Wednesday, June 10, 2009


Meeting with Beth:

This morning I talked with Beth about the CMD issue for a while. She found something wrong with my code (I knew it had to be something!) that was throwing off all the CMDs. So we fixed that and now the python matching code agrees with the idl matching code which is comforting. All in all, the two matched 19850 stars. There were still a few features on the CMD that looked fishy, but it looked much more like a CMD--including the thin disk and what could be a turn off. But it's still not as deep as we expected. In order to really test this, though, the magnitudes have to be calibrated so we know exactly how deep the CMD actually is.

Sharp and Round cuts in DAOphot

Before doing the calibrating stuff I spent some time fiddling with the DAOphot parameters. I took a look at the objects in the previous allstar files to see if I could improve upon the sharp and round limits that I applied in DAOphot. I plotted flux vs. round and sharp to narrow the limits and changed the daophot.opt files for each of the g and r bands. I ended up doing two cuts--one stricter than the other.

Boo on not being able to insert a table. But anyway, the more lenient one was:
...............g10 ....................r7
sharp..... 0.35 to 1. ..........0.3 to 1.
round..... -1 to 0.25........... -1 to 0.2

I tried to lower the upper limit on sharp but DAOphot wouldn't let me for some reason...

Originally, there were 33679 stars in the r band catalog and 27334 in the g band catalog. This first cut lowered the number in each catalog to 17335 and 17583, respectively. It then found 9299 matched stars.

The second, more strict cut used:

......................g10................ r7
sharp .......0.35 to 1. ...........0.3 to 1.
round ........0. to 0.25 .........0. to 0.2

so it just cut off the bottom end of the round measurements. This cut reduced the catalogs to 3138 and 2645 in the r and g bands, respectively. 540 stars were matched and put in the CMD.

The weird features--like the third, reddest peak--remain in both. But the stricter cut makes the CMD shallower so it might be best to use the first, more lenient cut.

Calibration and zero-point offset plot

I matched the CMD stars (from the first sharp/round cut) to SDSS stars and plotted gtrue-ginst vs. ginst (where gtrue is SDSS and ginst is KPNO) to get a sense of what the zero-point offset is. The offset doesn't seem to be dependent on instrumental magnitude (this is good), but it's not zeroed (this is also good!). In fact it's centered around a gtrue-ginst of 7. To get a more quantitative idea of where we are I ignored SDSS stars fainter than 22 and took the median of the offset (gtrue-ginst). Offsets for the g and r bands are 6.7059405 and 7.2057229, respectively. I added these offsets to the instrumental magnitudes and made a new CMD. I'm still concerned because it only goes to about 23rd magnitude. But it looks much cleaner and more CMD-like. (

I'm concerned with the fact that only 1300 or so stars are getting matched between SDSS's 25,000 stars and the 9,300 which made the sharp and round cuts on my data. To double check this I'm going to try matching before doing the sharp and round cuts to see if the number of stars that match SDSS changes. If it does, I'll investigate this further. I expected more to match.

I'm meeting with Suzanne and co. in the morning for a quick IDL intro. Then it's back to the drawing board on how to get this CMD deeper.

1 comment:

  1. Once you select the chi and sharp cuts that you want to implement - you don't need to go back and rerun DAOphot. You just implement those cuts on the catalog that you have in hand.