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Overheads

Overheads

The nominal GPI acquisition time is 15 minutes. This time is expected to apply to coronagraphic modes when the target is a bright (V <9), single star, and should be added into your overall time estimate. Non-coronagraphic modes i.e. direct imaging and NRM has an acquisition time of 10 minutes. Fainter targets, or targets in very crowded fields requiring the use of finder charts, may require longer acquisitions.  Please note that the field of view is very small; so accurate positions and proper motions will be essential for efficient acquisitions. 

The following will be included as baseline calibrations (for baseline telluric standard frequency, see the IR Calibrations page). You do not need to account for these in your total requested time:

  • Telluric cancellation standards (taken as calibrations every month)
  • Polarization standards (taken as calibrations every month)
  • Astrometric fields (taken as calibrations every time instrument mounted on the telescope)

The following are NOT part of the baseline calibrations: 

  • PSF standards
  • Any type of standard taken in conjunction with the science observation 

OT Time-line and time calculation

With the June 2015 OT release the timeline is accurate to within a few percent as detector overheads and half-wave plate changes are now correctly estimated. 

A minor correction is that the OT will assume a standard acquisition time of 15m independent of the mode, so in the case of Direct imaging and NRM observations the acquisition is overestimated by 5 minutes.  

Acquisition Overheads Breakdown

The overheads for GPI acquisitions can be broken down into:

 Step

Events 

Time in Coronographic Modes 

Time with non-coronographic modes 

1

Slew of the telescope and dome

5 minutes

5 minutes

2

Arc exposure for flexure compensation

2 minutes

2 minutes

3

Internal calibrations (AOWFS lenslet to AOWFS detector mapping, Focal Plane mask (FPM) calibration)

5 minutes

2 minutes

4

Closing of AO loops (one or two loops at the moment) 

1 minute

1 minute

5

Convergence of loop driving star on FPM (LOWFS/CAL) 

2 minutes

N/A

Total

 

15 minutes

10 minutes