Re: Curious Result

[Tom Droege <tdroege2@earthlink.net>]

Andrew and all,

I have finished the first round of investigation of this.  As you say, once 
you make VVH large enough it appears to make very little difference.

Since there are clouds, this was done looking at a light box field at about 
40% and 80% of full scale.  As near as I can tell, there is no difference 
in the gain at the top and the bottom of the frame as long as one is 
somewhat back from saturation.  There is a real problem doing this at 
saturation since there is all that extra charge and it goes 
someplace.  Once you get close to saturation, the data is trash.  With 
stars, you can look at points at various positions on the image without 
flooding the CCD with charge.  This allows saturated measurements to be made.

I have log book data that says that there is a big difference in the top 
and bottom saturation level.  That is you can have a different saturation 
level at the top and bottom of the frame.  By fussing with VVH one can get 
the saturation level at the bottom of the frame to be nearly the same as 
the saturation level at the top of the frame.  From my data, it is always 
higher at the top of the frame.  I have been running with a relatively low 
VHH (3.9 instead of mfgr's recommended 6) as determined by this experiment.

It should be noted that the 80,000 full well (32000 count) linearity 
specification for MPP mode is achieved at any possible VVH.  My log says 
that anything over 3 volts will do it.  We have been setting the saturation 
limit at 20,000 counts.  Because of the 16 bit data used, for practical 
purposes we can rarely get over 30,000 count signals.  All these things 
point to this not being the problem with the noise floor.

This earlier measurement was done after some advice from the experts at 
CCD_World.  Remember the full well limit for this device is something like 
200,000 e- in BCM where it is only 80,000 e- in MPP mode.  We go to MPP 
mode to get the lower leakage and thus much less noise for us.  We trade 
off dynamic range.  This is not a bad trade for us since the bright stars 
are less interesting since they are well studied.

Now there is this curious effect that there are more stars found at the top 
of the frame than at the bottom.  There is also the observed effect that 
the brighter stars hit a noise floor.  So I am wondering whether the two 
things are related.  The coincidence is too strong, and I think it will 
have to be investigated.

The plan is to take sets of images at different VHH levels.  Then I will 
plot mag sigma vs mag for each image set and see if the noise floor is 
related to VHH.  Sigh!  There is not a clear day in sight.

Tom Droege

At 11:52 PM 3/21/03 +0000, you wrote:
>On Thu, 20 Mar 2003 07:22:11 +0000, Tom wrote:
>
> >Those of you worrying about this sort of thing (Andrew, Michael) might
> >consider whether one can apply a top to bottom gain fit that would make a
> >correction.
>
>Yes.
>
>Unless voltages are so badly off that charge is leaking
>all over the place. It has never been that bad in the
>past ...
>
>I seem to remember that you played with this before and
>so long as the voltages were not grossly wrong it made
>damn all difference? Apparent saturation levels changed
>a bit but I was never convinced that you were actually
>changing the charge transfer efficiency appreciably. Yes -
>the gradients are real. But they calibrate out nicely.
>
>Andrew Bennett, Avondale Vineyard