[KIDS] Crosstalk on CCD96 exceeds bias level

[Konrad Kuijken]

Indeed, no option now but to mask the pixels where the information is lost. I did not see this behaviour in the commissioning data I looked at last year, the signal remained above zero.

Regarding the effect of changing the bias level: indeed, a higher bias level reduces the level of saturation.
But I do not believe bias level gives Poisson noise, it is purely a dial in the video chain I think, designed to hit the most linear part of the amplifier response. 
It is not clear to me that we have much freedom in setting the bias level - all CCDs talk to each other and I do not think Olaf can control them individually.

How are you doing the correction? Previously I showed that also unsaturated pixels can cause electronic ghosts (see commissioning reports).

K

On Apr 19, 2012, at 8:42, Massimo Dall'Ora wrote:

> Hi Ewout,
> 
> interesting plot....
> 
>> From the stellar photometry point of view (that is my point of view), to 
> increase the bias level has no scientific effects, apart of to slightly reduce 
> the dynamical range, and to increase the noise of the measurements at high 
> magnitudes.
> 
> Probably some issue could arise for the extended sources, since an increase of 
> the bias level of ~600 counts introduces an additional noise of  ~25 counts, 
> that could mask faint features.
> 
> Regards
> massimo
> 
> On Wednesday 18 April 2012 12:22:30 Ewout M. Helmich wrote:
>> Hi everyone,
>> 
>> We're investigating correcting the crosstalk of OmegaCAM. One issue that
>> may require action on Paranal is that the crosstalk on CCD#96 (upto -600
>> ADU from CCD#95) exceeds the bias level of ~400 ADU in CCD#96. See the
>> attached plot. Pixels exceeding ~50000ADU in CCD#95 will put that pixel
>> to 0 in CCD#96, destroying the information there (this in the absense of
>> sky background, the background in the attached example is about 100 ADU,
>> in g'). I see two ways to address this:
>> 
>> 1) We can mask pixels with value 0 in CCD #96 (will have to for existing
>> data).
>> 2) The bias level of CCD #96 can be increased by e.g. 300-600 ADU.
>> 
>> The question is if there are objections from a scientific point of view
>> to this bias level increase.
>> 
>> Regards,
>> Ewout
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