Rory’s Astrophysics Blog is going interstate! I am in Chicago now, waiting for my flight to Boston (home). I am going to fill my readers in on a few things that have been giving me a lot of trouble and some ideas I have for next steps.

The Mystery of 15 Chips

Hubble’s WFC3 camera from which we obtained the images of NGC6819 contains two CCD chips with which it images the sky. DOLPHOT’s splitgroups command is supposed to split the fits files into two files each containing the data from only one chip. Whenever I have run this command on my computer, it has saved not two but 15 images saving them as chip1 through chip15. At first I thought this was correct, but when I learned that WFC3 only has two chips and when Thom ran splitgroups and had the images split (correctly) into two chips, I realized 15 chips is incorrect. I spent all of yesterday and most of today so far trying to figure out where the error lies in my execution of splitgroups. I recompiled dolphot with different compilers, I downloaded fresh flc images, and I scoured the splitgroups source code to see if I could find where it was doing something wrong.

What I think is happening is that splitgroups is interpreting each extension on my fits file as a different chip. There are 15 extensions each of which represent different data products (as further explained here). The weird part is that I checked Thom’s .fits images and they also have 15 extensions yet splitgroups accurately only saves two chips from his files. Why does splitgroups behave differently on our different computers?

Ok, my flight boards momentarily, but I think I figured out why splitgroups is giving me trouble. I have tried running WFC3mask, but get an error because it isn’t able to find the PAM. turns out, I didn’t realize the PAM was not included in the wfc3 module, so I didn’t download it. With the PAM, WFC3mask should run properly and I think WFC3Mask prepares the files to be correctly split by splitgroups. I am fairly certain this is the issue, since it is a key difference between Thom and I as he did download those PAMs and the PSFs and splitgroups works correctly for him when he runs wfc3mask first.

The PSFs are fairly large, so I may not be able to download them before takeoff. I will try to play with dolphot inflight, and I will update when I land with what I have been able to accomplish.

The biggest lesson I have learned from this experience is that when I get stuck on something that someone else has got to work read their blog-THOROUGHLY-I didn’t pay much attention to Thom’s installation post since it came post-installation for me, but he did explicitly mention the PAMs which are the thing that tripped me up.

Hopefully tonight or tomorrow, I will quit this stagnation and blaze ahead with dolphot. I believe Thom is working through pure dolphot with no pydolphot, and I am interested in seeing if that will work well. I am going to continue running pydolphot as I think it may streamline the process, but by trying both methods, we will see which method is actual more efficient/effective and move forward accordingly.

UPDATE re: PAMs

I am so relieved that splitgroups and wfc3mask work correctly now. It has been one of the longest periods of frustration of this entire research process. I am happy to be through the woods of this situation and able to now run pydolphot with new issues! I am cramped on the plane but will continue this either tomorrow or tonight and see if I can get photometry out of pydolphot.