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Hubble Space Telescope
Differential Photometry Across the Detectors

Differential photometry across the detectors has been measured for a few filters in all three cameras as part of the cycle 7 calibration plan (proposal ID 7693). Images of the absolute standard P330E (solar analog) were taken at 144 different positions detectors using filter F160W for NIC1 and filters F110W and F222M for NIC2 and NIC3. Data with camera 3 were obtained during the first NIC3 campaign while this camera was on focus. All images have been calibrated using on-orbit darks and flats. Countrates were measured for all camera and filter combinations using a 0.5 arcsec radius aperture. Deviations (in %) from the mean value were computed and the 1 sigma of the distribution calculated (see table below).

Results of the Differential Photometry Calibration Test:

1 F160W 1.0 dph1_f160w.pdf
2 F110W 1.7 dph2_f110w.pdf
2 F222M 1.0 dph2_f222m.pdf
3 F110W 10.7 dph3_f110w.pdf
3 F222M 1.6 dph3_f222m.pdf

The results show that differential photometry for cameras 1 and 2 is better than 2%, in the filters for which measurements are available. Since the images obtained as part of this calibration program have been calibrated using the available on-orbit darks and flats reference files available to the observers, there is no reason to believe that a differential photometry at the 2% level, or better, can not be obtained for the rest of the filters in these two cameras, if the appropiate reference files are used.

The results for camera 3 have to be treated with more caution. For the F110W filter, the one sigma deviation from the mean (10.7%) is dominated by intra-pixel sensitivity variations (see section NIC3 photometry and pixel centering) and not by residuals in the large scale structure of the flats. The results for the F222M filter show that a differential photometry of better than 2% can also be achieved in this camera.