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Imaging Observing Strategies

The gaps between the three detectors in GMOS cause gaps in the imaging field about 2.7 arcsec wide, see the example. If continuous coverage of your field is needed, you will need to obtain multiple exposures and dither between the exposures. A minimum dither step of 5 arcsec in the X-direction (p-direction on the OT) is recommended.

For unbinned observations the pixel scale is 0.0727 arcsec/pixel (GMOS-N) and 0.0730 arcsec/pixel (GMOS-S). If you are requesting imaging in image quality 70-percentile or worse, consider binning the CCDs 2x2 giving an effective pixel scale of 0.1454 arcsec/pixel (GMOS-N) and 0.146 arcsec/pixel (GMOS-S). This cuts the overheads from the readout from 150sec to 55sec per frame.

The GMOS North CCDs have strong fringing in the z'-filter, and weak but still visible fringing in the i'-filter. The peak-to-peak amplitude is about 5% for the z'-filter. The GMOS South CCDs have strong fringing in both the i' and z' filters, the peak-to-peak amplitude is about 50% for the z'-filter.  For sparsely populated fields, fringe frames can be constructed from the science images. For crowded fields or extended objects, you may consider obtaining addition sky observations for construction of fringe frames.  Example fringe frames are available (GN, GS). To construct a reasonably good fringe frame you will need a minimum of six dithered science exposures.

Are the Baseline Calibrations sufficient for your program? If you need photometry to better than 5%, you will need to add standard stars to your observations to determine extinction.