Re: Accuracy of Centroid Measurement

Posted by karenacollins on Oct 06, 2014; 10:18pm
URL: http://astroimagej.170.s1.nabble.com/Accuracy-of-Centroid-Measurement-tp184p185.html

Hi Dave,

As far as the accuracy of the centroid measurement, I don't have much information to offer. I think it would depend on several factors, including star brightness and PSF shape (e.g. focused vs. defocused). Also, the measured centroid location depends somewhat on where the starting point is relative to the final determined centroid location. However, the "Howell" centroid option (see below) gives very repeatable results, no matter where the starting point is (within reason). Do you have the "Howell centroid method" enabled or disabled in aperture settings?

AIJ currently reports all measurements using 6 decimal places, so there is no intended precision due to the number of reported decimal places.

There are actually two different methods of centroiding in AIJ, which can be selected by clicking the "Change Aperture Settings" icon () above an image. That will open the panel shown below (from AIJ version 2.1.5). If "Use Howell centroid method" is DESELECTED, a basic "Center of mass (light)" calculation is done to find the centroid. If SELECTED, the method described in Howell, CCD Astronomy, 2nd Ed., p. 105 is implemented. Both of these methods require iteration to settle on the best centroid point. They both work about the same for focused stars, except that the Howell method finds almost the exact same centroid with fewer iterations, no matter which side of the PSF it is the starting from. The Howell method works best if you have focused and/or faint stars and you want to get consistent centroid results every time. If you have brighter stars with defocused doughnuts that are not exactly symmetrical in brightness around the doughnut, the "non-Howell" version finds a centroid that is a little closer to the center of the doughnut, which I think is what is desired in that case.

If you are able to characterize the precision, in one or both centroid modes, I would be interested to know about your findings, once they are public.




Karen