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Priorities for Design of Telescope Building
Overall Design
Structure of Foundations
Supporting Structure
Cladding and Insulation of Building
Thermal Properties of External Surfaces
Dome and Hatches
Observing Walls and Gates
Observing Floor and Heat Trap
Ground Floor
Building Bogies and Rail
Communication between Floors
Access to Heat Trap and Hydraulic Bearing
Facilities for Handling and Instrumentation
Some Precautions recommended for Improvement of Thermal Control and Image Quality
Some Work in Progress
Heavy Wind and Icing


A description is given of the enclosure of the Nordic Optical Telescope. In addition to protection of the telescope against adverse weather conditions, highest priority for building design was given to provision of stable thermal conditions and efficient handling and elimination of heat flows. The building has an observing floor, a false floor as part of a cooling jacket, a ground floor with control facilities and a basement. The building corotates with the telescope. The size of the building has been minimized. Telescope and building rest on separate foundations. The 11.1 metres wide dome has two hatches, remotely operated. The walls of theobserving floor have large gates, providing openings of up to 240 degrees, giving a combination of air flushing and wind protection. All heated areas are enclosed in a cooling jacket which eliminates thermal gradients and cools the observing floor. The building can withstand wind speeds up to 225 kilometres per hour and can be clamped. Heavy items can be installed on the observing floor via a cantilever crane.

Key words: Thermal Stability - Cooling Jacket - Air Flushing - Dome -Corotation


For the design of the Nordic Optical Telescope (NOT), highest priority has been given to image quality (Ardeberg, 1985, 1987, 1989). This decision hasseveral implications regarding optics and mechanical structure of the telescope. At the same time, emphasis on high image quality implies a number of importantconsequences regarding the design of the telescope enclosure. For the final output image quality, restrictions caused by the enclosure are, in practice, often more severe than those depending on opticsand mechanical structure. For this reason, it is highly essential that the enclosure is correctly designed and constructed and, also, that it is adequately handled during observations.

Priorities for Design of Telescope Building

The telescope building has to serve a number of purposes. It is constructed to protect the telescope against precipitation, humidity, wind and dust. At the same time, it must ensure freedom from excessive thermal variations of the telescope and its surrounding installations as well as significant departures of the temperatures of these units from that of ambient air during observations. Further, it has to provide an adequate air flow around the telescope during observations, defining a proper balance between thermal and turbulent influences. Finally, the telescope building must include space for necessary installations such as control rooms, electronics and motors, air circulation and cooling. In addition, it hasto fit the local topographic conditions in an adequate manner.

Among all factors determining the final design of the building for the NOT, great attention was given to the results of investigations of the telescope site with microthermal sensors (Ardeberg and Andersen, 1990). It was quite clear that the data obtained on microthermal activity showed the site to be of high quality, as did the turbulence data derived from trailing of stellar telescope images. At the same time, resulting detailed data on microthermal activity as a function of height above ground showed that there was a rather well-defined upper limit to the layer dominated by ground turbulence. This limit was quite constant over the complete diurnal cycle except for a few hours around meridian transit of the Sun.

The implication of the structure of microthermal activity on the design of telescope and telescope enclosure was that the primary mirror had to be placed at a minimum elevation of around six metres above local ground level. At the same time, it was clearly indicated that a corresponding elevation of close to eight metres would provide favourable safety margins against detrimental effects of local ground turbulence.

With the position of the telescope at the North end of the Cruz del Fraile ridge, the difference in height between the telescope and local ground level varies along the periphery of the telescope building. Seen from the North and from the prevailing up- wind direction, the primary mirror is around 13 metres above local ground, whereas seen from the South and from the prevailing down- wind direction, the corresponding altitude difference is around 9 metres.

As a result of the relatively high elevation of the telescope above ground, it was natural to choose a telescope building with facilities for control, electronics, motors, air conditioning and cooling devices located in the ground floor. The observing floor was then placed on the first floor of the building. At the same time, greatest weight was given to arrangements aiming at elimination of thermal gradients. One of the results of this was the installation of a cooling jacket around ground-floor facilities including a heat trap between the ground floor and the first floor (Ardeberg and Andersen, 1988, 1990). Finally, a base-ment floor was prepared for various purposes. The basement is partly below ground level to the North of the building, and close to fully below ground level to the South of it. In Figure 1, some of the principles of the enclosure of the Nordic Optical Telescope and adhering facilities can be seen.

