Define ccd exposure and h4 ramp time sequence

CCD exposures have two fixed-time parts: the pre-integration wipe and the post-integration readout. And of course a variable integration time. H4 ramps come in integral numbers of fixed-time reads with a few constraints; some known, some not.

The SPS sequencer needs to know how to schedule the two together.

• ramps will always start with one reset frame. Unlikely to be more.
• it is not impossible that the first read for some cameras will not be useable.
• read time is nominally 5.57s, but with the interleaved reference pixels ("IRP") it will be up to double that. 5.57 + (5.57/n) for n probably 2-4. Maybe 1.
• single reads are essentially meaningless. The minimal useful ramp contains two complete reads – i.e. not overlapping with shutter moves, etc.
• to evaluate persistence, we will take reads after the shutter is closed, while the CCDs are reading out. It is possible that we will also want to take two reads before the shutter is opened.
• The 5.57s units will certainly constrain the short calibration sequences...

Arnaud and I talked through much of this, and I promised to pin it down a bit better.