How is GAIN FITS keyword used during PSF photometric extraction ?

[gijsverdoeskleijn]

Dear SourceXtractorPlusPlus team,

Could you describe how the GAIN FITS keyword value in the MeasurementImage is used during the fitting procedure of SourceXtractorPlusPlus when it is performing PSF photometric extraction? We failed to locate a description in this SourceXtractorPlusPlus manual .

Reason that we ask: we use SourceXtractorPlusPlus PSF photometry intensively in the quality assessment of the ground-based surve coadded images for Euclid. We notice that changing the value of the GAIN FITS keyword changes the resulting PSF magnitudes for data that otherwise is identical. So identical pixel values in science image, RMS image and flagmap and PSF models and identical values of other FITS header keyword values.

We are happy to provide our data, software version and configurations files for SourceXtractorPlusPlus that we used: just let us know.

Kind regards, the Euclid ExtStackedFrame Quality Assessment Team,
Gijs Verdoes Kleijn (g.a.verdoes.kleijn@rug.nl), Pablo Corcho-Caballero, Nafiseh Mazoumzadeh and Eduardo Balbinot

gijsToSelf: counterpart issue ticket in Euclid Redmine: https://euclid.roe.ac.uk/issues/16879 , #469

[marcschefer]

Hi,

The value from the GAIN keyword is used when setting the weight of individual pixels in the loss function to account for the Poisson noise:

weight->at(x, y) = sqrt(1.0 / (back_var + pixel_val / gain));

I hope that helps