The basic design outlined above serves to place the telescope at a favourable elevation relative to ground turbulence and also to provide a structure allowingfor arrangements eliminating vertical thermal gradients. Another requirement, closely connected to the assignment of first priority to image quality, is an observing floor free from all significant thermal anomalies. Primarily, this implies a small telescope enclosure. Secondly, the enclosure has to provide effective diurnal thermal stability. This, in turn, calls for possibilities for efficient air flushing, cooling and insulation but also for absence of heat sources (Ardeberg and Andersen, 1988, 1990).

Added to the requirements detailed, the telescope building must provide pro-tection against wind and dust togetherwith adequate options for observations also under non- ideal wind circumstances. Naturally, a final requirement is pro-visions for good facilities for easy control and cabling for the telescope as well as for ancillary instrumentation and for rapid positioning.

For the design of the telescope enclosure, further evaluations were made through model tests in a wind tunnel (Bjerregaard, 1985). Analyses of air flows, wakes and ventilation were made. These included evaluations of air flow properties along the structure of the enclosure as well as inside it, notably around the telescope.

Overall Design

The building for the NOT has been designed as a low- cost structure with the principle aim to keep thermal turbulence down as much as possible. As guide lines for the design we defined compactness, thermal balance and simplicity.

The NOT enclosure has been constructed as a two- storey building also including a basement and a heat trap. The height of the building has been determined mainly from the results of microthermal studies, detailed above. The diameter of the building and the size of the dome have been minimized with respect to the working range of the telescope. It is pointed out that the telescope building and its foundation are mechanically completely detached from the telescope foundation.

The telescope enclosure proper is a dome of conventional design with two hatches resting on generous observing- wall gates and a cool observing floor. The dome radius is large enough for safe operation of the telescope but not more. In order to ensure a cool observing floor and avoid vertical thermal gradients, a cooling jacket heat trap of generous dimensions has been included under the observing floor. Below the heat trap and on the ground floor, control room, space for electronics, motors, devices for air conditioning and cooling and an entrance sluice have been included in a cooling jacket. Finally, in the basement, mainly below ground level, space has been made available for inspection and adjustments of building bogies and for installation of instrumentation. The Nordic Optical Telescope in its building, readyfor observations in the Cruz del Fraile site, can be seen in Figure 2. Both hatches are open as is one of the sections of observing wall gates.

With minimized space for control facilities, and with priority for convenient communication between control room and observing floor, allowing for flexible experimental instrument installations, cabling arrangements are essential. For these as well as for other reasons, the complete building rotates with the tele-scope, excluding basement installations.

In spite of the relatively high number of installations included, the telescope building is rather small for a telescope with a primary mirror of 2.56 metres. The outer diameter of the building is 12.5 metres at the base. The observing walls have an outer diameter of 12.7 metres and the dome a diameter of 11.1 metres.

The total height of the telescope is, on the average, 16.5 metres above local ground.

The telescope building has no provisions for offices. It does not include sanitary facilities. In order to save space and money and also in order to provide a building as adequate as possible for strict observing purposes, both offices and sanitary installations have been concen-trated to the service building.

Detailed design of the enclosure and supervision of its construction was executed by Danalith A/S. Project leader was Palle Søndergaard. The construction proper was made by Huarte. Local architects were J. Diaz-Llanos and V. Saavedra.

Structure of Foundations

A basic requirement for a telescope with high image quality and high pointing and tracking accuracy is that the foundations for both telescope and telescope building are of high quality. Finally, it is very essential that there are no significant effects of coupling between the telescope building and the telescope, neither direct ones nor indirect ones via the foundations and ground.

For the NOT, foundations for the telescope and for the telescope buildingare completely separated. The foundation for the building has been attached to the surface rock layer. This rock layer has a depth of around 1.5 metres. For the telescope, the foundation has been attached to a rock layer, the upper surface of which is at a depth below ground level of around three metres, and the penetration depth of which is considerable. Between these two rock layers, there is soft material of volcanic origin.

All conclusions regarding rock material are based on a detailed geotechnical site investigation (Lund, 1985). This investigation involved deep core boring followed by laboratory analysis.

