[Comm2011] archival OmegaCAM detector data?

[Gijs Verdoes Kleijn]

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 Hemisphere
Karl-Schwarzschild-Strasse 2,  85748 Garching bei Muenchen,  Germany 
Email: dbaade at eso.org    Tel: +49 89 3200-6388    Fax: +49 89 3202362