The dark current for GSAOI is of the order of 0.01 e-/s/pixel, implying on a total dark charge of ~12e- for the longest integration time allowed (20min). This very low dark current implies that daily darks taken with the same exposure time as the science frames (for dark subtraction as part of the standard reduction process) are NOT necessary, and in fact, subtracting a dark frame from the science and sky frames prior to subtracting the sky from the science proper will result in increasing noise. Both the dark current and the cross-shaped "warm" pixels are accounted for when subtracting the sky.

Darks (or rather "flats-off") are not required for Dome or Twilight Flats - this was tested during commissioning and attempting to subtract darks or flats-off from twilight and dome frames, respectively, have not produced any measurable improvement in the quality of the resulting flat-field. GCAL flats-off can be obtained as part of the GCAL flat sequence, if such is required (but users are reminded that GCAL flats are sub-optimal for data reduction).

If the applicant deems that darks are required for the data reduction, a daytime sequence can be added to the PhaseII and will be taken on a best effort basis within a few days of the observation execution (not charged to the programme).

Given the low dark current and the temporal behaviour of the dark frames, current plans are to create a library of dark frames with different exposure times, which can be downloaded from the GSA and used for data reduction if necessary.

Selected from the Persson (1988) list. Observations of standard stars are only obtained in broadband filters J, H, K, Ks, Kp. A more limited set of standards include the Z filter. Our goal is to obtain at least 2-3 standards at different airmasses each observing night. Approximate zero points are also available, and an study of the color effects originated from the differences in QE between the four arrays is ongoing. Photometric accuracy is limited to 5-10% due to uncertainties in the atmospheric extinction over Cerro Pachon, flat-fielding and sky subtraction, and the precision in the gain determination for each of the arrays. If the user is aiming at a higher photometric precision, it is necessary to self-calibrate the images, detector by detector.

Users should be aware that the standard star observations are NOT performed with the GeMS loops closed (NO AO correction - natural seeing only).  If the applicants would like to obtain the flux calibration stars with the AO loops closed, these should be provided as separate targets in PhaseI and included as Program Standards in PhaseII, therefore incurring in charged time to the program. The same procedure should be followed if the program requires a specific calibrator to be observed (e.g., for narrow band filters). Users should be careful to select standards which will NOT saturate in the fully corrected images, and check if the guide stars can be mantained for all offset positions. Please remember that the ODGW guide stars cannot be moved from one detector to another within the same sequence.