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Photometric Standards and zero points



Photometric Standard Stars

The photometric standard stars are selected from the Photometric Standard star catalogues listed in the NIR Resources web page. The photometric standards are observed as part of Baseline Calibrations for GSAOI. The observations are taken from the GS-CAL (shared) program, and not from the individual science programs. The observations of standard stars are only obtained in the J, H, Kp, Ks and K broadband filters. A more limited set of standards include the Z filter. The observations are obtained under natural seeing conditions (NO AO correction). If applicants would like to observe photometric standard stars with the AO loop closed, the coordinates of the selected standard stars and the time for observation have to be included in the Phase I proposal. Note that when the GeMS loops are closed, the standard stars may saturate even with the shorter exposure time allowed (5.3 seconds). Therefore, users should be careful to select standards which will NOT saturate in the fully corrected images. More detailed information is given in the GSAOI Baseline Calibration web page.

Photometric Zero Points and Sky Background

Approximate photometric zero points in the J, H and Ks broadband filters were measured from data collected during April 2012 GeMS/GSAOI commissioning run. The instrumental magnitudes in the GeMS/GSAOI system were calculated as follows:

mλ  = - 2.5 log Fλ

where mλ is the instrumental magnitude and Fλ is the flux in ADU/s. The resulting values for the instrumental magnitudes were then compared with the 2MASS standard magnitudes to define the zero points. Note that airmass effects were not considered in the calculation of the zero points (the observations were assumed to be taken at a “typical” airmass). The table below contains the approximate zero points for GeMS/GSAOI system. Extinction values were taken from Nikolaev et al. (2000, AJ, 120, 3340) . The sky brightness values, converted from the average counts in the sky frames, are also listed in the table.

A comparison of the values listed in the table with the values derived from the January 2012 and February 2012 commissioning runs indicates that the photometric zero-points are reasonably stable (within 3%). More detailed information about the photometric zero-points, including information about the color terms for each detector, will be posted in this web page as commissioning progresses. In the near future, more detailed information about the GeMS/GSAOI zero points will be provided through the Instrument Performance Monitoring project web page.

Filter Central wavelength
Zero point magnitude
Sky background
J 1.250 25.48 15.30 0.092
H 1.635 25.77 13.90 0.031
Ks 2.150 25.17 12.40 0.065

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