This weekend, I honed my star detection a bit and overlaid the detected stars on a reference image to see how well the stars matched the actual stars on the image. I didn’t have a ton of success matching stars well, and wasn’t sure how to proceed. I felt like I needed somethin like the “threshold” parameter from our earlier aperture photometry source detection codes.

Reading excerpts from a couple studies that use Dolphot I noticed they used signal to noise ratio as a way to select for good sources. I have been using SNR, but only cutting off at 5, which is very low. I tried a handful of different values for SNR cutoff and settled on 75 as a decent value that gets most of the stars in the image. I also added a crowding cutoff at 1 (keeping only rows with crowding below 1), as I think pretty much every source above 1 is in a diffraction spike of a brighter source that isn’t actual in the open cluster.

This is the result of that source culling, there are about 2100 sources identified here, many of them are in the diffraction spikes of larger sources, and there are some prominent sources not detected.

This image is the sources with sharpness between -.3 and .3, quality flags of zero, SNR above 75, roundness below 1, and crowding below 1. Although it is imperfect, I think it is as good as I can get.

Turning Pixels to WCS

With the source discretion out of the way (for now), I decided to turn my attention to converting the source position list from pixel coordinates to WCS coordinates which is the first step to finding corresponding sources in different images. I consulted Marta’s codes on github and I consulted Marta in person for help on using the astropy.wcs function. I made pixwcs.py a program that turns a list of positions into two arrays, one of RA and one of Dec.

I am working on a program, catmat.py that runs category matching for two fields of stars. Thom and I are exchanging data now so that we can both work on all of the data.

I am going to finish up for the night soon, but am ready to continue to catalog matching tomorrow morning, and I will hopefully be able to have a color magnitude diagram by the end of the day tomorrow!