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Hubble Space Telescope
DrizzlePac: Photometry Demonstration

AstroDrizzle: Aperture Photometry Accuracy

Demonstration goal

The goal of this demonstration is to assert the accuracy of aperture photometry measurements on ACS/WFC images using the new software package Drizzlepac. With this goal in mind, we compare stellar instrumental magnitudes calculated using individual FLT images from the HST pipeline with the instrumental magnitudes of the same sources estimated using the AstroDrizzle output.

Observational data

This example describes the combination of two sets of three ACS/WFC images of open cluster NGC 6791 (Program 9815, PI Ivan King) which were obtained with the filters F606W and F814W. The GO used three orbit visit for each filter, taking three short (30 sec) exposures with POS TARGS in order to achieve a 3-way sub-pixel dither pattern. The interchip gap was not covered in these observations. Table 1 provides some basic information on the images which are available from the HST archive for those who wish to repeat this experiment.
Dataset Filter POS-TARG1 POS-TARG2 Exposure time
j8ny01suq F606W 0.000 0.000 30.0
j8ny01t4q F606W 0.0665 0.1886 30.0
j8ny01teq F606W 0.1827 0.0813 30.0
j8ny02yuq F814W 0.000 0.000 30.0
j8ny02z9q F814W 0.0665 0.1886 30.0
j8ny02zqq F814W 0.1827 0.0813 30.0

Table 1: HST/ACS datasets used in our experiment. The exposure time is given in seconds.