Aperture Photometry script, question

CraigNZ

Well-known member
Jul 8, 2015
101
0
I used AP script to measure the flux of an image and saved the results in a Image table. The documentation shows the CATRA and IMGRA columns are the catalog RA position and the measured RA position on the image respectively. I used Exctract stars from catalog, and used the Gaia DR2 catalog. I then loaded the table into a spreadsheet and had excel calculate the difference between CATRA and IMGRA which should give the drift of the star from J2000 to JNow, but the results show 0 difference for all stars. Shouldn't I see some sort of proper motion drift or difference between catalog and actual position on the plate?


Craig
 

CraigNZ

Well-known member
Jul 8, 2015
101
0
I think I found the problem. Using "Catalog" for the star extraction method takes the Gaia RA and maps that on to the image. Using "Image" for the star extraction method uses the Gaia RA position as a starting point and then uses PSF to locate the centroid of the star, in this case the CATRA and IMGRA values are different. That is what I was looking for.

Craig