The telescope is located at the north-northwestern extreme of the Cruz delFraile ridge. As a result, the building foundation rests on inclined ground with a span in level of close to four metres. It is noted, that all over and around the building area, the surface rock layer is rather solid. The sole of the building foundation has been moulded firmly into the surface rock layer. The concrete foundation has a thickness of 50 centimetres and forms a circle with an outer diameter of 10.7 metres. To the West it has a manhole with a height of 130 centimetres and a width of 100 centimetres, providing access from the outside to the basement of the telescope building.

The telescope foundation has its sole firmly moulded into the rock layer starting around three metres below ground level. Because of the inclination of local ground, this is between 1.5 and 4.5 metres deeper than the sole of the foundation for the telescope building.

The telescope foundation is a concrete cylinder with a thickness of 50 centimetres and an outer diameter of 4.1 metres. At the level of the basement, it has an entrance with a height of 150 centimetres and a width of 70 centimetres, providing access to the inner space of the telescope foundation.

The northern part of the building foundation rests on a concrete base sole with a thickness of 100 centimetres, the southern part on a corresponding base sole with a thickness of 70 centimetres. The telescope foundation rests on a solid concrete cylindric base with a diameter of 6.0 metres and a depth of 3.5 metres. The base is mechanically detached from the surface rock layer by a slit filled with 50 mm thick neopolen.

The foundations of the Nordic Optical Telescope and of its building are shown in Figure 4. The two constructions can be seen, mutually separated. The central tower carries the telescope, the outer, circular foundation the rail for the telescope building.

Supporting Structure

The supporting structure of the telescope building is a frame construction made of iron beams. This construction makes the supporting structure rather light yet very stiff with respect to rotational move-ments. Further, it gives some vertical flexibility. This is essential for adequate compensation of possible imperfections in the relative vertical positions of the building bogies. The supporting iron-beam structure can be seen in Figure 5, showing a picture of the telescope building at Cruz del Fraile, taken just before the supporting structure was covered with interior and exterior surface material.

Cladding and Insulation of Building

The cladding of the telescope building consists of corrugated aluminium sheets with a thickness of 0.7 millimetres. The profile of this cladding has a depth of 40 millimetres. Inside the cladding, there is a composite insulation layer. Surfaced by a plastic foil of thickness 0.6 millimetres and covered on its inner side by 0.7 millimetres thick galvanized steel, the insulation has, as main components, two sheets of mineral wool, each with a thickness of 60 millimetres.

Inside the outer part of the external walls, there is a gap with a minimum depth of 160 millimetres. At the same time as it includes the supporting structures of the telescope building, the gap acts as an additional insulating layer. Moreover, it is part of our cooling jacket system, provided with forced circulation of air, thermally stabilized and adjusted to the temperature of ambient air. On the inner side of the gap, and of the supporting structures, an internal insu-lation layer has been mounted. This layer consists of three sheets of mineral wool, each 60 millimetres thick. On its inner as well as on its outer side, the mineral wool insulating layer is covered by a plywood sheet with a thickness of 18 millimetres.

With a total minimum depth of 540 millimetres, and composed of a number of different layers, external walls of the telescope building should provide adequate protection concerning thermal balance. With two, mutually separated, insulation layers, one internal air slot and one external slot system with forcedcirculation of thermally stabilized air, both insulation and air circulation should be rather effective.

It should be noted, that the air slot including supporting building structures is a highly active subsystem, as it is also part of the cooling jacket enclosing all facilities installed in the ground floor of the telescope building. With the forced air circulation in the cooling jacket, including powerful cooling, all thermal gradients in the walls should be effectively eliminated.

Thermal Properties of External Surfaces

For a proper thermal balance of the telescope, one of the important factors is the behaviour of external surfaces of the telescope building. In practice, we are concerned with the surface of the dome and with the cladding of the verticalbuilding walls. Whilst the dome is made of galvanized steel, the wall cladding is made of aluminium.

In practice, the dome as well as the wall cladding influence thermal balance of the building and telescope through interaction with ambient air and the sky. Whilst aspects of insulation have been discussed under "Cladding and Insulation of Building", we are here concerned with interactions between surfaces of dome and building, on the one hand, and with the sky, on the other hand, through radiative processes. This interaction has to be divided in effects during daytime and night time, respectively. In daytime, possible overheating of dome and cladding threatens thermal balance, whilst undercooling may be as serious, or even more serious, an effect during night time.

