Re: Long Term Drift

[Stupendous Man <richmond@a188-l009.rit.edu>]

  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