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I was wondering how to subtract the skylines from my image. I have vertical skylines across the entire field, and my object line runs horizontal. I used apall to define an aperture across the image, and then defined my background, but it did not remove the skylines....It's as though it only removed the background for one the one column, and not across the entire band. Any suggestions?

Hello,APALL is widely used with sky subtraction and should work for you. There are three standard ways of removing sky lines. In APALL there is fitting a background at each column from one or two background regions and defining a background spectrum by an average or median and subtracting that. These are controlled by the "background" parameter.Most methods, such as the ones above, depend on the sky lines being aligned with the rows or columns of the image. If they are significantly tilted or curved then either a rectification step is needed or the sky is extracted as an actual spectrum, dispersion calibrated along with the object spectrum and then the subtraction is done in wavelength space. For this method a useful task that lets you really try and tweak the subtract is SKYTWEAK.I hope this helps. If not you can post your apall parameters and maybe I will see that you have something set wrong.Yours,
Frank Valdes