Re: CCD Charge clearring, is it required?

[Tom Droege <droege@wwa.com>]

Reading out the CCD clears it, so the clear operation is 
something you only do at the start of the evenings runs,
or if you have been off for a while for some reason.  The 
longer you are off, the more charge accumulates.  When
things get really saturated, then you have to clear several
times.  The charge ends up in funny places on the chip and
it takes a while to migrate to where it can be cleared.  At
least this is what I have found experimentally.  Experts 
can comment.

This is one great advantage of the drift scan operation
over picture mode.  After a few drift scans, the charge 
conditions settle down, and the constant scanning at
constant intervals means you get very repeatable line
to line results.  I give up drift scanning on the Mark IVs
with reluctance.

My view is that in picture mode, you want to take pictures
at very regular intervals if you want comparable results.

Tom Droege



At 11:11 PM 1/27/98 -0800, you wrote:
>I'll raise this issue as a possible concern.  I don't know
>if it is really a problem.  What do the CCD experts think?
>
>Seems to me 40 seconds is a long time for a clear operation.
>I've read a few places that you want to do a clear a few times
>before each exposure.  (Is this true?)  If so This adds 80 to 
>120 seconds of unproductive dead time to the time between 
>exposures.  Maybe we can just take back to back exposures and
>not clear?
>
>I am thinking the a lot of that 40 seconds is taken up by the ADC
>delay.  I think we could skip this.  At least the Mk III driver
>does not bother to perform a conversion while clearing.  Norman's
>Mk III code drives the CCD at maximum rate for about three frames
>and never touches the ADC's "start conversion" pin.
>
>I'd like to clear and open the shutter very quickly so as to
>minimize the dark current.  With a 40 second time to clear,
>would a short integration period dark frame contain a "dark
>gradient"?
>
>
>
>Tom Droege wrote:
>
>> 
>> The only way I can see to clear the CCD is to read it out.  You
>> probably have to do it several times at the start.  This seems to
>> be a common problem for CCDs.  I hear grumbling from the Sloan
>> worker bees about this.  Each read out takes 40 seconds.  You can
>> determine when it is done by polling on the memory done bit.  Or
>> later an interrupt.  If it works.
>> 
>> One does a read out by sending a message to the stamp that says
>> "read out the CCD".  All the stamp does, is send a single pulse
>> to the read out circuitry.  It is off and running for the next
>> 40 seconds.
>
>-- 
>   --Chris Albertson             home: chrisa@wavenet.com           
>     Redondo Beach, California   work: chris@topdog.logicon.com
>
>
>