Re: Saturation Improvements

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Fri, 29 Sep 2000 18:44:48 -0500, Tom Droege <tdroege@veriomail.com> wrote:
*>A plea to CCD-World brought some advice on how to fix the problem with 
*>saturated stars.  As you may recall, a saturated star at the top of the 
*>frame has a larger signal than one at the bottom.  A first experiment 
*>showed that it was only saturation that was changing.  Signals below full 
*>well were the same at the top and the bottom of the frame.  Advice from Jim 
*>Janesick showed how to adjust the VVH level.  The procedure is to look at 
*>blooming and adjust VVH for the maximum signal.  I did this last night by 
*>looking at a saturated star near the bottom of the frame with various VVH 
*>levels.  4.1  (old value was 6) volts proved to be best.  With this level, 
*>the saturated star level is close to the same top and bottom, and the full 
*>well signal is somewhat larger.  Looks like another bug has been beaten to 
*>a pulp.

Not quite, Tom. For future reference, how do you determine when a star
(a set of pixels) are "saturated"? This is not a silly question. As, over
the course of time, you've adjusted the operation of the Mark IV, the
value of "saturation" in terms of ADU counts has almost certainly changed.
So I expect it would change in the future. COnsequently, anyone reviewing
old data or new will need to know your definition of saturation - to determine
it themselves on older images.

You may recall a dialog in this maillist some time ago, when there was
consideration of photometric evaluation of saturated stars. SOme members
felt that despite saturation they could make an accurate (or at least
consistent) measurement under those conditions. I'm NOT opening up that
subject for debate. However, even if members wish to avoid consideration
of "saturated" stars, they will need a consistent way to determine when
that occurs. As the hardware will likely be tweaked at each site or
in the course of your rebuilding, members will need to make this determination
based on what they can see in whatever images they obtain. Or, if a
camera owner is adjusting their camera, they also will need the same
criteria. Finally, if/when there are nonlinear effects near saturation,
again a working definition will be necessary.

If you have already defined saturation in a Tech Note, you may wish to
simply refer us to that Note. Given your most recent work, if there
is such a reference in a TN you might want to update it accordingly.

Believe me, this kind of information is MUCH HARDER to remember than
to record, so I hope you record it well. I've had some bad experience
in trying to determine these kinds of operating parameters after-the-fact,
so that is why I've gone to some lengths to ask you to nail down your
working definition of saturation.

Herb Johnson

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