| | 21 | '''UPDATE 2012.11.28''' : the discussion below is correct, but one wrinkle is that the airmass slopes (c1 in the equations below, ZP.AIRMASS in ppStack.config) have been left at 0.0. Thus, the airmass extinctions for the different input images have not been corrected based on the airmass of the images. Instead, the zero points have been tweaked by the relative photometry analysis in the assumption of some possible cloud levels. Thus, the output stack effectively has a zero point of 25.0 (the value for ZP.TARGET) and an airmass of 0.0. Also note that the photometry from the stack is re-calibrated relative to the reference catalog (currently, an ubercal-based reference). However, this calibration process ''also'' ignores the airmass (because it was originally written for the synthetic photometry database with poor photometry quality). Thus, the re-calibration tends to find zero points which are close to 25.0. In future (after the current, 2012.07.06, version of the LAP re-analysis is done), we need to set the correct airmass slope terms in ppStack.config and also include it in the zero point calibration. |
