Thursday, May 28, 2009

DAOphot, here I come.


Good news. When I look at the r-band sky levels without having swarp first do the background subtracting, I get some really crazy stuff happening. Exposure 181 drops nearly to 0 on chips 4, 5, and 6, and exposures 179 and 180 are much higher than the others.Yesterday I had only looked at the sky levels for the background subtracted frames. Now looking at the stack of 7 exposures (without the 3 weird ones) I don't see the bad patch that was there before in ds9 OR in atv, so I'll proceed using the 7 exposures instead of 10. I still have yet to decide which stack to use for the g-band--that decision probably won't be made until I see the CMDs.

I calculated the median of medians from the un-subtracted sky levels to figure out what value I should add back in. I did this for the 10-exposure and 7-exp stacks in both r- and g-bands--4 values in all:

10 exp:
g: 528.095
r: 1823.11

g: 493.916
r: 1817.94

It seems reasonable to me that the median levels differ slightly between the two sets of stacks, but not by too much within each band. This makes me think there's not anything really crazy going on. These are the constant values I'll add to the sky-subtracted stacks to put the sky back in.

I added the sky to both the 10-exp and 7-exp g-band stacks as well as the 7-exp r-band stack.

DAOphot ftw.

Wednesday, May 27, 2009

Update on where I am

I stacked the 7 exposures for the g-band and got them looking nice.

I then checked out the psfs of the sources on both the 10-exp and 7-exp stack of the g-band data to see if the sources looked okay. They looked decent--a little raggedy around the edges but otherwise circular. Neither looked particularly better than the other. I'll take a look at the CMDs of each to see if there are differences.

I took a look at the sky levels for the r-band data and discovered that exposures 138 and 139 are much different from the others. Even the ones with comparable values don't seem to follow any trend as far as I can tell. I will ask Beth about this tomorrow and in the meantime make a stack of the 8 exposures that seem consistent. The levels for the r-band are also much higher than those for the g-band, but I think this is okay as long as all 10 are in the same ballpark.

I also stacked the 10 exposures of the r-band and there's something funky happening with at least one exposure on Chip 3. The same shows up in the 8-exposure stack. Oddly enough, I'm seeing this in ds9, but it doesn't show up when I look at the stacks in atv. I'm not sure why this would be, but the images look fine in atv. I also looked at the psfs of both stacks using atv and they look fine.

I discovered that the problem I was having earlier with stacking the images was in fact a result of me averaging the exposures in the stack instead of medianing them. Because the averaged stacks looked so messed up, this is reason enough to use a median instead of an average, in my opinion.

Once I take a look into what's happening on Chip 3 in the r-band, I'll go about adding the sky back in. I'll do so by adding a constant background, the value of which will be the median of the median background level on each chip. Of course, this constant will be different for the g- and r-bands.

After the sky's back in, I should be set to daophot which I think I should be able to at least begin by the end of the day tomorrow.

Summer Research 2k9. This is going to be epic.

Long-term Goals:
Complete the Wil1 analysis I've been working on for nearly a year.
Create deeper CMDs of Willman 1 stars.
Help correct the paper Beth wrote in 2006 and resubmit.

Short-term Goals:
Figure out the sky subtraction and add the sky back in.
Create a CMD of Willman 1 by Friday, June 5, 2009.

Current task:
Yesterday afternoon I re-swarped all of the original files without background subtracting them. I then created a plot of the sky levels on each chip and each exposure to compare with the one I made when I used swarp to background subtract. I noticed that the two plots looked very different. Exposures 126, 127, and 130 have much higher sky levels than the other exposures, which seem to follow an expected trend assuming that the moon was rising for consecutive exposures while the data was taken. I talked about this with Beth who suggested that clouds could have interfered with those 3 exposures and could have affected the sky level in all sorts of ways. Reflection from clouds could explain a brighter sky.

Next I'm going to reswarp the 7 exposures that seem well behaved. I will then compare the stacks of 10 and 7 exposures (both created with background subtracting), respectively, to see if the 3 wacky exposures are affecting the CMD. From this, I will choose which stack is most robust to use for the remainder of the analysis.

After choosing which set of exposures to use, I'll decide whether a median or average would be best for making the stack, again by comparing the CMDs.