sky background subtraction

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Robin Leadbeater

Hi Ron,

I don’t have much experience in planetary imaging but in slit spectroscopy the sky background, measured above and below the target spectrum, is similarly subtracted from the spectrum image.  When this is done however it is important that the software preserves any negative pixel values that are generated (due to noise in the image). If the negative values are clipped to 0 you do not get the correct total flux values. I think the same will apply here.  Are the two images in your document before any sharpening ?  If so I would say there is clear signs of clipping. (You could perhaps check by taking a profile through the two images before sharpening . The brightness of the planet limb should merge smoothly into the background due to the finite resolution of the imaging system). It depends to what use the image is going to be put but if there is any clipping, I would expect any measurement of total flux or diameter for example not to be reliable.