Pixel Area Maps for ACS/WFC, ACS/HRC and ACS/SBC images

Richard Hook, February 2004

When HST/ACS images are flatfielded by the CALACS pipeline the resultant FLT files are flat if the original sky intensity was also flat. However, because there is very significant geometric distortion in such images, the relative photometry of point sources in FLT images cannot also be correct because the pixel areas on the sky vary around the field. After drizzling the geometric distortion is removed but the sky remains flat so both surface and point-source relative photometry are correct in the resulting DRZ files and the photometric zero-point in the header will allow conversion from electrons/s to absolute flux units.

Unfortunately users who may wish to perform point-source photometry directly on the distorted FLT files, rather than the drizzled (DRZ) data products, will require a field-dependent correction to match their photometry with the DRZ zeropoint.

The correction needed is to multiply the measured flux on the FLT image by the pixel area at the corresponding position and also to divide by the exposure time (DRZ images have units of electrons/s). The pixel area must be expressed in units of the reference scale (50mas/pixel for WFC and 25mas/pix for HRC). Once this correction has been applied the same zero-point as applied to the drizzled data products may be used to go to absolute flux units.

To facilitate this correction we are providing a pixel-area map for both the HRC and WFC channels of the ACS as well as an example script which will allow the construction of such a PAM when required. To correct a flux measured on the FLT file to that on the DRZ image it is necessary to multiply by the PAM value and divide by the exposure time:

DRZ_flux = FLT_flux * PAM / exposure time.

The PAM for the WFC is close to unity at the center of the WFC2 chip, close to 0.95 near the center of the WFC1 chip and close to 1.12 near the center of the HRC.