Goals for observing targets off center of track and Priority 4 targets

Rules of thumb:

• 5 min before/after Topt is always fine (<2% difference in pupil)
• 10 min before/after Topt is fine if the only other choice is P4 (<6% pupil loss)
• If >10 min before/after Topt, exposures should be extended (LRS2 only)
• Observe P4 targets only if:
  • all requested/anticipated standard/telluric stars have been obtained
  • all requested engineering tests are finished (nightly high-speed CWFS)
  • they do not interfere with observing a non-P4 target
  • Note that P4 can be observed at non-optimal times (even 15-20min before/after Topt)

Data to support these before/after time ranges: consider a 1200s exposure taken at three declinations:

At Dec=-10deg, optimal timing gives 59% pupil illumination.

Starting up to 5min early/late still gives 59% illumination.

At +/-6 min, pupil is at 58%

At +/-9 min, pupil is at 57%

At Dec=+15deg, optimal timing gives 92% pupil illumination

Starting up to 4min early/late gives 91% illumination

At +/-6 min, pupil is at 89%

At +/-9 min, pupil is at 86%

At Dec=+45deg, optimal timing gives 94% pupil illumination

Starting up to 3min early/late still gives 94% illumination

At +/- 6min, pupil is at 92%

At +/- 9min, pupil is at 90%

The off-center losses get worse for longer exposure targets, and will require the RA’s judgment. For example if there is a P1 target requesting a 2400s exposure, try to keep it as well-centered as possible. If a P3 target requests 2400s, it may have to be done off-optimal or else it may never be observed.

SJ, 2 Apr 2020