Importance of photometry calibration at CCD level for coadded images

This is probably a very basic question but I am wondering how the zero point determined while reducing individual CCD images are propagated to the coadded images ? In other word; how important is it to determine the zero point for individual CCD if one performs a calibration like stellar locus regression on the coadded images ?

For HSC, we have been using meas_mosaic to set a position-dependent magnitude zero-point (kinda like “ubercal”). @parejkoj is in the process of putting the same capability into jointcal.

Getting the zero-points aligned on the individual CCDs (so that sources have the same calibrated flux level) is important, even if you’re planning to calibrate the coadds independently: if they aren’t aligned you’re going to introduce additional noise in the coadd.

Ok, that makes sense. But are the coadd actually recalibrated in the current pipeline ? If yes where is it done ? measureCoaddSources ?

No, we currently don’t have a facility for recalibrating the coadd. I think QA on the coadd measurements indicates that this is not necessary.

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But you certainly shouldn’t let that stop you! If you put a stellar locus regression Task together, it’ll be a very useful QA tool.

We are working on 2 fronts for the DESC cluster analysis on CFHT data:

  • Improving photometric calibration at CCD level by using SNLS catalogs as in Betoule et al. 2013
  • implementing stellar locus regression at the coadd level

But I think that we should also improve getZeroPoint() in pipe_tasks/…/ as it does not take into account the errors on magnitudes from the reference catalog (unless this has been changed recently, but I don’t think so)

I’d suggest we skip that, and spend our efforts improving the photometric fit in jointcal (which I plan to start very soon). That would be our go-to source for calibration in the future, no?

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I agree that it is better to put the efforts on improving the photometric fit in jointcal now, but it’s a little bit embarrassing to keep a bugged CCD photocal in our pipeline. This is also bad for future multifit algorithms which will be running on individual exposures, not coadded ones.

I’m embarrassed by the current behavior of PhotoCalTask as well, but I also don’t think it’s doing much harm. As for multifit, it will definitely use JointCal outputs for both photometry and astrometry, so the initial CCD-level versions of those don’t matter there either. Where they will matter much more is in Alert Production.