Implement minimum exposure time constraint about desisurvey

[djschlegel]

The baseline design given to the focal plane systems was not to reconfigure more often than every 800 sec, so that's what we have designed to. The bright time survey isn't allowed to drive these requirements.

That said, we could reasonably expect to get a sign-off from Joe Silber to do this as frequently as every 400 sec. We could use that faster number, with the caveat that it's not guaranteed. Here's what Joe said today: "We nominally designed to 800s cycle time. At this point 400 sec is about as fast as I expect we'd want you to go. We'll be looking for power savings as we move forward (for example we believe we can run positioners about 20% lower power), but there are enough variables that I wouldn't guarantee any better than 400 sec, since there are other possible heat sources to resolve, such as if we need more power at the GFAs."