[KIDS] Improved background subtraction to remove CCD edge vignetting

[Hugo Buddelmeijer]

Dear KiDS members,

An improved background subtraction algorithm for RegriddedFrames was 
created to remove the vignetting effect at the edges of the CCDs. We 
(Groningen) plan to use the method for KIDS ESO DR1.

* Example *
See this example CoaddedRegriddedFrame of KIDS_184.0_-0.5 in r:
  before: http://tinyurl.com/arce9oj
  after:  http://tinyurl.com/aso2q5k
And in the i-band, KIDS_131.0_-1.5:
  before: http://tinyurl.com/b33pu47
  after:  http://tinyurl.com/a3muamz

* Procedure *
The method is selected by choosing BACKGROUND_SUBTRACTION_TYPE 4 for a 
RegriddedFrame. (Only in 'current', not yet in 'AWBASE'.) This performs 
the following:
1) Create a mask to flag all sources and bad pixels.
2) Calculate the median value of the background pixels of each row.
3) Subtract this median value from the row.
4) Let Swarp remove the rest of the background (as usual).

* KIDS ESO DR1 *
Based on our experiments the Groningen team suggests to use the new 
method for the KIDS ESO DR1 release. We plan to start processing next 
week so there will be time to reprocess bad cases, if they arise. Do you 
agree?

* Caveats *
There are currently no known cases where the quality of the 
CoaddedRegriddedFrames decreases. Chip-filling galaxies will probably be 
a problem, but that was already the case with the original method. The 
vertical bar pattern seen occasionally at the corner CCDs is not treated.

* Future *
The 'bar pattern' arises because the vignetting of the background is 
slightly different in the science images than in the calibration flats. 
It might therefore be possible to improve the flatfielding to remove the 
bar already during the creation of ReducedScienceFrames. This requires 
more investigation and will not be achieved in time for ESO DR1.

* Sources *
The flux of the sources is affected differently by the vignetting than 
the background. Correcting the source flux requires a separate solution 
and is therefore beyond the scope of this discussion. (There can be a 
flux error of about 1-2% in the affected regions of the individual 
exposures, leading to a potential flux error of 0.2-0.4% in the coadds.)

* More Information *
The wiki contains more information about the bars in general and 
examples of the improved background subtraction:
   http://wiki.astro-wise.org/projects:kids:tbarvignetting
In particular in the section "Improved background subtraction".

Greetings,
Hugo Buddelmeijer