Re: CCD Charge clearing, is it required?

[Chris Albertson <chrisa@wavenet.com>]

Thanks for the comments.  I think I know a bit more then I
did a few days ago.

It sounds like charge clearing is required if the CCD has been
sitting around for a while.  How long is "a while" and does it
need to be cleared once or several times.

I also hear that CCDs can be cleared quickly with some binning 
tricks.

Here is what we can do to perform the "expose" command:


Option 1, the Mk IV has no erase mode:

 0) When the memory board sends the interrupt, signaling that
 the CCD has been read out we start a timer running.  
 It counts in seconds.

 1) When we get an expose command we check if the shutter is open.
 if open we send an error back and quit.

 2) So now we know the shutter is closed.  First check the timer.
 if the timer has a large value or if it is not running then
 we do a read out operation and clear the timer.  This takes 40 sec.

 3) open the shutter. start an exposure timer.

 4) when exposure timer expires we close shutter and send the
 pulse to signal a CCD readout.

 5) When memory board is full (interrupt happens) do step zero 
 and read out the memory to a disk file.

 This will let us do back to back exposures but will force a
 clear operation if there has been some time between exposures.

Option 2, there is an erase mode.

 This is easy.  We just do an erase every time before opening
 the shutter.  We need not care how long it has been sense the
 last time we took an exposure.

The question I have is will bright stars leave their imprint on
the CCD from one frame to the next?  I think it may be a matter
of degree.  I remember the trail effect on the Mk III.  Saturated
stars leave trails.  Apparently some charge gets left behind
when you row shift if the star is bright enough. 
-- 
   --Chris Albertson             home: chrisa@wavenet.com           
     Redondo Beach, California   work: chris@topdog.logicon.com