There’s a conversation on the ASTAP forum about adding a circle / ellipse and how to measure the sky background. Perhaps others who know more about comet photometry than me (most people!) could advise and join?
https://sourceforge.net/p/astap-program/discussion/suggestions/thread/b1c1be085c/
Han has added the option to measure inside a circle / ellipse now in the development version (right mouse drag while holding down the Ctrl key).
Thanks
Mark
British Astronomical Association
Supporting amateur astronomers since 1890