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Dear Iraf experts, Hi, I am trying to compare the flux levels of two images of a galaxy taking with different instruments (WIYN and HST), that's why I need to transform one image to align with another one. I already considered fllter responses and things like that. So I tried to use geomap and geotran, the coordinate transformation works very well, but when I try to compare the flux of the two images, they differ in nearly a factor of 3 with a large scatter ( +-1 in ratio) This scatter indicates that this large factor is not due to a wrongly used scale factor (as otherwise the scatter should be small) , but due to the way I did this transformation. So I checked the geotran and the keyword (preserve total image flux) is 'yes' so presumably it should be allright.

The first thing that come to my mind is that geotran only preserve the total image flux, but not the fluxes of the individual objects. That's why I would like to ask if the experts here have due with this kind of problem before, or anyone know the accuracy of the flux after geotran transformation.Thank you very much.

You're right that geotran only preserves the flux of the total image (output pixels are multiplied by the Jacobian of the transform) and not individual objects (which would require modelling of the PSF and background). Beyond that I can't really say what this does to the accuracy of the flux of an individual object, I believe though that the flux preservation accuracy was checked long ago.If somebody has looked into this more closely I hope they'll post another reply.