Yet more MDS testing!

(This page is linked from Durham_MDS_Testing, Durham_MDS_Testing2 and Durham_MDS_Testing4 )

These are outstanding issues from the improved MD08 stacks provided by Paul and Heather in October 2009. You can click on the images to see large versions:

Background problems due to removing the correlated read noise

The above is a portion of MD08.skycell.041.stk.29871, Heather's stack of the 41 best i-band images using the latest version of IPP. The vertical dark streaking around all the bright images is due to the background polynomial fits used in removing the correlated read noise. The displayed image is about 12 arcmin from top to bottom.

Clipping of stars at the stacking stage

Above is one of Paul's nightly stacks of 8 i-band images, MD08.skycell.041.stk.25532 . This is the best seeing (~1.0" FWHM) of any of them. Masked pixels are shown in red. Note how most of the centres of the stars are masked. This masking happens at the stacking stage, and is not in the warps. The scale here is about 6 arcmin from top to bottom.

Another of Paul's nightly stacks of 8 i-band images, MD08.skycell.041.stk.26482 . The seeing is worse here (~1.4" FWHM) but the clipping is also worse. There are virtually no unmasked stars on this image, and several galaxies are also masked.

Photometry on these new stacks

Here we show the comparison with SDSS for four stacks of MD08 skycell 041 i-band. The original 200907 release; Paul's nightly stack 25532 (8 frames); Heather's new stack of 41 frames; Paul's version of 25532 fixed to remove the problem of clipped stars. All except the original have the convolution correlated noise problems fixed. The x-axis is SDSS magnitude - blue crosses are SDSS classified stars, red circles SDSS classified galaxies. We have used the Panstarrs PSF mags for this comparison, so don't be surprised that the galaxies don't agree.

Original 200907 stack (20 frames. 1.3" FWHM)	Nightly stack 25532 (8 frames, 1.0" FWHM)

Heather's stack 29871 (41 frames, 1.4" FWHM)	Nightly stack 25532 after Paul's fix for clipped stars

Points to note: the stars on Heather's stack show a scale error wrt SDSS. Pauls nightly stack before the clipping fix was problematic. After the fix the photometry is much improved, but still shows more scatter than the original data.

Below is an example of a remaining problem with the clipping. This is the weight map round a star on Paul's fixed 25532 8-exposure stack. Successive gradations in the levels indicate how many warps are contributing to a particular pixel (with at most 8 warps it is easy to see the difference in levels) - brighter means fewer warps. Notice how, as one approaches the star radially, successively fewer and fewer warps appears to contribute (note: in the very centre the star is no longer sky noise dominated so the quantisation is no longer seen - I have set this area to be white).