For at least some INT-WFS FITS images in detector counts, this is an example of how to derive the effective zero-point, i.e. taking the exposure time into account.

Most of the required quantities are given in the Data Quality Control (DQC) catalogue, see help file for explanations of these terms. The DQC can be queried via the AstroGrid portal.

MAG_ZEROPOINT = magzpt - (am-1)*extinction + 2.5*log10(exptime) - apcorr 

magzpt

is the standard zero point for the band in question

am

is the airmass

exptime

is the exposure time in seconds

apcorr

is the aperture correction

SExtractor also uses the BSCALE, BZERO and GAIN, found in the FITS header

You also need the

seeing

from this catalogue if you want SExtrator to attempt star-galaxy separation

For the average

extinction

and other notes see WFS Photometric Calibration

-- AnitaRichards - 14 Mar 2005