Re: Aperture radius settings in the Seeing Profile dialog

Posted by karenacollins on Mar 23, 2020; 2:32pm
URL: http://astroimagej.170.s1.nabble.com/Aperture-radius-settings-in-the-Seeing-Profile-dialog-tp1304p1317.html

As it turns out, the method used before the recent change to calculate suggested photometric radii in the seeing profile code was *not* a bug afterall. It was a deliberate change to attempt to accommodate both focused and defocused images. I have now reverted to the original code starting with the 3.4.0.05 and later.

The method I have reverted back to looks for the radius where the average radial flux level drops by ~1% of the peak ADU in the aperture (by default). This radius r is the suggested aperture radius. It has nothing to do with FWHM.

If you prefer a different value of flux cutoff threshold, the default value can be changed in the "Aperture Photometry Settings"  "Radial profile model normalized flux cutoff" setting. The default value is 0.01 (i.e. 1%). To make the suggested aperture radius smaller, change to a larger number such as 0.02 (2%) and to make the suggested apertures larger, change the value to a smaller number such as 0.005 (0.5%).

Then the suggested inner background radius is defined as r1 = r x 1.75, and the suggested outer background radius is defined as  r2 = r1 x 1.5.

Finally, please note that these values are only suggested starting points. If you need the absolute best photometric results, you should try a series aperture radii smaller and larger than the suggested value to see which produces the lowest model residual (RMS).

Note: The current AIJ paper states that r = 1.7 x FWHM, r1 =1.9 x FWHM, r2 = 2.55 x FWHM. This is not correct and has not been correct for many years. However, it is the paper that needs updating and not the code.

Karen