Do we need a faster readout?

["Albertson, Chris" <CAlbertson@primeadvantage.com>]

All this talk about reading images out of the RAM card and
saving to disk really is looking to save time where it does
not matter much.  What we should be concerned with is the
shutter open/closed ratio.  That's all that matters.  Read
out speed matters only in so far as it effects the above ratio.

The way I look at it, the image moves through an assembly line.
While it takes Ford 200 man hours to assemble a car they can
produce more then one car every 200 hours.  We can do the same
thing.  My real-time controller attempts to use Ford's method.
The software moves an image through a pipeline:

1) Shutter opens, Image in on the CCD array,
2) Shutter closes, Image is moved from CCD to RAM Buffer Card,
3) At any later time, Move image from RAM Buffer Card to internal 
   arrays, one for each color
4) Move image arrays to the current list of "output locations"
   these may be any or all of the following:
   a) FITS file
   b) A soft copy display device (i.e. some window on 
      some computer's screen)
   c) A (possibly reduced size) tiff, gif, or jpg file.
   d) Some program (like a data reduction pipeline).

It tracks which location contains an image or is empty.  For
example I can't open the shutter if the CCD still contains
an not yet read out image.  I can't read out the CCD if the
RAM Buffer Card has not yet been read into the internal
array.  I can't read out the RAM card if the internal array
has not been saved.  So in the worst case we have to delay
the shutter opening a few seconds while the pipeline clears.

If the exposure times are reasonable we have as I said
"all the time in the world" as all this gets done when the
shutter is open.  A 100 second exposure gives us 100 seconds
to mess with the last image.  Even QBASIC is fast enough.
If you are taking 60 second exposures I really don't see
why the readout and save to disk operation needs to be fast.

The only bottle neck in the system is the CCD readout time.
For a survey you'd like to have the shutter open 100% of the time.
Increasing the shutter open/closed ratio is as good as
increasing the lens' f-ratio.
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