Accuracy of Centroid Measurement

Posted by DaveCotterel on Oct 06, 2014; 9:18pm
URL: http://astroimagej.170.s1.nabble.com/Accuracy-of-Centroid-Measurement-tp184.html

I am involved in a double/binary star measuring project using 'lucky imaging' stacked jpeg images.  I have been using Astroimage J (AIJ) to find the centroids of the two stars.  I then use simple Pythagorean geometry to find the separation and position angle of the stars.  The scientific accuracy of my work depends a great deal on these centroid calculations.

The centroids given for a typical star by AIJ are given to six decimal places (!) of a single pixel.  How many of these digits are scientifically significant?  (The pixels on my camera are 4.2 microns square so the sixth decimal place puts us into the realm of the larger organic molecules in size which is a bit much I think you will agree...)  TPerhaps more troubling is that there is always some variation of these values.  Because of this variation I take 5 centroids and then find the mean of the values.  

Here is a typical set of (x-values only) centroids for a star.  y-values vary in a similar fashion.

                   
250.373143
250.371879
250.363256
250.375980
250.365331

I am using an aperture wide enough to include all the pixels involved in the star image.  I only use stellar images which are not saturated so all pixel values are below 255.

How, exactly, are the centroids calculated?  Why is there variation at all if there are no saturated pixels in the star image?  Should be a fairly simple algorithm - take the value and location of each pixel and....

Thanks you for the excellent software and for your time taken to answer the questions.

Dave Cotterell, Blackwater Observatories, Toronto, ON