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So this seems to suggest that the error in color is not a function of stellar color? I will try this again with unstacked ipp data soon.

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Aug 22

I've tried looking for the effects of water vapor (greater deviation from mean i-z color for redder stars) in the unstacked data, but found nothing. Here's the graph (only stars which have no close neighbors and accurately calibrated colors from stacked data were plotted):

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To ensure that this is no fluke, I looked at the changes in stellar color of a particular star (chosen because we have many observations of it). I found that on a single night, the g-r color (which is not supposed to be affected by water vapor) varied as much as (if not more) than the i-z color (which water vapor is supposed to affect):

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I also checked for the possible effects of airmass. That does seem to affect anything either:

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So unfortunately, I haven't found any evidence that water vapor affects stellar color (or I guess I should say, I have not found any evidence that water vapor is the limiting factor on the precision of PS1 photometry) so far 

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Data access on Odyssey:

1. Run JAuth.jar to get login key
2. ssh -Y into to odyssey.fas.harvard.edu, or herophysics.fas.harvard.edu, using the electronic key. 
3. run tcsh
4. source .myrcstubbs
5. data are at /n/panlfs/data/MIRROR/ps1-md/gpc1/
6. nightly science uses individually warped images, nightly stacks run on stacked frames
7. image types: wrp is warped. 
8. see available modules with "module avail"
9. load a module with "module load hpc/ds9-5.6"
10. photometry is in .cmf files, as FITS tables. 
11. in python: 
    1. import pyfits as p
    2. p.open('filename')
    3. print a[0].header
12. or, imhead on command line
13. a[1].data.AP_MAG for aperture magnitudes
14. PSF_RA and PSF_DEC are in the skycell files. 
15. make a scratch directory for data in /n/panlfs

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