Thursday, June 10, 2010

Bright star tests

Today I spent some time trying to understand how Addstar is working and where things are going wrong. In a word, that was a fail. Addstar seems to be skipping over some artificial stars that I've indicated in my input file and I couldn't find a reason for it to do so. As I mentioned yesterday, Addstar was inserting only those stars in every other row. I found that when I decreased the spacing of the artificial stars it was then inputting alternating columns and rows to give a diamond grid effect.

While I couldn't solve the mystery of Addstar, I decided to go ahead and do more testing using only bright artificial stars in order to find a way to troubleshoot the problem. I found that Addstar added the same pattern of artificial stars for a given spacing of stars. So as long as I am careful to keep my spacing the same or double check the resulting artificial star pattern in a bright star test for a new spacing, I figure I can reliably make a new file with only those stars addstar is actually inputting for comparison later when I calculate the completeness levels.

In addition to the spacing tests, I also went back to the original artificial star input to make sure that I was only inputting stars that appear on all exposures of the same chip. When I do my Allframe analysis, I require stars to have both g and r information (all of my input artificial stars do) and to be found in at least 2 exposures. (Because the exposures are dithered from one another, there could be stars near the edges that appear on only one exposure. My method for choosing my artificial stars before today would've meant this would only be the case near the edges of my reference frame, so it should not have had a huge effect, but I did this correction to eliminate the edge factor altogether.) It ended up just being easier to ensure that all of my artificial stars appear on the overlap of all exposures. This means I'm inputting fewer stars per Allframe run than I had previously. This is fine because I can always run more than 10 artificial star tests or further decrease the artificial star spacing per run to get more artificial stars for the end statistics.

My next project will be completing the effective Addstar files (including the positions and mags of only artificial stars that actually appear in the fake images). I've been thinking about whether it will be using to run Allframe on the bright artificial star tests. I'm not sure it will provide much more insight than running the actual tests will. Instead, I'll practice making the effective Addstar files and double checking the art star positions. Then I'll apply that technique to the real files and Allframe the real files again. I'll then go back to Beth's method of looking at the number of stars per magnitude on the true and artificial frames as compared to the input. If there are still major problems, it might worthwhile to do the full Allframe analysis on the bright artificial star images.


For now, I've settled on a 50 pix spacing in both the x and y directions. I've determined that the area that overlaps all exposures has coords
xmin = 439.4917
xmax = 3989.424
ymin = 251.866
ymax = 1840.46

I rounded the min values up and the max values down for good measure. In all, I'm inserting 70 stars in the x direction and 31 stars in the y direction. r181 still have signficant astrometry problems, so I've ignored it when calculating these values (it's one of the 2 exposures that's not used in the analysis, so I haven't even bothered to run it through DAOphot and Allstar. r183 is the other, but it will at least generate an allstar output in the usual amount of time, so I run it with the others for simplicity, though it is not used at all in the Allframe analysis.)

Ideas about Addstar:

Just talked to Beth about a few other things to consider when figuring out the best way to work with Addstar. First, I'll generate a fraction of the stars I currently have to see if we're somehow exceeding a limit Addstar has for the number of stars that can be input at one time. The second idea is to just cut the spacing in half to see if I can get the spacing I had wanted in the first place. I mentioned this idea yesterday and it's the desired way to do things because we want to fit a reasonable max number of stars into each AST so that we can cut down on Allframe computing time. Additionally, it'll be easiest to generate the effective Addstar file using this approach. Right now, the easiest spacing for that seems to be 56 pix in either direction, but tomorrow morning I'll try larger pixel spacing, perhaps 28 pix (half the original) to see what the pattern looks like.

No comments:

Post a Comment