Even more MDS testing!

(This page is linked from the other Durham testing pages Durham_MDS_Testing and Durham_MDS_Testing2 and Durham_MDS_Testing3)

Here we look at stacks from the ps1-md release on November 18th 2009.

Photometry

Shown is the SDSS comparison for PSF mags on the ps1-md MD08 skycell 041 i-band stack 30204 (left) and Heather's previous stack (29871) on the same cell. Blue points are SDSS classified stars. Both have nominally the same exposure time (equivalent to 41 240sec exposures) and we assume come from the same set of exposures. However, the seeing is 2.0" on 30204 but only 1.4" on 29871, and the zero points differ by about 0.4 mags (note: the zero point for 29871 has the expected value). Clearly there are problems with the stellar photometry being non-linear.

However - above we show a direct IPP aperture magnitude (AP_MAG) comparison between 30204 and 29871. This is gratifyingly linear, from which we deduce there must be some problem in fitting the PSFs.

This is demonstrated here. The above shows the difference in PSF magnitude as a function of x position on the stack for two reductions of the same stack (30204). One is the provided cmf file, the other is our own run of psphot (code from SVN as of Nov 25th 2009) on 30204. Note the shift in difference with x.

Astrometry

Show above are the x and y position offsets between two reductions of the same stack (30204). One is the from the standard cmf file provided, the other is from our own run of psphot on 30204 (from code taken from the SVN on Nov 25th). The rms is about 0.4 pixels, so as both methods used identical datasets this is a measure of the reproducability of psphot. The offset is of order 0.1-0.2 pixels. NOTE: although not shown, 29871 v 30204 shows a similar scatter, athough with a different overall offset.