2013-08-28

2013-08-27

Summary of previous discussion of this issue ==

2013-05-13 ​Confluence page

2013-05-20 ​Non-zero SKY on stacks

2013-06-03 ​Update to non-zero sky issue

2013-06-17 ​More on SKY residuals on stacks

Initial checks

The initial thought for this issue is that the stack sky levels are incorrect either due to an error in the sky statistics, or due to an error in the fitsio data storage. To investigate this, I used the set of warps for a very deep stack (~450 inputs) and pulled statistics out of logs and compared them with direct measurements.

1. Internal background code check. Manually run psphot to fit the background level, and confirm that this matches the value recorded in the stack log.

2. External background code check. Compare the psphot background levels to a Gaussian fit to the histogram of image values.

3. Database check. The GPC1 database contains a background value recorded for the warps. I do not have a good explanation as to why these match so badly.

4. BSCALE check. If the fitsio routines are to blame, it's likely an issue with the quantization. There's a correlation here, but it is more likely due to the fact that the background level is correlated with the distribution width (bg_sigma), as is BSCALE.

The initial thought for this issue is that the stack sky levels are incorrect either due to an error in the sky statistics, or due to an error in the fitsio data storage.