In collaboration with Per Kjaergaard (CUO), the Multi Object Spectroscopy
(MOS) mode for ALFOSC will be made available for the community beginning
with the October 2001 - April 2002 semester.
General description
-------------------
The MOS mode requires the manufacture of aperture plates with slitlets
centered on selected objects in the field, pointing the telescope so
that these objects coincide with the slitlets and obtaining an image
with the spectra of these objects.
Information
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Before Easter 2001, the MOS User Manual will be available on the NOT Web
Site. This will contain documentation mainly on target aquisition.
When and How to used Multi Object Spectroscopy
- some simple notes for this ALFOSC-option on the NOT.
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1. You can 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 the
"mosplate" programme.
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. We will try to provide a
partial night for this purpose before your spectroscopy run. If this
is weathered out, some of the spectroscopy run may be used instead.
Since a significant amount of time is needed to make the MOS mask, it
is advisable to have a short subsidiary programme to fill the delay.
Note that in order to manufacture the MOS-plates, a non-trival
transformation is made from CCD pixel coordinates to mm-coordinates on
the MOS-plate (to be manufactured in a CNC controlled milling
machine). Therefore it is not possible to artificially shift your
field on the CCD, nor to use absolute alpha and delta coordinates.
You will need to identify at least 3 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 interfer 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 apertures for
the alignment stars should preferably be square, about 4"x4".
For the sizes of the apertures for your science targets you should
consider the following:
a) width: The width will as usual determine your spectral
resolution. However, it is difficult to make slits smaller than 1"
(corresponding to 140 microns) and the aligning procedure will
not allow you to center better than say 0.2" - 0.3". So it will
generally be more safe to use a slit width larger than 1".
b) 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 eventually obtain more science spectra.
2. When you have aligned your MOS-plate following the procedure
described elsewhere you will need to adress the following points
carefully:
a) line spectra for wavelength calibration;
b) flat field comparison spectra (internal halogen lamp).
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. 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
science targets.
Absolute calibration (using spectrophotometric standard stars) will be
dificult 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.
3. Reduction of mos-spectra:
Consult "A User's Guide to Reducing Slit Spectra with IRAF".
When using "apall" or the similar individual tasks, set
format=multispec. A rough guide the mos-reduction will be
provided later.
Practical procedure for preparation of a MOS observing run.
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The following procedures are set down in order to avoid
misunderstandings and situations where lack of time becomes the
determining factor. Adherence to these procedures will result in the
best possible preparations for your observing run.
- More than 3 MOS masks per night are not advisable. The time used to
offset the telescope is around 10 min. for an expert user and probably
on the order of 30 min. for well prepared users.
- It is not practical to have more than 20 and typically 8 - 12 objects
per MOS plate. In practice the 3 aquisition stars will fill some of the
useful space on the plate.
- No later than a week before the first night of the MOS observing run
the NOT staff must have:
1) 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.
2) A numbered list of ALFOSC ccd image pixel X and Y coordinates for these
objects e.g.
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
identified.
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. The
visiting astronomer is more than welcome to be present at the NOT in
this stage of the run preparations.
- If an ALFOSC image of the field is not available, the NOT staff may be
requested to obtain one.
After this information is received, the NOT staff can proceed with the
fabrication of the MOS plates. The plates are laid out using a special
IDL program that places the slitlets superimposed on an ALFOSC image and
then generates a CNC machine file used for machining the MOS plate.
Nordic Optical Telescope, March 30. 2001
Jacob W. Clasen
Software Engineer
jclasen@not.iac.es
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