Tuesday, May 1, 2012

M100 processing part 4 - FWHM evaluation

I have now finished working on rejection of blue and luminance data. In total I acquired 180 light sub-exposures through LRGB filters, representing 36 hours of integration time. Of this I wounded up rejecting roughly half due to a fading deep sky signal or high background level. I knew this would happen and that is partly why I took so much data - even after rejection I still wanted to have enough for a low noise image.

So, from a total of 1064 individual files prior to any processing (most of which were calibration data) we are now down to 96 light frames of reasonable quality! Next step is to look at the average stellar FWHM to see if more rejection is needed. For this I use CCDInspector:
FWHM on remaining sub-exposures (click for larger version)

Average FWHM with RGB filters is around 3.0" while it is 3.8" for the luminosity data (UV-IR rejection filter). Is this just because of poor seeing on those nights or is it typical a broader bandwidth results in fuzzier images? I suspect the latter, but I am not sure. I use a RC astrograph with no refractive elements except for the deflection plate in my AO-8 tip-tilt guider.

For the RGB data I will not reject any images due to FWHM since none of them differ significantly from the mean. On the luminosity data I want to pursue maximum sharpness and here I might choose to explore if something can be gained by stacking only the best half. Stay tuned!

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