughly triangular in
shape. These sections move three and three together, closing in an overlapping
fashion, sealing both section joints and the sky baffle
For a telescope intended for use in the optical as well as in the infrared
wavelength ranges, the design of the top end and the secondary mirror has to be
discussed with special care. This is especially emphasized by the fact that
some of the requirements due for observations in the optical and infrared
wavelength regions, respectively, tend to be somewhat incompatible.
In order to keep the light path as clean as possible for work in the infrared
wavelength region, the top unit should be constructed with a diameter smaller
than that of the secondary mirror. This is not in conflict with requirements
for observations at optical wavelengths, as long as there is good coordination
between mechanical and optical designs. For the NOT, such coordination has been
given high attention.
Normally, the major conflict of interests concerning observations at optical
and infrared wavelengths, respectively, iscentred on the decision whether or
not to install a device in the top unit allowing wobbling of the secondary
mirror. A wobbling secondary mirror is, for observations at infrared
wavelengths, a straightforward solution in order to provide an improved
discrimination between radiation from the target object proper, and that from
the surrounding sky. However, to be of significant help, chopping must have
both a frequency sufficiently high to compensate for variations of the
background radiation, and a throw large enough to allow also for observations
of extended objects and/or objects in contaminated sky regions. For the same
reasons, chopping should be possible with respect to two axes.
Ideally, the chopping frequency should be higher than 30 Hz. Further, maximum
throw should be at least several arc minutes. Concentrating on observations at
infrared wavelengths only, these requirements can be met, although theyare,
inevitably, in mutual conflict. With a top unit
outbalancing the secondary mirror and its cell and being wobbled in
counterphase to this unit, static and dynamic balance can be maintained,
keeping the tube structure free from significant effects of vibration.
Even for observations at optical and near infrared wavelengths, a wobbling
secondary mirror may, in itself, be of use. However, in general terms, a
wobbling secondary is a device causing much concern for such observations.
First of all, it may be rather detrimental to image quality, violating the
principle of fixed optomechanical elements. Also with sophisticated design, it
is hard to avoid completely a negative influence on image quality. The
situation becomes especially conflictive when image quality is given high
priority, as is the case for the NOT. Further, introduction of a wobbling
secondary will almost inevitably cause problems regarding pointing and
tracking. Again, the higher the requirements, the more serious become the
detrimental effects. It should be noted that decrease of pointing and tracking
quality is, inmost respects, even more serious for work at infrared wavelengths
than for corresponding work in the optical wavelength region.
Whereas a wobbling secondary mirror is rather useful for observations at
infrared wavelengths, especially in the thermal infrared part of the spectrum,
it is not the only alternative for improvement of discrimination between
radiation from the target object and the surrounding sky, respectively. Also
devices for focalplane
chopping can today be made rather effective, especially if wavelengths above
several micrometres are given lower weight. In many cases, a focal plane
chopping device seems a perfect alternative to the problem normally resulting
from solutions involving installation of a wobbling secondary mirror. Further,
and most important, larger two dimensional
detectors for imaging at infrared wavelengths are rapidly becoming commonly
available. This fact implies that the practical consequences of the choice
between different solutions for sky chopping are heavily decreased.
Nevertheless, for the NOT, the alternative sky chopping
solutions involving a wobbling secondary mirror and a focalplane
chopping device, respectively, were thoroughly studied and compared. The
outcome was, that for wavelengths shorter than around ten micrometres, the two
alternative solutions did not seem to give very different results for sky
suppression, whereas for longer wavelengths the wobbling secondary mirror tends
to give increasingly superior performance. With highest scientific emphasis on
wavelengths shorter than ten micrometres, and taking into account the
limitations posed by atmospheric watervapour for longer wavelengths, it was
decided that the most reasonable overall solution was to adopt focalplane
chopping, leaving the secondary mirror fixed. For further details regarding
alternative sky chopping
solutions for the NOT, reference is made to Olofsson (1984).
To improve image quality, the top end may be moved not only in focusing mode
but also laterally. In addition, it can be tilted, although this should,
normally, not be necessary. These movements are all discussed below, under
The NOT has two sky baffles, one for the primary mirror and one for the
secondary mirror. The sky baffle for the primary mirror has a total length of
2053 millimetres, a base diameter of 445 millimetres and a top diameter of 385
millimetres. The sky baffle for thesecondary mirror has a length of 630
millimetres, a base diameter of 620 millimetres and a top diameter of 715
Together with the quality of mirror blanks and optical figuring, the degree of
sophistication of the mechanical structure define the limiting performance
level of the telescope, especially with regard to image quality. In
consequence, considerable attention has been paid to design of these
structures. In particular, the complete mechanical structure has been the
subject of detailed finite element
With the fast, f/2.0, primary mirror, optomechanical deviations are critical
and have to be avoided to any significant extent. The adoption of a Cassegrain
focus station further stresses the importance of an excellent mechanical
structure. This is due both to the weight of ancillary instrumentation behind
the primary mirror cell and to the need for a comparatively large distance
between the rear plane of this mirror cell and the fork base, leading to
relatively long fork arms.
Requirements on the fork can be fully taken into account in a relatively
easyway. From the finite element
analysis, the size of the fork can be chosen on the "safe" side. In practice,
this means that the fork be made rigid enough to keep its resonance frequency
above critical level. This has been fully accomplished for the NOT. Details are
given below under "Resonance Frequencies".
