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Observing Strategies



The gaps between the three detectors in GMOS cause gaps in the spectral coverage, see the example. 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 central wavelength configurations of the grating. The recommended wavelength dither steps for the Hamamatsu detectors are >~ 20 nm (R150 grating), >~7.5 nm (R400 grating), and >~ 5 nm for all other gratings.

The GMOS long-slits have bridges that cause gaps in the spatial coverage, see the example. The bridges are approximately 5 arcsec wide (Hamamatsu) and 3 arcsec wide (e2v). If continuous spatial coverage of your long-slit observations is needed, consider dithering in the Y-direction (q-direction on the OT). A minimum dither step of 5 arcsec is recommended.

If you are requesting long-slit observations in image quality 70-percentile or worse, consider binning the CCDs by two in the spatial (Y) direction giving an effective pixel scale of 0.1614 arcsec/pixel at GMOS-N and of 0.160 arcsec/pixel at GMOS-S.

You may also consider binning in the spectral (X) 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 ~10 unbinned pixels (Hamamatsu) or 12 unbinned pixels (e2v). Use the grating information to derive this information for other configurations.

If your science target cannot be detected in imaging mode in one of the GMOS filters in about 5 min of exposure time, you will need to supply coordinates for a nearby brighter target (R<19 mag. and preferably a point source) and accurate offsets between that brighter target and your science target. The accuracy of blind offsetting is better than 0.1 arcsec for offsets less than 20 arcsec. For the blind offsetting to work it is essential that the same guide star can be reached for the bright object and the science target.

If your science target is fainter than about R=18, you will have to supply a finding chart at the time of Phase II submission.

If you are observing very faint objects in the red, you may want to consider if your program would benefit from using GMOS in Nod-and-Shuffle mode.

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. If you need radial velocity standards, then these need to be added to your program.