modelling the background

Robin Leadbeater

Yes the sky background has to be subtracted somehow. The same problem crops up in spectroscopy of supernovae and some programs have various ways of fitting a model to the sky background in the surrounding region. In my limited experience of photometry I think programs which model the star point spread function against the background work better in this situation, though there  could still be a bright/or dark patch directly under the target of course.  The transient survey instruments get round this problem by subtracting an image taken when the transient was not there.