For the telescope tube, the problem of optomechanical performance is somewhat
more difficult. The basic requirement is that, in all operating positions of
the telescope, the relative positions and orientations of the primary and
secondary mirrors be kept constant. In total, the telescope tube has to carry
not only its own weight but also that of mirror, mirror cell, instrument
adaptor and ancillary instrumentation. This calls for a high quality
primary mirror cell, dealt with above under "Cell for Primary Mirror". However,
it also requires a stiff tube and an adequate counterweight system.
From results of finite element
analysis, the structure of the telescope tube has been chosen so as to
guarantee necessary rigidity. (Further details regarding telescope rigidity may
be found under "Resonance Frequencies"). Four remotely adjustable
counterweights are attached to the center section. The counterweights are moved
in a simultaneous fashion. This will adequately take care of the problem of
varying weight and size of ancillary instrumentation. As the instrument adapter
rotates to compensate for field rotation, the telescope counterweights can not,
in a proper way, take care of asymmetric weight distribution caused by
ancillary instrumentation. This type of effects has to be compensated by
counterweights on the adapter. The weight of each telescope counterweight is
200 kg and its total working range is 800 millimetres. It is added, that some
unbalance can be accepted without negative effects. Assembly drawings can be
found in Nordic Telescope Scientific Association Techn. Rep. (1986).
Even with maximum efforts concerning primary mirror cell, tube
structure,counterweight system and top end, there will, with changing telescope
position, be certain inevitable relative displacements between the primary and
secondary mirrors. To compensate for these effects, the secondary mirror can be
moved in various ways.
First of all, the secondary mirror can be moved along the optical axis, for
focusing. The total focusing range is 8 millimetres. Further, the secondary
mirror can be moved laterally with respect to the optical axis. The lateral
movement is along a direction perpendicular to the altitude axis. As a result,
the centre of the secondary mirror can, laterally, be moved freely along a line
in the top-end
plane with a stroke of 0.6 millimetres. Finally, if needed, the secondary
mirror can be tilted with respect to the optical axis. The maximum tilt is one
degree. Theoretically, no tilting of the secondary mirror should be necessary.
For ultimate fine tuning of image quality, an autoguider is available. At the
same time as the use of this facility is recommendable, it should be
stronglyemphasized that autoguiding is no substitute for an adequate mechanical
structure. For optimum image quality over longer exposure times, the
appropriate solution is an adequately working mechanical system, including an
integrated servo system taking care of critical movements, plus an autoguider.
Reference is also made to details given in "Instrument Adapter".
An integrated servo system (Jannerup, 1986) under computer control manages all
movements and handles all data relevant for proper alignment and image quality.
This means that the electronic system automatically controls
of telescope with respect to azimuth axis
of telescope tube with respect to altitude axis
rotation of instrument adapter
support system for primary mirror
movement of secondary mirror
displacements of secondary mirror
of secondary mirror (ifnecessary)
of autoguider corrections
of data on temperature, pressure and relative humidity of ambient air
plus continuous calculation and implementation of refraction data
The bandwidth of the main servo loops is around 5 Hz. This is sufficiently high
to guarantee a tracking accuracy, as seen by the detector, of better than a
tenth of an arc second RMS.
The total image quality
specification for the telescope is that in a passive mode and over a field of
ten arc minutes, the integrated optomechanical system should give resulting
energy concentration corresponding to 80% geometrical energy within 0.4 arc
seconds for all positions with zenith distances smaller than 60 degrees. For
larger zenith distances, the corresponding image quality
specification states 80% geometrical energy within 0.7 arc seconds. These are
instrumental image qualities well matching the best atmospheric image qualities
For maintenance work, it is necessary to lock the telescope in certain
positions. For this reason, a safety lock pin is available on one of the fork
arms. Withthis lock pin, the telescope tube can be locked in zenith position
and at four degrees above the horizon.
Cassegrain Focus Station
With high priorities for optimum image quality and minimum effects of
instrumental polarization, there was high preference for a single focus station
implying a minimum of optical elements and a maximum of optical symmetry. As a
result, it was decided to make the NOT a single focus
telescope of Cassegrain type, at the same time not precluding later addition of
a Nasmyth focus station.
Some of the instrumental consequences of the choice of a Cassegrain focus
station are discussed above under "Cell for Primary Mirror" and "Mechanical
Structure". It is added, that the simpleoptical solution for the Cassegrain
focus tends to be favourable also for pointing and tracking accuracy. Finally,
together with a telescope with altazimuth mounting and a telescope building
rotating with the telescope, choice of Cassegrain focus gives favourable
conditions for cabling, especially for experimental setups. More details
regarding cabling aspects are given in a description of the NOT building
For improved flexibility, especially regarding rapid and comfortable exchange
of ancillary instrumentation, provision is available for pseudo-Nasmyth
connection of instruments. This solution is based on a fibre optical
feed through the Cassegrain instrument adapter. It allows rapid exchange of
several pieces of ancillary instrumentation in a standby
fashion. Regarding connection and exchange of ancillary instrumentation, more
comments can be found elsewhere.
The distance between the vertex of the primary mirror and the focus is 962
millimetres. For the Cassegrain focus, the free distance behind the
instrumental adapter is 1500 millimetres. The corresponding maximum weight of
a piece of ancillary instrumentation to be attached in Cassegrain focus is 250
kg. In both cases, the values can be regarded as generous. Also instrumentation
considerably larger and/or heavier can be used, connnected via optical fibres.