As possible materials for dome and wall cladding, we considered originally alu-minium and galvanized steel, in both cases with and without painting. With respect to paints, these ones are usually based on TiO2 pigments, being white or dyed with other colours. Attention was also given to some specialized paints providing low emissivity in the infrared part of the spectrum. The latter type of painting has been of special interest to space probes.Paints specialized to have low emissivity at larger wavelengths are relatively new on the market. Thus, the durability of these paints seems still uncertain. Moreover, they are very expensive. For these reasons, we have decided to refrain from such paints, although they are intrinsically very interesting. In practice, we have therefore concentrated on a comparison of untreated aluminium and galvanized steel and of white painting based on TiO2 pigments. For these surfaces, we have considered solar absorptance, dominating the daytime interaction with sky, and emissivity at infrared wavelengths, determining the corresponding interaction during night time.

Given the facts related and taking into account also stiffness and durability, as well as availability and price, we have chosen an untreated dome made of galvanized steel and dome wall cladding made of thin aluminium sheets, corru-gated with a profile depth of 40 millimetres for improved thermal per-formance. We feel that this choice provides for a rather decent control of thermal balance around the telescope. It is added that on-site practical tests of our surface materials, with and without paint-ing, have provided excellent confirmation of our conclusions as given above.

Dome and Hatches

The dome has been delivered by Ash Dome, Plainfield, Illinois, U.S.A. The choice was based on earlier experience of (smaller) domes delivered by the same manufacturer. At the same time, price considerations were taken into account.

The dome is made of galvanized steel. It has a diameter of 11.1 metres and a total weight of approximately four tons. The observing slot has a vertical extension of 105 degrees and a width of 2.9 metres. It is covered by two hatches.

In closed position, the upper hatch covers the observing slot down to 24 degrees above the dome horizon, corresponding to approximately 2.5 air masses, as seen from the base plane of the dome. This implies that, if the upper hatch is open and the lower hatch is closed, the telescope can be pointed as low as approximately 35 degrees above the horizon before vignetting is imposed and the lower hatch has to be opened. This position corresponds to around 1.74 airmasses. The lower hatch covers the slot down to the horizon.

The upper hatch moves along the dome structure. The lower hatch opens in a drop- out fashion. Both hatches are remotely operated.

The dome is insulated with 5 centimetres of polyurethane foam sheeting. The minimum free distance between telescope top end and dome insulation is around 25 centimetres. The angular freedom in azimuth between telescope and building is five degrees in both directions.

The exterior surface of the dome is of high importance for the diurnal exchange of radiation between dome and sky. This has been discussed in some detail under "Thermal Properties of External Surfaces".

Observing Walls and Gates

The observing walls surrounding the observing floor have a total inner height above the floor of 170 centimetres. They are provided with a total of 16 gates. Eight of the gates have a width of 30 degrees, or 320 centimetres, each. The remaining eight gates have a width of 15 degrees, or 160 centimetres, each. All gates have a height of 150 centimetres. The gates are placed in a way alternating wide and narrow ones. They can be opened in an overlapping manner. Overlapping can be made along three parallel peripheries. This way, the observing walls can be opened to various degrees and in various orientations. This should, for all conditions, provide possibilities for optimum combination of air flushing and wind protection. The maximum opening of the observing walls corresponds to 240 degrees.

The efficiency of the wall gates for air flushing has proven very high. Operation of these gates gives immediate thermal adjustment for the air contained in thedome volume and influences in a most favourable way the thermal characteristics of other elements and structures, the behaviour of which is critical for high image quality (Ardeberg and Andersen, 1990).

Observing Floor and Heat Trap

The observing floor has a diameter of 12.5 metres. Excluding the space occupied by staircase and the telescope fork, there is a total of 95 square metres of floor space available. The observing floor consists of steel plates, mounted on a framework of iron girders. The floor is painted with special paint, giving a non- slip surface.

Between the observing floor and the control floor, there is an extension of the cooling jacket into a heat trap. The heat trap is an intermediate floor with a height of 1.5 metres. It is provided with an air circulation device, giving a flow of around 8000 cubic metres of air per hour. This corresponds to a complete recycling of the total air volume in the heat trap in somewhat more than one minute.

The air flowing through the heat trap can be cooled down to -6 degrees C. Adjustment of the temperature of this air flow is used to eliminate vertical temperature gradients originating in the ground floor but also tostabilize the observing floor at ambient temperature or slightly below. With the temperature conditions prevailing at the Observatorio del Roque de los Muchachos, the temperature range of the air cooling system is sufficient for all cases so far experienced.

