Mark – Thanks for signalling that ASTAP might be suitable for comet photometry. Please pursue this and see where it leads.
Paul, yes – circular apertures are just a special case of elliptical ones, which can also be adjusted for any tilt angle. Interestingly, there is also another form of photometric aperture used by pros called the ‘capsule aperture’, which is like a rectangular aperture but it has a hemispherical form at each end of the narrower sides of the rectangle. It is used for the photometry of trailed images.