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"Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Conservation of flux

Exploring the PSFPosted by Peter Thejll Jan 22, 2012 01:37PM
During convolution of synthetic images with various PSFs we need to consider if flux is conserved. We calculate such images for a range of alfas and show here the difference in percent of the total flux of the image relative to the ideal image.

Percentage-change PSF-file
------------------------------
-1.6768524 % out1p4.fits
-0.060953825 % out1p6.fits
0.014277556 % out1p8.fits
0.027423956 % out1p9.fits
------------------------------

That is, in an image convolved by the broad PSF with alfa=1.4 1.67% of the flux is lost, while only 0.03% of the flux is changed when convolving with a PSF with alfa=1.9 (the narrowest possible).

The loss (and gain) in flux is not understood yet - but is perhaps related to clipping (in the case of the broad PSF) of halo when the images are padded 3x3 in order to perform the FFT, or roundoff (narrow PSF - many values are rounded below 1 to 0).



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Posted by Peter Thejll May 29, 2012 09:29AM

I think you are right Chris - some part of the lost flux may well 'come back' if we do flux normalization - but let's check. We now also understand some of the non-linearity issues and have opted for a 'flux normalization' strategy, rather than a 'scale to some point in the BS' strategy.This is in the EFM Method: FFM considerations to follow! BBSO also, but not really our fight.

Posted by Chris Flynn Jan 23, 2012 04:06AM

I am not sure if this is a practical issue, because we'll have to scale the analysed frames by the total amount of light on the frame -- because we don't know the true luminosity of the source -- so won't this just scale away? I'll look into this as well.