Ground Floor

The ground floor of the telescope building contains four rooms, mutually separated by insulated doors. Of these four rooms, three house facilities for control of telescope and instrumentation, computers and electronics and various support installations. The fourth room is a specially designed entrance.

The largest ground- floor room is the control room. It is intended for observing activities. From this room, observers control the telescope and verify its position in the sky via terminals and dedicated screens. In the same way, though via separated terminals and screens, they can control and monitor the function of ancillary instrumentation attached to the telescope. In parallel to these direct observing facilities, the control room includes hardware and software aimed at limited on- line reductions of observing data. Although limited in space, the control room can accommodate up to four persons working together or in parallel. It is recalled thatpresence of observers on the observing floor during observing is, normally, strongly discouraged.

Electronics equipment for control of telescope and telescope building, including the main control computer, is installed in a second room in the ground floor. To a limited extent, operation of this equipment is included in normal observing routines. The electronics room allows regular working space for one person.

In the third ground- floor room, a number of installations serve to support air conditioning, air and water cooling, cooling with liquid nitrogen, function of the hydraulic bearing system and of the bellows floating the primary mirror. In addition, this room contains tools and equipment for mechanical and electronic maintenance work.

An air conditioning plant serves all rooms in the ground floor, maintaining them at well-defined temperatures. The systemdelivers excess heat to the connected cooling system having water as cooling agent. A powerful parallel air cooling system operates the air circulation in the cooling jacket. Via the heat trap, part of the cooling jacket, this air cooling system also maintains the temperature of the observing floor slightly below that of ambient air. Also from the cooling jacket, excess heat is transferred to the water cooling system.

Facilities for water cooling are provided also for ancillary instrumentation. This system allows cooling to approximately 5 [o]C. For ancillary instrumentation re-quiring more powerful cooling, notably detectors for the red and infrared part of the spectrum, additional facilities are available. This includes a cooling system in the ground floor, based on liquid nitrogen. For future applications, a system for cooling with liquid helium can be installed.

Also the hydraulic bearing for the telescope azimuth axis is controlled from the ground floor. Installations here include an oil sump, pumps andregulators. For the pneumatic bellow supports of the primary mirror, a control unit includes a pressure pump, regulators and an air drying column.

The entrance of the telescope building is constructed as a thermal sluice. During night time, the temperature of this space is maintained somewhat below that of ambient air. This prevents occurrence of rising turbulent air reaching the observing floor level when the entrance door is opened.

Building Bogies and Rail

Corotating with the telescope, the telescope building rests on a framework of iron beams. This framework carries the total weight of the building and its installations, approximately eighty tons. The framework, in turn, rests on four bogies. Each of these bogies has a total of three wheels. Two of the wheels run on horizontal axes, one on a vertical axis. One of the first-mentioned wheels is provided with a drive motor, whilst the last-mentioned wheel has no drive. It is installed to control positioning of the building in the horizontal plane and dimensioned to withstand wind forces of up to around twenty-two tons. One of the bogies is seen in Figure 11, resting on the supporting rail.

The bogie wheels act against a rail, defining the position of the telescope building. The rail is circular with a centre diameter of 10.25 metres. This diameter is accurate to less than three millimetres. The width of the rail is one hundred and twenty millimetres and its upper, polishedsurface has a horizontality accurate to one millimetre. In order to avoid excessive contact stresses between the edge of the rail and the wheels, the upper rail surface has been made slightly convex with the maximum departure from a plane being 0.6 millimetres. The rail rests on the building foundation. A shock-absorbing rubber layer has been inserted between the concrete foundation and the rail.

In order to protect the drive system against penetrating dust, the cladding of the telescope building overlaps the corresponding foundation some twenty centimetres. In addition, a rubber skirt hangs from the inside of the cladding with its lower part immersed in a water sump. This liquid seal is attached to the building foundation. In addition to water it contains glycol brine protecting the seal against freezing.

It is added that provisions have been made for both maintenance and replace-ment of bogies from the basement.

In the case exchange of a bogie is judged necessary, such an exchange can be made, during which the telescope building can be supported by jacks.


The basement of the telescope building is relatively spacious. It has a total interior diameter of 9.8 metres. Space is divided in two main parts. One part is defined by the interior of the telescope foundation, the other by remaining, outer basement space.

