Question about annulus setting in the drop-down menu at the Photometry tab

[Andy Howell]

Hello Han, the annulus setting (ASTAP version 2026.10.08, Intel 64 bit) in the Photometry tab has a drop-down menu with settings that range from 20px to 60px. I am not clear how to pick the correct setting. I am a little confused because I was accustomed to seeing annulus expressed as a multiple of stellar HFD. Thanks in advance for the clarification.

Best, Andy

[han.k]

Hi Andy,

The aperture is in multiplies of stellar HFD
Annulus is now in pixels.

I changed annulus size unit some time ago into pixels. There is no need to make it variable because the background does not change as the stars do. There was discussion some time ago and it was compared with AstroimageJ and other programs which also use fixed annulus. I'm not at my usual desktop computer to retrieve the discussion but the assumption was that it is more stable. There is no chance that to many bright stars or nebula reach the annulus area resulting in a variation during measurement. The chance this spoils the measurement is very low since the background is calculated using repeated sigma clip loops to throw out any star in the annulus and measure the true background.

Select for annulus something average like 30 or 40 pixels. the size would not make much difference unless you have a nebula around the variable star.

cs, Han