= 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)

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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%

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 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%

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 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%

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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