The MOS mode requires the manufacturing of aperture masks with
slitlets centered on selected objects in the field. A MOS mask will
replace one of the standard slits in the ALFOSC instrument. When
observing, the telescope is pointed so that the science objects
coincide with the slitlets in the mask. Subsequently an ALFOSC
exposure will result in spectra of many objects obtained
As of a few semesters ago, the MOS masks have been fabricated by staff
of the Telescopio Nazionale Galileo (TNG) at the observatory. As the
TNG workshop will prioritise urgent TNG work, the design of the masks
has to be started timely, e.g. not later than about 4-6 weeks before the
run. This implies that any required pre-imaging has to be planned
about 2 months before the run, at the latest.
When to use Multi Object Spectroscopy
For a single target, or for 2 close targets, standard long-slit
observations are preferred.
You may consider to use the MOS option if you have 3 or more objects
within the "central section" (y=750-1250) of your field. If objects
are situated outside this field, the spectra will run out of the top
or bottom of the CCD. This means of course, that the "central section"
depends on the length (dispersion) of your spectra. If you use low
dispersion grisms like 10,11, or 12, the central section will be
larger; if you use high dispersion grisms like 6, 7, or 8, the central
section will be smaller. You will get information on this when using
mask-design validator .
How to use Multi Object Spectroscopy
- Before the run:
A necessary pre-requisite is that you obtain a WELL-CENTERED image of
your field - for most practial applications the centering accuracy
should be around 10" in alpha and delta. The pre-imaging needs to be
done well in advance of the run, as the design and fabrication of the
masks typically takes several weeks. We will try to obtain the
necesary pre-imaging during technical or service nights.
You may want to optimise the position angle of your field to best
allow for atmospheric dispersion. E.g. if you have 3 masks a night,
then it may be good to optimise the field angle of the first and last
according to the parallactic angle.
On the other hand, you may
want to optimise the field rotation to fit most of your targets in the
central region of the chip in y-direction (see above), in order to
obtain the most objects with full-wave-range spectra.
If you want to optimise the field rotation to get the slits along the
parallactic angle while observing at the North-South meridian, for standard
horizontal slits, use:
in the TCS,
or equivalently specify a sky position angle -90 in the OB generator
when preparing for ALFOSC pre-imaging.
The pre-imaging should be done without a filter in the ALFOSC filter
wheel. However, if the spectra are to be taken with a filter in the
beam, then that filter should also be in the beam when doing the
pre-imaging. This is because filters inside ALFOSC tend to change the
transformation function discussed below. Filters in either of the
FASU wheels do not affect this.
You will need to identify 3-5 stars in your field (here full
field can - and should - be used) for aligning purposes. Inevitable
spectra of these stars will also be obtained, so you must make sure
that the spectra of the stars do not interfere with the spectra of you
science targets. Therefore you should not use too many stars for
aligning; usually 3 well distributed stars will do.
The aligning, or fiducial,
stars should not be too bright (e.g. V=16-17), in order to
limit scattered light.
For the sizes of the apertures for your science targets you should
consider the following:
The slit width is determined by the fabrication process and is
typically about 1.6 arcsec.
- length: Usually it is important to obtain a good subtraction of
the sky background. Therefore be sure to make slits long enough
for this to be possible. Usually one will be tempted to make too
short slits in order to obtain more science spectra.
However, the slits should not overlap spatially, as any overlapping
part will be unusable for sky subtraction and spectra extraction.
During the run:
When you have aligned your MOS-mask following the procedure
described elsewhere you will need to adress the following points
- a) arc-line spectra for wavelength calibration, preferrably
without FASU calibration lens
- b) flat-field spectra with internal halogen lamp, possibly with
FASU calibration lens
For MOS spectra it is very important to obtain these calibration
spectra at exactly the same pointing of the telescope as for the
science targets. The FASU calibration lens flattens the lamp-light
distribution over the full CCD, but care should be taken as to not
introduce spectral shifts (this needs to be checked in the
If you want to determine the slit function (for each
MOS slit) you should obtain spectroscopic sky flats.
If you need standards (stars or galaxies) for e.g. radial velocities
you should obtain these through one (or more) of the MOS slits, to
ensure that the spectra will have the same resolution as for the
Absolute calibration (using spectrophotometric standard stars) will be
difficult and is probably not to be recommended unless you are an
expert user. Ideally the standards should be observed through each
individual slit. This is of course not feasible in practice. The
second best is probably to take the standard through (one or) two of the
slits and to obtain spectroscopic sky flats, which then can be used
for determining large scale spatial variations of the sensitivity of
the whole system.
Practical hints for preparation of a MOS observing run
The use of more than 3-4 MOS masks per night is not advisable. The time used to
offset the telescope is around 10 min for an expert user and probably
on the order of 20 min for well prepared users.
- It is not practical to have more than 20 objects, and one typically uses
less than 15 per MOS mask. In practice the 3 aquisition stars will
fill some of the useful space on the mask.
- Designs allowing for nodding have proved to be succesful in
optimising sky subtraction. Such a design has two sets of slits
seperated by a fixed offset, such that when alternating observations
are made using each set, the other serves to collect sky background.
- For mask design it is advisable to have:
The reason for both the printout and the list is that if the object
are faint, they cannot be identified in the image display of the
layout program unless the pixel coordinates are available.
- A hardcopy of the ALFOSC field(s) in question with the objects for which
slitlets should be made marked in a clear way.
The printouts should be numbered.
- A numbered list of ALFOSC ccd image pixel X and Y coordinates for these
Field 1 (grism 4, slitwidth: 1 asec, slitlenght: 10 asec)
obj 1 x: 1267 y: 739
obj 2 x: 1898 y: 866
. . .
obj n x: 1634 y: 802
The lists should be numbered so the accopanying field printouts can be
For mask design one may use our WWW
mask-design validator. This program
takes as input an ALFOSC image, and the ascii lists of objects. Using
the program the slitlets of the masks are laid out superimposed on the
The program creates output files in the observing-system directory
dedicated to the observing program.
Note that the output of the mask-design program comes in 4 types
- two slit-position files in mm and pixel
- two PNG plot files, showing the mask layout and the projected layout
of the spectra on the CCD
The first two files are used for mask fabrication. But also note that in
some cases the ASCII lists and plot files are helpful for target
acquisition at the telescope.