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General description

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 simultaneously.

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 our 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:
      field-rotation 0
    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:

    1. width: The slit width is determined by the fabrication process and is typically about 1.6 arcsec.
    2. 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 carefully:

    1. a) arc-line spectra for wavelength calibration, preferrably without FASU calibration lens
    2. 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 afternoon).

    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 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:
    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.

    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 ALFOSC image.

    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

    1. two slit-position files in mm and pixel
    2. 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.


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