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Imaging Observing Strategies

The gaps between the three detectors in GMOS-S cause gaps in the imaging field about 4.9 arcsec wide. For GMOS-N, the inter-chip gaps between the three Hamamatsu detectors are 0.5 arcsec wider (5.4 arcsec). If continuous coverage of your field is needed, you will need to obtain multiple exposures and dither between the exposures. Due to additional bright columns at either side of the chip gaps, a minimum dither step of 10 arcsec in the X-direction (p-direction on the OT) is recommended. 

(The gap width for the old GMOS-N e2v DD devices was 2.8 arcsec, and the recommended minimum dither step in X-direction 5 arcsec.)

For unbinned observations the pixel scale is 0.0807 arcsec/pixel (GMOS-N Hamamatsu) and 0.080 arcsec/pixel (GMOS-S Hamamatsu). (The pixel scale of the old GMOS-N e2v DD devices was 0.0728 arcsec/pixel.) If you are requesting imaging in image quality 70-percentile or worse, consider binning the CCDs 2x2 giving an effective pixel scale of 0.161 arcsec/pixel (GMOS-N) and 0.160 arcsec/pixel (GMOS-S). This cuts the overheads from the readout from ~83 sec to ~24 sec per frame(*).

The original GMOS EEV CCDs had strong fringing in the z' and i'-filters. The peak-to-peak amplitude was about 5% for the z'-filter. The new GMOS-N and GMOS-S Hamamatsu CCDs have less fringing in both the i' and z' filters. The peak-to-peak amplitude is about 2.5% for the Y-filter. For sparsely populated fields, fringe frames can be constructed from the science images. For crowded fields or extended objects, you may consider obtaining additional sky observations for the 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.

(*) Final characterization of the read-out overheads for the GMOS-N Hamamatsu detectors is ongoing.