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MOS Observing Strategies

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General MOS considerations (GMOS only)


  • Wavelength dithers: The gaps between the three GMOS detectors cutout small wavelength intervals (grating dependent) from the spectra. If continuous spectral coverage is important for you, then at least two sets of observations with slightly different wavelengths are needed. The recommended minimum shifts are ~20nm for the R150 grating, ~7.5nm for the R400 grating, and ~5nm for all other gratings.
  • Spatial binning: 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 2 in the spatial (Y) direction giving an effective pixel scale of 0.16 arcsec. If you are using very short slits (3 arcsec or less), then spatial binning is not recommended (not even for nod&shuffle mode).
  • Spectral binning: Consider spectral binning (X direction) if the spectral resolution can be lower than that provided by the grating and the slit width. For example, with the B600 grating and a 1" slit, the resolution is about fwhm=0.54 nm, which is equivalent to ~11 unbinned pixels. Use the grating information to derive this information for other configurations.
  • When to use Nod&Shuffle mode: If you are observing very faint objects in the red or need slits that are very densely packed, your program could benefit from 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.
  • Nod&Shuffle offsets: MOS Nod-and-Shuffle programs which are nodding along the slit need to make sure that the slit length specified in GMMPS is compatible with the total offset distance defined in the Observing Tool's Nod-and-Shuffle component. GMMPS displays the correct numbers when loading the ODF mask design file. Nod-and-Shuffle MOS observations should always use symmetric offsets about (0,0) in the Observing Tool.
  • Calibrations: 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.
  • Acquisition: 2-3 stars are centered in 2x2 arcsec acquisition boxes. With pre-imaging 2 alignment stars are needed; with catalog masks 3 stars are required. The acquisition objects should be bright enough to give a good S/N (larger than 200) in a 30s 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. For observations with gratings B600 and R400, 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.
  • Guiding: For catalog masks, please make sure that there is an appropriate guide star available for the coordinates and PA of the mask design. Check also that this guide star is not also one of the mask acquisition stars!
  • Fringing: GMOS CCDs show fringing in the far red. If the objects are faint and the spectral region of interest is located where there is fringing, 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. An alternative is to use N&S.