They can then be placed in or on the fork structure, on the observing floor or
in more remote places. Further details concerning pseudo-Nasmyth
connection of ancillary instrumentation are given under "Instrument Adapter"
and in instrument descriptions.
In most types of engines working with bearings, smoothness of running depends,
to a large extent, on the quality of the bearings. This is especially true for
telescopes. The reason is that for telescopes, requirements on smooth operation
are hard, at the same time as the critical movements are very slow. Inpractice,
this means that telescope bearings have to operate with very low friction. It
is equally important that significant effects of play are avoided.
Friction values low enough to be acceptable for telescope applications are
provided by ball bearings of good quality,as long as diameters are small
enough. With bearing diameters exceeding one metre, it is increasingly
difficult to guarantee adequate friction values. In this case, hydrostatic
bearings constitute an attractive alternative. If hydrostatic bearings are
chosen, measures must be taken to minimize effects of heat dissipation from the
hydraulic power supply. Adequate temperature control is essential both for the
thermal stability of the telescope environment and for the viscosity of the
The NOT has azimuth and altitude bearings. For the azimuth axis, horizontal
loads are taken up by a ball bearing. This has a diameter of only 750
millimetres and is not exposed to more than marginal forces. As a result,
friction is not a significant problem for this bearing. Also, with a small
internal preload, effects of play are, in practice, fully eliminated. Both the
friction and the heat dissipation of the azimuth ball bearing are fully
For vertical loads, the azimuth axis has a bearing with a diameter of
4000 millimetres. With a diameter of this size, adoption of a ball bearing would
imply a considerable risk for significant and unpredictable friction. In
consequence, as a safe solution, a hydrostatic bearing has been chosen.
The hydrostatic azimuth bearing has three circular pads fixed to the yoke
structure. The bottom part of these pads is a cylinder with an outer diameter
of 250 millimetres and an inner one of 230 millimetres. To the five millimetres
high space inside the supporting cylinder, oil is pumped under a pressure of 30
bar. This raises the pads to the moving position, with the supporting cylinders
floating on an oil film with a thickness of around 30 micrometres.
The pads of the hydrostatic bearing rest on a surface consisting of a steel
ring, ground to high precision. This steel ring defines the horisontal support
plane for the yoke, and it has an oil sump for the hydrostatic bearing. The
steel ring is supported by a steel structure with a machined surface. This
steel structure rests in the concrete telescope foundation.In normal running
mode, the total heat dissipation in the hydrostatic bearing is somewhat below
one kilowatt. Water cooling of the hydraulic power supply is used to eliminate
the most immediate potential heat dissipation problem. The oil temperature is
kept constant at +5 degrees C. Thermally, the oil will
not affect the telescope structure and only marginally the telescope base. It
is recalled, that the complete hydrostatic bearing system as well as the base
and upper part of the telescope foundation are subjected to forceful cooling by
air flushing in the cooling jacket below the observing floor.
The altitude axis bearings have an outer diameter of 750 millimetres. This is
small enough for ball bearings to constitute a perfect solution. Radial groove
ball bearings have been chosen. The two altitude bearings are aligned to high
precision in order to avoid excessive friction. Both bearings are preloaded
with the yoke structure acting as a preload spring.
With the bearing and preload systems described, total friction in azimuth is
ofthe order of 10 kilopondmetres. In altitude, the total friction is around 20
kilopondmetres. Both friction values can be regarded as rather favourable. It
is noted, that, with drive motors disengaged, the telescope can be easily moved
manually in azimuth as well as in altitude. Including ancillary
instrumentation, the total moving mass of the telescope is close to 35 tons.
The NOT is driven in azimuth and altitude via gears. For each axis, there is a
large gear wheel with a diameter of 2400 millimetres and two small pinions on
two motors. The pinions have diameters of 100 millimetres.
The gears have been machined to very high precision with special tooth
compensation to assure smooth running. The tooth-to-tooth
accuracy of the two largest gears is guaranteed to be better than four
Accurate pointing of a telescope is an important feature for many reasons.
First of all, for all types of observations, good pointing is a prerequisite
for safe work, especially regarding fainter objects. Further, even if
relatively bright objects are observed, high pointing accuracy means faster
work. Especially for brighter objects, pointing and setting of objects can, if
the pointing is of low quality, often occupy more time than the observations
proper. Finally, for observations in the infrared wavelength region, accurate
pointing is mandatory, as objects are often hard or impossible to locate. For
observing in day time, these difficulties are greatly emphasized.
The basic prerequisite for accurate pointing is that the mechanical structure
reacts in a manner predictable within limits correspondingly smaller than the
pointing accuracy aimed for. Above, under "Mechanical Structure", details have
been given regarding rigidity of fork and tube. Further, it has been shown
howthe integrated servo system, under computer control, handles corrections
necessary to compensate for inevitable relative displacements between the
primary and secondary mirrors.
Naturally, high pointing accuracy can be obtained only with encoders of good
quality and sufficient angular resolution. This is dealt with below under
"Encoders". In addition, correction for effects of refraction has to be made
quite exactly. For this purpose, temperature, pressure and relative humidity of
ambient air has to be monitored continuously with sufficient accuracy. More
details regarding corrections for effects of refraction are given below, under
For the NOT, the accuracy of blind pointing has been specified as two arc
seconds RMS over the complete sky available. For differential pointing over
limited fields, the corresponding accuracy should be of the order of a few
tenths of an arc second.
