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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 consider central wavelength dithers of 5-10 nm.
  • 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 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 12 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.