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MOS Observing Strategies

General MOS considerations (GMOS only)

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.

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 0.16 arcsec.

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, 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.

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.

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.