Re: Long Term Drift

[Tom Droege <tdroege2@earthlink.net>]

Michael and all,

I think you misunderstood my point.  My assumption is that the CCD is being 
kept at constant temperature, but the ambient temperature that the 
electronics sees is changing.  This results in a drift because the 
temperature of the resistors, capacitors, etc., is changing.  Because the 
whole telescope and all it's electronics is sitting out in the open, this 
will be quite large from summer to winter.

There is actually a nice way to check this with the Mark IV.  One can set 
all the clock DACs to the same value.  This results in "null" operation of 
the CCD.  No charges are moved.  The temperature of the CCD does not affect 
what happens then.  What is left is the drift of the amplifiers and the like.

Actually, big telescopes do pretty awful things when it comes to 
controlling the temperature of the CCD.  The systems I have seen use a 
heater because things do not work very well at LN2 temperature.  So they 
heat the CCD up a little.  The schemes I have seen were not very well 
designed, but perhaps some do it with care.

I think I was really asking the question "Do the professionals separate out 
electronic(temperature) drift from CCD(temperature) drift and how do they 
do it?"  Note that we have a sensor for each because they are different 
effects.

Michael makes a good point about the difference in use between the covered 
(dark current sensing) and the overscan (electronics drift sensing) 
pixels.  So perhaps this answers my question above.  I had not thought of 
this difference before so it was worth running on in public.

Note that this is one reason for using dual slope integration.  I saw this 
used in Jim Gunn's electronics and shamelessly copied it's use.  (Never 
mind, it is a well known scheme in the biz, I just recognized why JG was 
using it.)  The dual slope scheme removes (mostly) capacitor temperature 
coefficient.  This is one of the harder parts to make stable.

Tom Droege

At 03:39 PM 1/26/02 -0500, you wrote:

>   Tom wrote:
>
> > Those of you that are offended that the ADC drifts around with temperature
> > should consider  ...
>      [various effects]
> > ....  There are a dozen or so
> > parts with similar effects.  It is thus not surprising that we observe 6
> > counts of ADC shift per C.  While I can think of ways to greatly reduce
> > this drift, I doubt that they are used by designers at the big
> > observatories.  I suspect that they just calibrate the shift with dark and
> > bias frames - what say experts?
>
>   Big telescopes usually have cameras with dewars cooled by liquid
>nitrogen.  I suspect that a clever design can keep the CCD temperature
>pretty darn constant over the course of a night by tying it directly
>to evaporating LN2.
>
> > I suspect that with a little effort, one can predict the bias value from
> > the VCO temperature and the CCD temperature.  This can then be compared
> > with the covered pixels as a check.
>
>   I think that a lot of astronomers do the opposite, in a sense:
>they use prescan/postscan pixels to track changes in the bias,
>and covered pixels to track changes in the dark.  Some of them probably
>notice that there are good correlations between these changes and
>temperatures reported in the FITS headers :-)
>
>                                                  Michael