Monday, June 1, 2009


Mag limit issues:

Last Friday afternoon I was having trouble because my CMD was only going to about 17th magnitude. I tackled that problem for most of the day.

I discovered that I'd lied to DAOphot a bit about how I had made the stack. So, after fixing this, my plots of sharp, round, and flux versus magnitude jumped down to about 20.5 magnitudes. Beth still expected this to be deeper. I then DAOphot'd a single exposure on one chip--Exposure 126 on Chip 7 to compare. I made the same sharp, round, and flux plots to check out the instrumental magnitude limits and they were showing stars only out to about 19 magnitudes. Still deeper than the 17 I was getting at first, but not quite down to the 20.5. This might be expected because the stack should get deeper than a single exposure, but not totally sure what the numbers should be.

I made a CMD of the new stack with proper DAOphot'ing and that looks to go down to about 19 mag. A few magnitudes better than before, but still not as good as I was expecting.

Email from Dave:

Dave sent an email today bringing up the issue of the distortion in our images. My scamp distortion output shows only one chip instead of all 8, so he was worried that we were assuming each chip has the same distortion which could easily be very untrue. At his suggestion, I overplotted SDSS stars before and after star subtraction to make sure that my stars were lining up with the ones SDSS knows. It looked like they were lining up pretty well, though there were a few places where SDSS said there should be a star but my image had nothing. I'm still not clear on what was causing this, but in general things lined up very well. I'll check again tomorrow to see if there's anything else I can do to check off on the distortion problems, though things look pretty good to me.

Dave also got back to me today about my DAOphot questions. He agreed with most of our parameter values: He used a single value for gain assuming it wouldn't affect any calculations we were actually going to use. He also used a variable psf and suggested that the KPNO Mosaic imager is probably especially prone to variable psf because it was one of the first wide-field imagers. He set percent error and profile error to 0 like we did, too. But just like us he doesn't really understand what this does either.

As far as Analytical Model PSF, he uses an option that's in the code, but not in the manual. Setting the parameter equal to 7 gives:

C Penny function --- Gaussian core plus Lorentzian wings.
C The Lorentzian and Gaussian may be tilted in different
C directions.

This is the fit he used, but he said it was because his LBT data was particularly bad and suggested we might not need to use it. He said we should try out some other things first. I emailed him back to ask how I could tell if the fit I chose was a good one. He suggested checking by eye to see if the residuals look good and checking to see if the chi-squareds were legit. I'm not sure I totally understand how to do those things, but that'll be a project for tomorrow.

1. Sign off on the distortion problems--make sure everything's cool there.
2. Explore some of the analytical models of psf to make sure I have a good one. This will pretty much conclude the discussion of DAOphot parameters pending future weirdness in results.
3. Talk with Beth in the morning about CMD calibration and get moving in that direction.
4. Email Dave back.

1 comment:

  1. To get a sense of the depth of the CMD - I would compare the depth of the CMD in the 2006 paper as it exists with the depth of your new photometry. You saw that the instrumental magnitude corresponding to the MSTO was about 16.5 (right?). You now go almost 4 mag deeper. See how many mags fainter than the MSTO the CMD in the paper went. Its a good sanity check that even with this initial sloppy round of photometry, you are 1.5 mags deeper in the stack than in the single image (did you input 1,1 for average,summed?).

    I wonder if we can look at the quality of the PSF fit as f(position) in the image to assess whether the stack worked correctly. I'll ask dave's opinion on this.