Zero points

For pipeline behavior and file layout, see User guide. This page summarizes magnitude systems and zeropoints.

hst123 reports magnitudes in the AB system by default. DOLPHOT uses the Vega system; hst123 converts using header keywords.

All HST images provide PHOTFLAM and PHOTPLAM per chip. The AB zero point used in hst123 is:

ZP_AB = -2.5 * log10(PHOTFLAM) - 5 * log10(PHOTPLAM) - 2.408

References:

• WFPC2 zeropoints

• ACS zeropoints

• WFC3 photometric calibration