[Comm2011] archival OmegaCAM detector data?

[Gijs Verdoes Kleijn]

Hi John, Gies,

Many thanks for your very fast response - very helpful!

Meanwhile, we've better news from here:  An exposure has been made through
the mask with the CCD names.  The saturation on all CCDs checked is at 
65,535.

However, the exposure was made from a console in the dome and so used a
different instant of the S/W.  We've yet to understand what causes the
differences (another one is that the auxiliary CCDs made the S/W in the 
VLTI
control room crash while from the dome they are working OK).  But the 
solution
shouldn't require a soldering iron.

Many thanks again and best regards,

Dietrich.

________________________________________
Von: John P. McFarland [mcfarland at astro.rug.nl]
Gesendet: Montag, 28. März 2011 13:26
Bis: Dietrich Baade
Cc: verdoes at astro.rug.nl; Ewout Helmich; Konrad Kuijken; Olaf Iwert; 
Christoph Geimer; Ueli Weilenmann
Betreff: Re: archival OmegaCAM detector data?

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
>
>