Re: Saturation Improvements

[Tom Droege <tdroege@veriomail.com>]

The thing that I have learned in going to CCD-World is that some of the 
saturated charge can be lost.  There seem to be two full wells, Surface 
Full Well (SFW) and Bloomed Full Well (BFW).  In SFW, some of the charge 
can be lost due to recombination.  This is apparently what is happening 
when saturated stars at the top and bottom of the frame have different peak 
values.  I would be very cautious about using saturated measurements.

This is something we can investigate in the future (or you can do it now - 
Disk 15 will do it).  By using a short and long exposure sequence, one 
measure stars on the long exposure that are saturated and compare them to 
the same stars on the short exposure where they are not.   Seems like a 
good thing to look at on Disk 15.  If someone does this, I will provide a 
similar measurement after the VVH adjustment to see if there is any 
difference.  In fact it would be interesting to compare Disk 15 with some 
new data taken at several different settings for VVH.

Those of you that are always asking for a project, here is 
one.  Hmmmm!  Not sure I have a copy of disk 15, so you may have to get it 
from someone else.  I think I gave away the copy I had kept for the archive.

Tom Droege

At 08:43 PM 10/2/00 -0700, you wrote:
>Herbert R Johnson wrote:
>
> >
> > 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.
>
>It's easy to know if a star is saturated.  You don't need to know
>much about the camera either.  If you think of an image as a three
>dimensional surface with stars as bumps the shape of the bump tells
>you if the star is saturated.  Normal stars have rounded tops while
>saturated stars will have the tops clipped off making them look like
>a plateau.  A histogram will show you where the system saturates too.
>
>Yes you can do photometry on saturated stars.  You look at the shape
>and size of the plateau and fit a curve to the bottom part of the
>plateau then look to see where the peak would have been if it had
>not been truncated.  Likely a lot of messy details involved.
>--
>    Chris Albertson             home: chris@albertson-home.net
>    Redondo Beach, California   work: calbertson@primeadvantage.com