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Hubble Space Telescope
DrizzlePac: Photometry Demonstration

AstroDrizzle: Aperture Photometry Accuracy

Aperture photometry comparison

NGC 6791 is a nearby (~4000 pc), old (8-12 Gyr) open cluster with an unusual metallicity ([Fe/H]=+0.3) which has been studied in great detail (see e.g. Stetson, P. et al. 2003, PASP, 115, 413 and King, I. et al., 2005, AJ, 130, 626 ).

The HST images obtained with the Advanced Camera for Surveys WFC for GO program 9815 (PI I. King) provide an excellent set of images to study the quality of the photometry performed on the AstroDrizzle output.

In the previous two sections we estimated the instrumental magnitude of ~1500 stars using the AD output and the original FLT images. Figure 6 shows the difference in magnitude (AD-FLT) for each star in our catalog as a function of instrumental magnitude. The stars of interest lie in the instrumental magnitude range [-10.0, -6.0]. We observe that most of the stars in this range are matched within 0.01 mag. Fainter sources present larger dispersion, but also larger errors as expected from individual 30 sec exposures.

Figure 6: Star by star instrumental magnitude comparison.

Figure 7 provides a comparison of our results with the published CMD in King, I. et al. (2005). The horizontal scales of the plots are the same. The vertical scales are different but the red boxes could be used as a visual aid for the comparison.

The left panel shows the [F814W, F606W-F814W] CMD from King, I. et al. (2005). The magnitudes in this plot were calculated by the authors using DAOPHOT, ALLSTAR and, for the brightest stars, they used the method of Anderson & King, 2000, PASP, 112, 1360 . The zero points were determined with the procedures described in Bedin et al. 2005, MNRAS, 357, 1038. The right panel is our instrumental aperture photometry performed on the AD output images.

Figure 7: [F814W, F606W-F814W] CMD comparison. (Left) CMD taken from King, I. et al. (2005). (Right) CMD obtained with simple aperture photometry on the AD output.

Our analysis shows that it is possible to achieve optimal aperture photometry using the AstroDrizzle output, provided that the combined images are created with carefulness and that cosmic rays are properly removed.