As stated above, image quality is a matter of high priority for the NOT. This
immediately calls for very accurate tracking. It must be added, that accurate
tracking is, in itself, of high importance for most types of observations. As
with pointing, tracking of high quality is a necessity for observations in the
infrared wavelength region, and especially so for observations carried out in
The tracking accuracy of a telescope depends on the quality of the mechanical
structure, mirror cells, mirror support systems and servo systems. These
details have been discussed above under "Mechanical Structure" and "Cell
forPrimary Mirror". Further, tracking accuracy is affected by uncontrolled
friction and defects of gears. This is taken up below under "Bearings" and
"Gears". Finally, wind, and especially wind with high frequency
variations, can influence the tracking quality in a rather significant way, if
adequate precautions are not taken. More details regarding this are given below
under "Sensitivity to Wind".
The specification regarding tracking accuracy for the NOT was defined as one
tenth of an arc second RMS. This refers to the complete sky available and to
effects with frequencies of 0.01 Hz and higher.
With altazimuth mounting, field rotation is inevitable. For many types of
observations, this is of no consequence.However, for certain types of
programmes, field rotation must be fully avoided. One example is longslitspectroscopic
observations over the surface of a galaxy, aiming at spectral and/or radialvelocity
analysis as a function of distance from the centre of the galaxy. Another,
rather simple, example is spectroscopy of a star with one or more nearby
Several types of optical rotators have been constructed. Most of them easily
take care of effects of telescope field rotation. However, at the same time
rather negative effects are added, such as light loss and decrease of image
quality.Especially the light losses can be considerable.
In order to avoid completely these effects, the NOT has been equipped with a
field rotator as an intermediate instrument item between the cell for the
primary mirror and the instrument adapter. With this rotator, residual effects
of field rotation should be fully negligible over the entire field accessible,
30 arc minutes. For further details regarding the field rotator, reference is
made to "Rotator" below.
For a telescope with altazimuth mounting, a zenith singularity is unavoidable
due to the fact that there is a practical limit to the azimuthal velocity of
the telescope and of the telescope building.
For the NOT, the azimuthal velocity has an upper limit, corresponding to the
slewing speed, of three degrees per second.As a result, the minimum zenith
singularity possible has a size of around 30 arc minutes times 20 arc minutes,
or about 0.2 square degrees.
It should be noted, that close to the zenith, the dome will introduce
vignetting, due to limitations in the maximum angle to which the upper hatch
can be opened.The surface around the zenith position, for which vignetting is
introduced, has an extension of 1.3 degrees for an object on the optical axis.
From a practical point ofview, the zenith singularity as well as the sky
surface around the zenith affected by vignetting are of negligible
For good tracking and image quality, a stiff mechanical structure with high
resonance frequencies is an absolute necessity. This need is highly emphasized
in the presence of significant wind buffeting. Further, great care must be
taken to avoid transmission through the foundations and/or ground of effects of
lower resonance frequencies of the building and dome structure.
The azimuth axis has its lowest resonance frequency at 7.4 Hz. For the altitude
axis, the lowest resonance frequency is 14 Hz. These are values which can be
classified as very safe, taking into account basic structural rigidity as well
as effects of wind buffeting. Coupling between building and telescope has
been minimized. Asa special precaution, foundations of telescope and building
have been completely separated. The foundation of the building rests in a
surface rock layer with a thickness of around 1.5 metres. The foundation of the
telescope goes down to another rock layer with its upper surface about three
metres below ground. The material between the two rock layers consists of
rather soft material. Further, the rail for the rotating telescope building has
been placed on rubber sheets.
Encoders and Transducers
Correct choice and matching of encoders are essential for accurate positioning
of the telescope. The NOT has a total of five encoders. Two encoders are
installed for position readings of the azimuth axis and two for corresponding
readings of the altitude axis. Finally, one encoder is installed in the rotator
for readings of rotator position.
For readings of the azimuth and altitude positions, the choice is a combination
of one static and one incremental encoder. The two identical solutions include
one absolute Stegmann encoder and one semiabsolute
Indoctosyn encoder. The Stegmann encoder has an absolute accuracy of 80 arc
seconds. The Indoctosyn encoder repeats itself and is absolute within one cycle
of one degree. Combination of the two encoders results in an absolute
positional accuracy somewhat better than two arc seconds. Over limited fields,
differential accuracy should be considerably better.
The rotator has been equipped with a Stegmann absolute encoder only. The
resulting accuracy of 20 arc seconds is fully sufficient for this purpose. With
the largest telescope field possible, 30 arc minutes, the corresponding maximum
error over a two hour
exposure corresponds to only a few hundredths of an arc second.
In the top end unit, two linear transducers are installed. These transducers
transmit data for positions of the secondary mirror in focus direction and in
lateral direction. In both cases, the accuracies delivered by the linear
transducers are beyond what is needed for practical purposes.
For telescopes with conventional pointing and tracking accuracies and for which
fine object tracking is done either manually or applying direct corrections
from an autoguider, refraction effects are of minor importance. In such cases,
effects of refraction are noticeable only for longer exposures of objects at
large air masses and for which exposure and guiding, respectively, are made
with light of significantly different wavelengths.
With high blind pointing and tracking accuracy, effects of refraction are much
more noticeable. For the NOT, already blind pointing should be accurate to
around two seconds of arc (Laustsen and Klim, 1985). This means that one might
expect to achieve accuracies for differential pointing in limited fields which
are of the order of a few tenths of an arc second. This, in turn, implies that
the telescope, as spinoff
data, can deliver astrometric positions of reasonable quality. Further, and for
many purposes of high importance, we should be in aposition to achieve tracking
accuracies better than a tenth of an arc second RMS.