The interior of the telescope foundation has a diameter of 3.1 metres and a height of 6.0 metres. It provides mechanical and thermal conditions of high stability, both on a diurnal basis and on longer time scales. This makes it an attractive place for precision instrumentation connected to the telescope via optical fibres.

The outer part of the basement is a cylindric space with inner and outer limiting diameters of 4.1 and 9.8 metres, respectively. The height varies from 2.2to 4.8 metres. This space is provided with scaffolding giving a cylindrical platform with inner and other diameters of 6.5 and 9.8 metres, respectively. The scaffolding is supported on its outer limiting diameter by the foundation of the telescope building and on its inner limiting diameter by the surface rock layer. As a result, it is mechanically completely detached from the telescope base.

Below the northern part of the scaf-folding, remaining space is 2.8 metres wide and between 2.2 and 3.0 metres high. Also this part of the basement is characterized by stable mechanical and thermal conditions. With some preparation, instrumentation may be installed below the northern part of the scaffolding. It is noted that the scaffolding in itself is convenient for inspection and maintenance of the supporting and rotation system of the telescope building.

Except for the door between the interior part of the telescope foundation and the outer basement space, the latter part of the basement can be accessed through two man holes. These connect thebasement to the exterior and to the control room, respectively.

In the possible case of malfunctioning of seals for the rotating joint, cooling water may spill onto the floor of the room inside the telescope foundation. For this reason, care should be taken not to store valuable instrumentation in this space.

Communication between Floors

Normal communication between the ground floor and the observing floor is via a narrow staircase. This staircase allows easy passing of people and smaller pieces of equipment but is not adequate for equipment which is large and/or heavy. Large and/or heavy pieces of equipment can be transported by other means (see below).

Communication between the ground floor and the basement passes via a simple staircase below a slab in the ground floor.This arrangement has been chosen for lack of space. Frequent communication between the ground floor and the basement is not foreseen. The staircase installed should be seen as a measure of safety and as an extra convenience. Normally, the basement should be reached via its exterior manhole. Again due to space and construction require-ments, this door has been made rather low.

Access to Heat Trap and Hydraulic Bearing

The heat trap below the observing floor can be accessed through two different manholes. Most convenient is to enter the heat trap from the observing floor, using the floor manhole. This manhole has two doors and a ladder leading down to the floor level of the heat trap. Alternatively, the heat trap can be accessed from a vertical manhole reachable from the staircase leading from the entrance of the telescope building to the observing floor. This manhole is normally kept closed.

With its limited height and with abundant installations, the heat trap does not pro-vide comfortable space for longer visits. At the same time, the space available issufficient for all inspection purposes as well as for all maintenance work necessary regarding the heat trap, installations for air circulation and hydraulic bearing system. Illumination is available in the heat trap.

Around the telescope base, there is sufficient space for inspection of and working access to the hydraulic bearing of the telescope. As a protection against dust, the bearing is surrounded by a thick but transparent plastic skirt. To a first approximation, inspection of the hydraulic bearing and the pads can be made without removing the skirt.

Facilities for Handling of Instrumentation

Instrumentation with modest size and weight can easily be entered in all spaces, including the observing floor, viastaircases. For instrumentation of larger sizes and/or with higher weight, it may be less convenient to reach the observingfloor this way. In such cases, another approach should be chosen.

Large and/or heavy instrumentation is most conveniently entered on the ob-serving floor via the cantilever crane,which swings out through the observing wall gate below the hatches. This crane handles up to 500 kg. For very large and/or very heavy instrumentation, use can be made of the elevator.

Some Precautions Recommended for Improvement of Thermal Control and Image Quality

As discussed above, a number of facilities are available for enhancement of thermal control and resulting image quality. In order to take optimum advantage of these facilities, the observers are requested to take into account some basic precautions. These precautions are shortly outlined below. It is noted that, in a more permanent system, the control system will either adjust facilities automatically or give corresponding instructions to the observers.

It is most essential to verify that the cooling-jacket system functions properly at all times. Until the control system is ready to take over in an automatic mode, it is advisable to make a careful check of the temperature of the air flowing through the cooling jacket.

Further, it is important to verify that the dome-air temperature stabilization system functions correctly during daytime in order to avoid heating of dome air andtelescope structure. Pending proper functioning of the automatic control system, the temperature of the dome air should be checked against expected night-time temperature of ambient air.

