Photometric Transformations

On the evening of 01 March 2019 (UTC), I collected a new set of photometric data using my 'G' and 'R' filters.  The target was, once again, Messier 46.  I like this particular object because it's a rich open cluster with nice selection of stars and it has a bonus object NGC 2438.

I took 200, 4 second images with 2x2 binning with each filter.  Running it through my stacking program, about 180 individual images were actually used for each stack.

Here is the Green stacked image:

and the Red stacked image:

After running the stacked image through Astrometry.net to get the WCS transformation (which is used to convert x,y pixel locations to RA, DEC coordinates), I use the IRAF task 'starfind' to locate the stars in the field of view.

The next step is to perform aperture photometry on each of the detected stars.  The result of this will a measured flux for each star.  The flux can then be converted to astronomical magnitudes following the transformation algorithm described Chapter 6 of the CCD Photometry Guide published by the AAVSO.  To calculate the transformation coefficients, I used the Sloan g' and Sloan r' photometric standards from the The AAVSO Photometric All-Sky Survey Data Release 10.

Here is my plot for T_gr = 1.570281 (slope = 0.636828)

and for T_g_gr = 0.291380:

and finally T_r_gr = 0.022768:

So then I can compute the magnitude of each star relative to a bright standard star I've chosen within the field of view.  After computing these magnitudes, I can compare my measurements to the Sloan g' and Sloan r' magnitudes.  The results are shown in the following two plots:

Along the x-axis is the Sloan g' or r' magnitude of the star, and along the y-axis is the percent difference between my measured magnitudes and the catalog magnitudes. As you can see, the accuracy is less than 1.5% even for stars as faint as magnitude 14.5.  The accuracy drops to less than 0.5% for stars brighter than about 12th magnitude.

So overall I'm fairly pleased with these results.  I'm going to take one more set of data using my 'B' filter, which I can then compare to the Johnson B and Johnson V magnitudes to see if I get similar results.

Not that it will be scientifically viable, but at that point I'll have 'R', 'G', and 'B' images of this region, so I'll probably combine them to produce a color image as well.  It might be pretty to look at.