With expectations for differential pointing and tracking down to and better
than a few tenths and one tenth of an arc second, it is obvious that effects of
refraction have to be taken quite seriously. In practice, this calls for
inclusion of accurate refraction corrections as an integrated part of the servo
system for the telescope. In the case of the NOT, such corrections are based on
local meteorological data obtained from a meteorological station placed at a
distance of 50 metres from the telescope building. These data are received and
processed by the telescope control system, appropriate corrections being
executed through the servo system.
The sky available to the observer is, in principle, limited by the geographical
latitude of the site, the altitude of the telescope, surrounding landscape and
restrictions imposed through hardware and software arrangements. In the case of
the NOT, practical limits to the sky available are defined by the geographical
latitude and software limits.
With a geographical site latitude of +28°45'20.5", and a
software limit at 4.5 degrees above the horizon, the sky available for
observations includes the complete Northern Hemisphere plus the Southern
Hemisphere north of declination -56. For practical purposes, the corresponding limitation in
declination should be regarded as around -50.
It must be pointed out, that for normal observations it is prohibited to
attempt to override the software limitation in height above the horizon. For
absolute safety, a mechanical endstop is set at four degrees above the horizon.
Also for altitude positions beyond the zenith, the telescope has a software
limitation set at 3.5 degrees beyond zenith. The corresponding mechanical
endstop is at four degrees beyond zenith.
The mechanical endstops in altitude consist of shock absorbers acting between
the moving part of the telescope and the stationary counterpart. A compression
of the oil-filled
shock absorbers results, via a current limiting switch, in cutting of the power
to the altitude motor.
With telescope positioning under computer control, warnings will be issued via
the control system, as soon as the altitude limits are approached. Positioning
orders including passing of the lower altitude limit will not be executed.
Instead, an explanation will be delivered via the control system.
All turbulence with the exception of intrinsic atmospheric site turbulence may
be described as man-made
turbulence. This can, in turn, be divided into observatory turbulence,
enclosure turbulence, telescope turbulence and instrumental turbulence. All of
these four components of man-made
turbulence are very important (Ardeberg and Andersen, 1990). However, in this
context we are only concerned with telescope turbulence.
Telescope turbulence can be seen as composed of turbulence resulting from
telescope structure and from the primary mirror, respectively. Both components
can give considerable contributions, especially important in the case of
telescopes with tight image quality
Turbulence from the telescope structure has two main sources. One is the
telescope structure proper, the other consists of devices attached to the
telescope structure. The telescope structure in itself produces turbulence
dueto heat exchange with ambient air and sky.
Most conventional telescopes have unfortunately large amounts of electronics
devices and motors attached to the telescope structure. Further, the heat
dissipation from these installations is often quite considerable, a severe
disadvantage emphasized by the closeness to the optical path.
Nocturnal heat flow from the primary mirror is a very prominent obstacle to
high image quality. Normally, the heat capacity of primary mirrors is very
large, with thermal time constants being of the order of a number of hours,
including several days in some cases. The critical position of the primary
mirror highly emphasizes its role as a source of severe thermal inbalance.
A hot primary mirror threatens image quality in two ways. First, a nonuniform
temperature distribution within the mirrortends to deform the mirror surface.
This is not, in itself, to be referred to as telescope turbulence. Second, a
very important contribution to telescope turbulence is provided by the hot air
layer above the mirror surface. The convective activity of this air layer can
easily dominate the resulting imaging performance for telescopes with tight
total error budgets in image quality.
For the NOT, effects of telescope turbulence have been taken very seriously.
Decided attempts have been made to minimize to the extent possible all sources
of telescope turbulence discussed.
A possible temperature gradient along the telescope structure has been given
special attention. Obvious sources of such a gradient are the hydrostatic
bearing and general heating of the telescope base and yoke.
Introduction of a thermal gradient along the telescope structure via the
hydrostatic bearing has been minimized in two ways. First, the hydrostatic
power supply iscooled. Second, the hydrostatic bearing as well as the lower
part of the yoke and the base of the telescope are enclosed in the heat trap
between the observing floor and the ground floor of the telescope building.
General heating of the telescope base and yoke has been minimized through a
series of precautions. First, all spaces in the ground floor of the telescope
building are kept at low temperature, carefully maintained by air conditioning,
excess heat being ducted away. Second, these spaces are heavily insulated.
Third, a "cooling jacket" constitutes an effective protection against all
remaining vertical heat leakage.
In order to minimize the risk of temperature gradients along the telescope
tube, the complete air volume around the telescope is subjected to active
temperature control. For this purpose, both floor cooling and cooling of the
air in the higher part of the dome have been established. For the heat exchange
between the telescope structure and nightsky, further studies on site seem
necessary. Further, the centre section of the telescope has its inner surface
The amount of electronics devices and motors attached to the telescope
structure has been restricted to an absolute practical minimum. Cabling and
remotely placed electronic devices have been preferred whenever possible. At
the same time, heat dissipation of remaining units has been kept as low as
On the telescope structure itself, a total of 15 motors are present. These are
two azimuth motors, two altitude motors, one focusing motor for the secondary
mirror, one motor for lateral displacement of the secondary mirror, two motors
for possible tilting of the secondary mirror, six motors for the flaps of the
mirror cover and, finally, one motor for the telescope counterweights.
Normally, only the four main drive motors and the two rotator motors are
running during observations.
