[KIDS] Crosstalk on CCD96 exceeds bias level

[Ewout M. Helmich]

Oh one more thing: the reason that you didn't see this effect during 
commissioning is because the crosstalk has increased. It jumped after 
the broken cable for CCD 93 was fixed. And it appears there was a 
(smaller) jump around new year as well.

See:

http://wiki.astro-wise.org/projects:kids:crosstalk#crosstalk_trend

Regards,
Ewout

On 04/19/2012 11:27 AM, Ewout M. Helmich wrote:
> Hi Koen,
>
> Well, what's what with the crosstalk correction I've put on our wiki:
>
> http://wiki.astro-wise.org/projects:kids:crosstalk
>
> The plan is to implement essentially what you did during commissioning.
> I have functional code (see results on the wiki) but not yet a proper
> way to store what was done in the database, which is what I'm working on.
>
> It became clear during yesterdays KIDS telecon that masking will result
> in gaps/holes in the coverage, because the masked pixels are at the same
> RA,DEC position for all exposures of a dither.
>
> Regards,
> Ewout
>
> On 04/19/2012 11:15 AM, Konrad Kuijken wrote:
>> 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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