Re: Accuracy of Centroid Measurement

Posted by DaveCotterel on Oct 07, 2014; 2:20am
URL: http://astroimagej.170.s1.nabble.com/Accuracy-of-Centroid-Measurement-tp184p186.html

Thanks for getting back to me so quickly, Ms. Collins (if I may be so bold...)

I am using a MacOS version of AIJ and, to the best of my knowledge it does not offer the Howell Centroid method..  I use very accurately focused images which appear quite round to slightly oval (depending on seeing and atmospheric dispersion) and have a distinct central brightening like a proper PSF should... I try very hard to avoid saturating any of the pixels by careful attention to exposure times and ISO settings.  I'm working at 0.275 arc seconds per pixel plate scale.....

When I press the Aperture settings I get this screen:




I have done some experimentation just now and, not to bore you with the details, it seems that as I increase the radius of the aperture beyond the radius of the star image, the range of 5 centroid values, in both x and y,  seems to converge on a constant value.  i.e  making the aperture greatly larger than the star offers no accuracy improvement.  

In my experiment a star that was 20 pixels by 20 pixels (10x10 radii) gave centroids that ranged no more than 0.025pix once the aperture was increased beyond 10 pixels.   At the plate scale I am working with this gives an error bar only 0.0069 arcseconds which is beyond fabulous!!!   (It's actually waaaay down in the noise....)

If my version of AIJ does not do 'Howell' centroids, can I assume that it is calculating the centroids by the "Center of Mass" method?

This is interesting!

Dave