[KIDS] Improved background subtraction to remove CCD edge vignetting

[Nicola R. Napolitano]

Hi Hugo and all,

the corrected images look very good, nice job.
We had a quick check to the image that looked worse than the others with 
clear severe features for CCD vignetting, i.e. KIDS_175.0_-0.5i from the 
wiki page.
In attachment the catalogs before and after the correction. Red are all 
sources, green the "good" ones.
The situation is indeed improved and also if there remains some slight 
horizontal features in the coadd in the area of CCD82, the catalogs are 
note significantly affected. Of course we want to look in more details 
at the extracted sources in that area.

All other images you sent in your examples with residual CCD gaps did 
not actually present spurious detection in catalogs, thus we expect the 
improvement here is on the photometry measurement uncertainty.

Cheers,

Nicola



On 1/9/13 9:56 AM, Hugo Buddelmeijer wrote:
> 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
> _______________________________________________
> KiDS mailing list
> KiDS at astro-wise.org
> http://listman.astro-wise.org/mailman/listinfo/kids


-- 
Nicola R. Napolitano
Research Astronomer
INAF - Observatory of Capodimonte
Salita Moiariello, 16, 80131, Napoli (Italy)
phone: +39 081 5575509; fax: +39 081 456710
email: napolita at na.astro.it
web page: http://www.na.astro.it/~napolita

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