From late afternoon, the entrance sluice of the telescope building should be temperature stabilized. This is managed automatically. As soon as the upper dome hatch is opened, cooling of the entrance sluice is engaged at the same time as cooling of the air around the telescope is disengaged.

For reliable thermal control, the observing wall gates are highly essential. Thus, it is important to open them, in the late afternoon, as fully as possible, weather permitting. During night time, the wall gates have to be positioned with regard to both thermal control, wind, local relative humidity and dust in the air. This has been detailed in a description of the Nordic Optical Telescope (Ardeberg, 1989).

Optimum air flushing of the telescope ambience requires, in addition to openingof wall gates, also opening of dome hatches. Both hatches should be opened in the late afternoon, as soon as weather permits.

The importance of the primary mirror for resulting image quality is obvious. Thus, the mirror cover should be opened in the early evening. On the other hand, the open structure of the mirror mounting makes the primary mirror much less critical than for conventional systems.

Presence of people on the observing floor should be avoided from the early evening. As much as possible, the observing floor should be void of people all night. At the same time, the air flushing possible assures that shorter presence of people on the observing floor has thermal consequences, the duration of which are rather limited (Ardeberg and Andersen, 1990).

Finally, for optimum functioning of our thermal control system, doors should be kept closed as much as possible. This is due for the staircase door on theobserving floor as well as for all doors of the ground floor installations.

It is added, that during all operations taking place in daytime, including ob-servations, it is of utmost importance toavoid completely that sunlight hits the telescope or parts of it. If it should occur, there is all reason to be prepared for long duration of thermal relaxation and significant effects of this on image quality.

Some Work in Progress

In addition to regular fine-tuning of facilities available, some work packages have been initiated, aiming at further improvement of observing conditions and operation. Among the projects in course, special mention can be made of those regarding thermal monitoring and control and upgrading of the operation system for the upper dome hatch.

At the present time, thermal monitoring is exercised with a rather basic system. In addition to our weather station, we have, as a temporary monitoring arrange-ment, installed thermal probes outside the dome, in the upper part of the dome, on the observing floor, on the telescopestructure and on the primary mirror. These probes are connected to a multi-colour recorder. The resulting graphical output is, in a first approximation, useful in a dual way. First, real-time checking of the thermal behaviour of the elements included serves to optimize immediate thermal performance through adjustments of cooling and devices for air flushing. Second, off-line study of temperature behaviour of the elements provides valuable information for actions on a more extended time scale.

Whilst very useful for our present operation and as a starting point for further undertakings, the system nowexisting for thermal monitoring is far from satisfactory as a more permanent instal-lation. Design of a permanent installation has been made. This installation includes units for active thermal monitoring and control.

Thermal monitoring will include all elements judged important for tempera-ture control and resulting image quality. In consequence, thermal probes will be installed for monitoring of temperatures of external air as well as the air volumes in the dome, close to the observing floor, in the cooling jacket, including the heat trap below the observing floor and the basement of the building. Similarly, thermal probes will monitor the tempera-ture of the air in the primary mirror cell and in various parts of the telescope structure. Further, thermal probes will be attached to the dome structure, including its exterior as well as its interior surfaces. Corresponding installations of probes will be made for the observing walls and gates as well as for the observing floor. In the same way, the cladding and the basement of the telescope building will be equipped with thermal probes.

The thermal behaviour of the primary mirror and of all parts of the telescope structure is of prime importance for resulting image quality. Thus, these elements will be generously covered with thermal probes. This will include the secondary mirror and its supporting structure as well as the rotator and instrument adapter. Also, the hydraulic bearing and the telescope foundation will be included. In addition, we will keep track of the temperature of the ground and the ground layer surrounding the telescope building.

All thermal probes installed will be connected to a central unit coordinating temperature monitoring, data retrieval and measures for continuous adjustment of the temperatures of cooling agents and of air coolers. This unit will contain a small computer and peripheral devices for storage and display of measurement data and actions provoked.

The system for operation of the upper hatch, delivered with the dome is not more than marginally satisfactory. Whilst the system has, so far, behaved reasonablywell, the risk for sudden malfunction seems significant. Especially unfortunate is the risk, that the hatch may be impossible to close in connection with adverse weather conditions. In order toimprove the situation, the operation system for the upper dome hatch will be upgraded, including an additional system for emergency closing.