The two motors for azimuthal movement of the telescope have heat dissipations
which are very low. Further, both motorsare cooled. Moreover, they are located
in the cooling jacket environment. As a result, thermal influence on the
telescope structure should be negligible, even if it has to be noted, that high
wind forces can deteriorate the picture somewhat.
Also the two motors installed for altitude movement of the telescope have very
low heat dissipations. They run in a continuous mode during normal observing.
Like the azimuth motors, the altitude motors are cooled. This should reduce the
thermal influence of these motors to negligible amounts.
The motor installed in the top end for focusing movements of the secondary
mirror has a very low heat dissipation. Further, during normal observing
activity, it is activated only in an intermittent mode. It seems fully
justified to regard thermal influence of this motor as insignificant.
The necessity for lateral movements of the secondary mirror is very limited.
The motor for this purpose, installed in the top end, has very small heat
dissipation.The thermal effects of this motor can be regarded as insignificant.
Angular adjustment of the secondary mirror is achieved through axial
corrections in two of the three support points. For these corrections, two
motors are installed in the top end. They have very low heat dissipations.
During normal observing, these motors are not active. Thus, they have no
The six motors installed in the centre section for operation of the flaps of
the mirror cover are inactive during observing work. Thus, they have no thermal
influence. It is noted, that it belongs to normal prudence to open the mirror
cover well in advance of observing initiation.
On the centre section, there is finally a motor attached for movements of the
counterweights. This motor is activated only in connection with exchanges of
ancillary instrumentation attached directly to the Cassegrain focus station.
For this reason, it has no thermal influence.
In addition to the azimuth and altitude motors, a total of four encoders are
installed. All have heat dissipations which are negligible.
The last item to be mentioned in connection with heat dissipation along the
telescope structure is the rotator unit. The rotator has two motors and one
absolute encoder. The two motors have very low heat dissipations and the
absolute encoder a negligible heat dissipation. For some observing programmes,
the rotator can be maintained inactive, in which case it has no thermal
It is added, that the top end unit contains two linear transducers. These are
used to measure displacements of the secondary mirror along and perpendicular
to the optical axis. The heat dissipation in these transducers is
Finally, there are certain possible thermal effects from the instrument adapter
and from ancillary instrumentation. The thermal influence of the adapter is
discussed under "Instrument Adapter",whereas those of ancillary
instrumentation are detailed elsewhere.
For the NOT, mirror blanks are of Zerodur. The very low thermal expansion
coefficient of this material is an excellent guarantee that distorsions of the
surface of the primary mirror will be below detection level even for worst
possible practicle temperature differences between the primary mirror and
Creation of a heated, convective air layer above the primary mirror is a
problem much more difficult to solve. Flushing of the mirror surface with cool
air is far from simple and is not evident as remedy. For a primary mirror as
large as that of the NOT, air flushing of the surface may well add more
convective activity than it eliminates.
Clearly, the most efficient cure against mirror turbulence, at least in
principle, is to maintain the mirror temperature equal to that of ambient air.
This involves two difficulties, both hard to solve. These difficulties concern
thermal relaxation and temperature prediction.
The heat capacity of the primary mirror is quite low because of the high aspect
ratio. The resulting time constant is of the order of a couple of hours. Still,
thermal relaxation has a time scale far from insignificant compared to diurnal
and nocturnal cycles.
Problems resulting from the time scale of thermal relaxation of the primary
mirror are severely emphasized by difficulties encountered in forecasting
nocturnal ambient temperatures. Unfortunately, these difficulties imply that
also with optimum preparations, measures against telescope turbulence
originating in the air layer above the surface of the primary mirror will,
unavoidably, contain a component not fully within control. The best possible
remedy is to keep the temperature of the telescope ambience as constant and
close to forecasted night temperature as possible with a preference for
slightly lower temperatures. This is the policy followed for the NOT. It is
added, that a study of local temperature conditions is foreseen. This will
improve both forecasting of night temperature and measures to maintain
Sensitivity to Wind
Wind affects observing conditions in many ways. In general, complete absence of
wind is favourable for telescope pointing and tracking but not necessarily the
optimum condition concerning the microthermal climate of the site and for
degradation of image quality. Wind of modest speed should be of no consequence
for the operation of an adequately constructed telescope, at the same time as
it might well improve the microthermal climate of the site andfurther
turbulence due to the telescope. High wind speeds tend to influence both
telescope operations and microthermal site climate in a negative way.
Regarding telescope behaviour, the primary negative effect of high wind speed
is buffeting. Clearly, wind with a low bandwidth influences telescope operating
parameters less than wind with a spectrum characterized by a prominent high frequency
part.The telescope parameters most immediately influenced by wind (buffeting)
are pointing, tracking and sensitivy to dust. To some extent, also telescope
turbulence will be affected. The tracking and telescope turbulence directly
influence image quality.
Concerning pointing and tracking, a minimum sensitivity to wind buffeting can
be achieved with a mechanical telescope structure with maximum high resonance
frequencies. In this respect, more details can be found under "Resonance
frequencies" and "Mechanical Structure" above.
The dependence on wind characteristics of sensitivity to dust are evident. For
more details, reference is made to "Sensitivity to Dust" below.
Both the degree and effects of telescope turbulence may be significantly
by wind. This type of wind sensitivity depends on various parameters. In the
case of the NOT, given its rather low level of telescope turbulence, such
effects should be close to negligible.
For the NOT, protection against effects of wind can be achieved through
adequate combination of the positioning of gates and hatches. At the same time,
great care should be taken for wind speeds higher than 10 metres per second,
especially in case of gusty wind.
