[Comm2011] archival OmegaCAM detector data?

[John P. McFarland]

Hi Dietrich,

Here is a histogram of the maximum pixel values of ESO_CCD_#66, the one the
was replaced that had two hot columns.  All maximum values are above 60000
ADU and 25 of them are at the maximum possible of the data: 65535 ADU.
These are taken from 1345 ILT images of all types.  I can look at all the
other CCDs to see if they are similar.  Hopefully sometime this 
afternoon we
will have a large amount of the commissioning data thus far ingested into
our system and be able to do a similar investigation on that data from here.

Cheers,


-=John


On Mon, 28 Mar 2011, Dietrich Baade wrote:

> Hi Gijs, John,
> 
> we have run into some apparent problem with the OmegaCAM science mosaic, and
> Konrad amd Ewout suggested that I contact you.
> 
> What we see
> Bright stars saturate well below 2^16-1 ADU.  Often, this happens at 57,000
> ADUs; only the brightest stars reach 62,500 ADU.  The cores of such images
> have a flat plateau.  This is true for all CCDs (quite a few) in the mosaic
> that I've checked. 
> 
> What else we know
> The CCD test reports from e2v show full-well capacities of between 220,000
> and 340,000 e-/pixel.  You have previously determined gains of 2.2-2.4
> e-/ADU.  A quick on-line estimate here suggests ~2, which I consider
> consistent.  A gain of 2.3 e- would imply 150,000 e- @ 65,535 ADUs, well
> below physical full well.  FORS2 CCDs from the same batch have a gain of 2.4
> and don't run into physical saturation before the limit of the A/D converter
> is reached.
> 
> What we suspect
> The numbers for OmegaCAM do not match.  Since the entire science mosaic
> seems affected, the problem must be basic.  It could be a wrong gain setting
> by S/W or H/W or similar.  Olaf had no time yet to look into this. 
> 
> What could help us
> It would be great if you could search your archive for images containing
> saturated pixels with a value of 65535 (=2^16-1).  Don't send us such
> images.  It will be sufficient if you could provide a brief description. 
> Ideal would it be if you found a CCD with a (partial) hot column.  In that
> case, give as us the location or name of the chip and the column number. 
> The faulty CCD that was replaced in 2009 might also have shown such
> saturated columns. 
> 
> If you do not find anything quickly, suspend your work and let us know how
> you've proceeded.  We would, then, tell you whether it makes sense to
> continue.  
> 
> Many thanks in advance for your help. 
> 
> Best regards,
> 
> Dietrich. 
> 
> -- 
> Dietrich Baade 
> ESO - European Organisation for Astronomical Research in the Southern Hemisp
> here
> Karl-Schwarzschild-Strasse 2,  85748 Garching bei Muenchen,  Germany 
> Email: dbaade at eso.org    Tel: +49 89 3200-6388    Fax: +49 89 3202362 
> 
>
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