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MOS Observing Strategies

This pages outlines aspects of MOS observing that PIs may like to take into consideration when planning their observations. Go directly to:

General MOS considerations

The user may want to assess carefully the observing conditions needed for the imaging of the field (if pre-imaging is to be used to design the mask). Requesting very good observing conditions for the imaging will lower the chance of getting the imaging done early during a semester, and therefore lower the overall chance of getting the MOS observations completed within a given semester.

The gaps between the three detectors in GMOS cause gaps in the spectral coverage, see the data examples. The gaps are roughly 37 unbinned pixels wide. The size of the gaps in wavelength space depends on the grating used, but is typically a few nanometers. If continuous spectral coverage is essential for your program, consider using two configurations of the grating with central wavelengths 3-5 nm different.

If you are requesting MOS observations in image quality 70-percentile or worse and your slitlets are longer than 3 arcsec, consider binning the CCDs by two in the spatial (Y) direction giving an effective pixel scale of 0.1454 arcsec/pixel (GMOS-N) and 0.146 arcsec/pixel (GMOS-S).

You may also consider binning in the spectral direction depending on how well-sampled your spectra need to be. For example, with the B600 grating and a 1" slit, the resolution is about fwhm=0.54 nm, which is equivalent to 12 unbinned pixels. Use the grating information to derive this information for other configurations.

If you are observing very faint objects in the red or need slits that are very densely packed, you may want to consider whether your program would benefit from using GMOS in Nod-and-Shuffle mode.

The overheads for Nod-and-Shuffle are significant, but can be minimized by nodding along the slit, keeping the science target(s) in the slit(s) for both the A-position and the B-position. Small nod distances have lower overheads than large nod distances. Very small nod distances (< 2arcsec) can further reduce the overheads by employing electronic offsetting.

If the program requires nodding off to sky, ie. not having the science target(s) in the slit(s) in the B-position, nodding in the q-direction as defined in the observing tool has the lowest overheads.

MOS Nod-and-Shuffle programs which are nodding along the slit need to make sure that the slit length specified in the mask design process is compatible with the total offset distance defined in the Observing Tool Nod-and-Shuffle component. Nod-and-Shuffle MOS observations should always be defined symmetrically about (0,0) in the Observing Tool Nod-and-Shuffle component.

If you are using very short slits (3 arcsec or shorter) for Nod-and-Shuffle MOS observations, it is recommended not to bin the Y-direction of the detectors.

Consider if you can take advantage of one of the configurations for which the special Nod-and-Shuffle darks will be available. If you require a special configuration, you may define a Nod-and-Shuffle dark observation in your Phase II program and it will be taken for you on a best-effort basis.

Are the Baseline Calibrations sufficient for your program? If you need accurate telluric line removal, you will need to add telluric standard stars to your program, or if possible you can add a few blue objects to your mask design. You will have to be sure that these stars are spread across the CCDs so that they cover the spectral range sampled by your observations. If you need radial velocity standards, these need to be added to your program.

MOS Target Acquisition

The target acquisition for MOS observations is slightly more complicated than described in the generic target acquisition scenarios.  The aim for the target acquisition for MOS observations is acquisition with a precision better than half the slit width over the full field covered by slit-lets. To achieve this it is necessary to include a minimum of two, preferably three, acquisition objects in the MOS mask design. It is recommended that these objects are point sources. The apertures cut in the masks for these objects are square and 2 arcsec x 2 arcsec. Further, the objects should span as much as possible of the field of view. This will allow centering of the acquisition objects in the apertures.

The acquisition objects should be bright enough to give a good S/N (larger than 200) in a one minute imaging exposure with GMOS. They should also be faint enough that the spectra of them do not significantly saturate the CCDs during the science exposures. To ensure efficient acquisition, point sources with V magnitudes in the interval 16 mag to 20 mag are recommended for observations with grating R150_G5306. For observations with gratings B600_G5303 and R400_G5305, it is recommended to use point sources with V magnitudes in the interval 15 mag to 20 mag. Fainter acquisition objects may be used, but additional time should be budgeted for target acquisition.

Fringing in Long Slit and MOS Spectra

As mentioned earlier, the GMOS-N CCDs show strong fringing longward of 800nm and the GMOS-S CCDs show severe fringing longward of 700nm. If the objects are faint and the spectral region of interest is located where there is severe fringing, we recomend two observing strategies to minimize the effects of fringing during the sky subtraction.

Good sky subtraction can be obtained by observing the objects in at least two positions along the slit. In this way, one can use one image to subtract the sky from the other image and obtain two sky subtracted spectra of the objects. This is a primitive way to do Nod & Shuffle.

The other method to directly do the sky subtraction and the fringe correction is to use Nod & Shuffle.