In itself, the telescope should maintain its pointing and tracking quality even
for wind speeds up to 22 metres per second. However, due to the risk for
presence of dust in the air at higher wind speeds, the dome should be
completely closed (gates and hatches) for wind speeds above 20 metres per
second. For the same reason, all gates and the lower hatch should be completely
closed for wind speeds higher than 12 metres per second. Further, for wind
speeds above 15 metres per second, the telescope should not face the upwind
direction nor any direction within 45 degrees of the upwind
Regarding the safety limits detailed above, instructions will be given via the
control system. This includes warnings when limits are approached. Further, the
control system takes over in an automatic safetymode, if instructions are not
duly followed. In this way, possible temptations to continue observing beyond
safety limits should not affect the telescope adversely.
It is important to remember that the dome must be completely closed
(gatesand hatches), as soon as the telescope is subjected to wind vibration.
This applies also to wind speeds lower than the limits stated above. Continued
exposure to wind vibration will seriously affect gears and bearings. Details
regarding hatches and gates are given elsewhere.
Sensitivity to Humidity
No part of the telescope nor of the ancillary instrumentation should be exposed
to water, neither in the form of precipitation nor as condensation. For this
reason, the dome should be completely closed (gates and hatches) as soon as
there is risk for condensation and/or precipitation.
First of all, when the sky is overcast, the upper hatch should be maintained
closed. Further, the dome should be completely closed (gates and hatches) as
soon as the local relative humidity is 95 per cent or higher. The same applies
when condensation and/or precipitation is/are observed, whatever the reading
for the local relative humidity. It is highly recommendable to pay special
attention to circumstances as soon as the local relative humidity is above 80
per cent and/or when it is in a state of rapid increase.
If the local relative humidity has reached values of or above 95 per cent
and/orprecipitation and/or condensation has/have been noted, the dome should
remain completely closed (gates and hatches) until the local relative humidity
has fallen to values below 90 per cent and signs of precipitation and
condensation do not appear.
Higher levels of humidity are especially dangerous when coinciding with high
amounts of dust in the air. This is discussed below under "Sensitivity to
Dust". Concerning hatches and observing wall gates, details can be found
Sensitivity to Dust
Already small amounts of dust in the air are detrimental for telescopes.
Accumulated effects on sensitive parts can be very serious and heavily decrease
operational quality. Examples of such sensitive parts are gears, bearings and
In principle, the most direct way to control the presence of dust in the air is
through use of dust meters. Such meters are available commercially. Some of
them are rather reliable. However, most of the dust meters are designed to
operate in conditions rather different from those at sites for optical
telescopes. Many of them are integrating devices. If long integration times are
necessary for consistent data, the applicability to site conditions is
In the absence of reliable dust meter
data, the presence of dust in the air has to be taken with greatest concern. It
is especially alarming when coinciding with significant wind speeds and/or
higherlevels of humidity. As soon as the wind speed exceeds 10 metres per
second and dust is present in the air, the gates and lower hatch of the dome
should be closed. Further, in these circumstances, the telescope should not
face the upwind
direction nor any direction within 45 degrees of the upwind
direction. If dust is present in the air, the dome must be completely closed
(gates and hatches) when the wind speed exceeds 12 metres per second.
Presence of dust in the air may provoke condensation at levels of local
relative humidity much lower than those normal for air free from dust. This
type of condensation may occur very rapidly. Therefore, coincidence of higher
levels of humidity and presence of dust in the air calls for special attention.
Details regarding hatches and gates are given elsewhere.
With a telescope with altazimuth mounting, field rotation has to be taken into
account. For many types of observing programmes, field rotation is hardly
noticeable, and for even more types of programmes it is without consequences,
except for autoguiding. At the same time, for some types of observing
programmes, field rotation is rather important. This has been discussed above
under "Field Rotation".
The field rotator provided for the NOT rotates the field to an accuracy of 20
arc seconds. For all practical purposes, this is more than sufficient, as
discussed under "Encoders and Transducers". At the same time, the rotator can
be used to position ancillary instrumentation to selected angular positions at
corresponding accuracy. This may, for example, be used in case of long-slit
spectroscopy of galaxies and nebulosities.
With a blind pointing accuracy of two arc seconds for the NOT, use of a finder
telescope is superfluous for normal observing. However, for testing and
debugging in initial phases and after reassemblies in connection with
aluminizations of telescope optics, access to a finder telescope is often
helpful. For this reason, the NOT has been provided with a finder.
The finder telescope is a refractor with a free aperture of 153 millimetres and
a focal length of 2130 millimetres. It has a rigid tube and is firmly attached
to the centre section of the NOT. A modest CCD camera is used as detector. The
image resolution is close to one arc second and the limiting magnitude with
normal scanning frequency is around V=7.
The instrumental adapter for the NOT is intended to provide interface between
the telescope and ancillary instrumentation. At the same time, it includes some
observing functions common to most types of observation. Among the functions
possible, one may mention field viewing, periscope viewing, autoguiding,
image quality monitoring, offset guiding, autocollimation
for alignment verificationof telescope optics, complete checking of prevailing
image quality, framegrabbing
and limited photometry. In addition, the instrumental adapter holds a
permanently mounted standby
CCD camera, possible to switch in within a matter of seconds. The main purpose
of this latter arrangement is to permit the observer to take advantage also of
excellent image quality prevailing for shorter periods only.Further, the
adapter carries a Hartmann-Korhonen
device for online
determination of optical quality.