Heavy Wind and Icing

The telescope building has been constructed to withstand the climate at the summit of the Caldera de Taburiente, even when it reaches its most severe limits. In this respect, special consideration has been given to heavy winds and icing.

Wind- speed limits for operation of the telescope have been discussed in connection with the telescope proper (Ardeberg, 1989). When out of operation, the building can withstand wind speeds up to at least 225 kilometres per hour. At least to our knowledge, such wind speeds have never been observed at the Roque de los Muchachos area. As an extra measure of safety, the telescope buildinghas been equipped with special clamping devices. At the same time, it should be noted that calculations, in practice, clearly show that negative forces on the support bogies can never occur.

If hard winds, raining and low tem-peratures coincide, horizontal under-cooled rain may give rise to icing. In some cases, this may produce consider-able and asymmetric ice loads on structures like our telescope building. If wind speeds are below 120 kilometres per hour, the building may be slowly rotated to decrease effects of asymmetric ice loading.


Special safety measures are adequate due to the rotating building and icing of the dome. Safety devices have been installed. However, certain safety measures should be observed in order to avoid accidents.

When the building rotates in slew mode, presence on the catwalk and outer staircase is strictly prohibited, if special arrangements have not been made. Corresponding presence in the basement is equally strictly prohibited. Except for tracking movements, all rotation is preceded by loud warning signals from a horn. Noting such signals, anybody present on the catwalk should leave it immediately. Anybody on the outer staircase should immediately enter the building. As a further measure of safety, emergency stop switches are available at the staircase. Pressing the switch contacts disconnects all power to telescope and building.

After icing of the dome, melting may imply serious risks. Ice and snow fallingdown on the staircase and catwalk may constitute severe threats. In such cases, utmost care should be exercised. Special precaution should be taken before entering the staircase. Above the staircase and just below the dome, a grid has been installed to stop larger blocks of ice and snow from falling down on the staircase.

For protection against fire, all doors in the ground floor as well as the door to the observing floor are heavy and basically fire proof. Further, the ground floor has two separate exits. In addition to the main entrance, the communication staircase to the basement provides an escape facility via the exterior door of the basement. All exit doors are marked with fluorescent tape to be visible also in darkness conditions.

From the observing floor, there is, in addition to the entrance door, marked with fluorescent tape, an escape possibility through the wall gate below the hatches. Here, a break- jump device is available,possible to use together with the cantilever crane. As a further escape possibility, a rope ladder has been installed in the same place.

Similarly, also for the basement two escape possibilities are provided. These are the exterior manhole and the manhole above the ladder leading to the ground floor. Finally, escape from the heat trap should pass via the manhole leading to the observing floor. Again, all exits are marked with fluorescent tape. It should be emphasized that presence in the heat trap is not foreseen except for in connection with special inspection and/or work.

All users of the telescope and facilities in the telescope building are requested to familiarize themselves with escape possibilities. These are graphically shown in Figures 12, 13 and 14 for the observing floor, the ground floor and the basement, respectively. Likewise, users are requested to get familiarized with other safety devices available and with all working spaces which will possibly be used by them.

It is noted that all transports of liquid nitrogen have to be made with great care. The transport dewar has to be well secured and transported in a vehicle with secured luggage compartment. Transport of dewars from ground level up to the observing floor should be made with help of the cantilever crane.

For handling of batteries and possible charging, safety goggles are available as well as an agent for rinsing of eyes.

Previous to all work on or around the building rail, the emergency switch has to be activated and locked. Because of fire risks, storing of packing material in the basement will not be accepted.

All responsibilities for car driving rest with the drivers under Spanish law. Users of cars are requested to familiarize themselves with risks along the roads, including difficult parts of roads, heavy inclinations of the ground around roads, possible landslides and avalanches. Special care has to be exercised in case of fog, snow, heavy rain, ice on the road and possible under-cooled rain.


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. 1989: Some Properties 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.

Bjerregaard, E.T.D. 1985: The 2.5 m Telescope. Flow Visualization in the Wind Tunnel. Model Tests for Two Telescope Domes. Report of the Danish Maritime Institute, Lyngby, Denmark.

Lund, N. 1985: 2.5 m Nordic Optical Telescope at Roque de los Muchachos on La Palma. Site Investigation. Geotechnical Report from Dansk Geoteknik A/S.

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