What do I need to know about photometric zeropoints for images and source lists?
ACS 1. ACS images are in ELECTRONS/SECOND. Zero-points have been published in Sirianni et al. 2005, PASP, 117, 1049, Table 10; updated values are included in the ACS Instrument Handbook and can be found at http://www.stsci.edu/hst/acs/analysis/zeropoints. Zero-points can also be computed from the values of PHOTFLAM and PHOTPLAM in the header of the image file using the equation: ABMAG_ZPT = -2.5 Log (PHOTFLAM) – 21.10 – 5 Log10(PHOTPLAM) + 18.6921 Note that the HLA source lists are in units of ABMAG. 2. If performing aperture photometry, the relevant aperture correction is applied. Published ACS zero points are for an infinite aperture; details of the aperture correction depend on filter and photometry procedure. See the Sirianni et al paper for more details. 3. Convert to the desired magnitude system. ACS zero-points are provided in three systems: ABMAG, STMAG, and VEGAMAG, see the ACS Data Handbook for more details on the systems. Note that the conversion between these systems depend