With a considerable number of electronic devices, the instrumental adapter has,
intrinsically, a heat dissipation far from negligible. For this reason, a
special cooling unit has been designed. With thisunit active, resulting heat
dissipation from the adapter is negligible.
For further information regarding the instrumental adapter, reference is made
to a detailed description by Florentin Nielsen (1989). Assembly drawings can be
found in Nordic Optical Telescope Scientific Association Techn. Rep. (1986).
In addition to improvements of alignment, tracking and thermal control, some
projects are in course for further upgrading of our telescope. Among these
projects we mention, first, installation of a system of temperature probes
designed to monitor temperatures of ambient air, enclosure, air volume around
the telescope, on and below the observing floor and of various parts of the
telescope, including both optical and mechanical elements. Resulting data will
be used for further improvements of routines for thermal control. Second,
withan aspect ratio of 1:13.5, the primary mirror is well suited for active
mirror support corrections. We have taken up design of corresponding
modifications and supplements. Third, advanced laboratory simulations,
supported by activities for the design of the Large Earthbased Solar Telescope,
entrusted to our telescope group, strongly indicate that air flushing of the
surface of the primary mirror can significantly improve turbulence conditions.
We intend to follow this up, and a possible design is presently under
Andersen, T.B. 1985: Optical Specifications and Performance of the Nordic
Optical Telescope, Technical Report from the Nordic Optical Telescope
Andersen, T.E. 1981: 2.5 m Telescope Design Study, Birkerød,
Andersen, T.E. 1982: 2.5 m Telescope, Addendum to Design Study,
Andersen, T.E. 1986: Mirror Cell Pressure Regulator, Technical Report from
the Nordic Optical Telescope Scientific Association
Andersen, T.E., Jessen, N.C. 1985: Deformation Calculations of the Primary
and Secondary Mirrors of the Nordic 2.5 m Optical Telescope, Technical Report
from the Nordic Optical Telescope Scientific Association
Ardeberg, A. 1983: The Case for a 2.5 m Telescope, Proc. Nordic Astronomy
Meeting in Oslo, Aug. 17, 1983, Inst. Theor. Astrophys. Blindern, Oslo, Report
No. 60, p. 7
Ardeberg, A. 1984: Ancillary Optical Instrumentation - Provisions and
Options, Proc. Nordic Astronomy Meeting, Sept. 3-5, 1984, Obs. and Astrophys.
Lab. Univ. Helsinki, Report 6/84, p. 121
Ardeberg, A. 1985: Nordic Optical Telescope, Vistas in Astronomy 28, 561
Ardeberg, A. 1987: On the Nordic Optical Telescope, Observational
Astrophysics, Methods and Techniques in Optical Astronomy, Proc. Nordic
Research Course, Brorfelde, June 1-12, 1987, ed. R. Florentin Nielsen, p. 105
Ardeberg, A. 1990: The Enclosure of the Nordic Optical Telescope, Nordic
Optical Telescope Scientific Association.
Ardeberg, A., Andersen, T. 1988: VLT Design Implications of the Nordic
Optical Telescope, Proc. ESO Conf. on Very Large Telescopes and their
Instrumentation, Garching, 21-24 March 1988, ed. M.-H. Ulrich, p. 183
Ardeberg, A., Andersen, T. 1990: Low Turbulence - High Performance, SPIE
1990 Symp. on Astronomical Telescopes & Instrumentation for the 21st
Century, Advanced Technology Optical Telescopes, to be published
Florentin Nielsen, R. 1989: Cassegrain Adaptor for the Nordic 2.5 m
Telescope, Technical Note from the Nordic Optical Telescope Scientific
Haarala, S., Korhonen, T., Lappalainen, T., Sillanpää, A. 1988:
Optical Tests of the Primary Mirror for the Nordic 2.5 m Telescope, Proc. ESO
Conf. on Very Large Telescopes and their Instrumentation, Garching, 21-24
March 1988, ed. M.-H. Ulrich, p. 589
Jannerup, O. 1986: Design of Digital Main Servos for the Nordic 2.5 m
Optical Telescope, Technical Report from the Nordic Optical Telescope
Korhonen, T. 1987: Optics for the Nordic Optical Telescope, Observational
Astrophysics, Methods and Techniques in Optical Astronomy, Proc. Nordic
Research Course, Brorfelde, June 1-12, 1987, ed. R. Florentin Nielsen, p.
Korhonen, T. Haarala, S., Piironen, J., Sillanpää, A. 1985:
Manufacturing Optics for the Nordic 2.5 m Telescope, Departm. Phys. Sci. Univ.
Turku, Report R 84
Laustsen, S., Klim, K. 1985: Telescope Pointing and Tracking, Technical
Note from the Nordic Optical Telescope Scientific Association
Nordic Optical Telescope Scientific Association 1986: 2.5 m Telescope
Assembly Drawings, Technical Report NOTSA
Nordic Optical Telescope Scientific Association 1986: 2.5 m Telescope
Adaptor Assembly Drawings, Technical Report NOTSA
Olofsson, G. 1984: Do we need a Wobbling Secondary for NOT?, Proc. Nordic
Astronomy Meeting, Sept. 3-5, 1984, Obs. and Astrophys. Lab. Univ. Helsinki
Report 6/84, p. 171
Svärdh, I. 1989: User Manual for the Nordic 2.5 m Telescope Control
System, Technical Report from the Nordic Optical Telescope Scientific Association