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Overheads

For Phase I observing proposals, you can use the telescope setup times given below, plus 2 min per exposure to cover readout, GRACES cassette motion, etc. 

For more exact calculation of the overheads where these are critical to your observing program (for example time-resolved observations), the detailed listings on this page may be used. The detailed overhead information is also useful for the Phase II planning of your observing program. 

 

Telescope setup (acquisition) time

The setup times include slewing to a new target, starting guiding, and accurately centering objects on the GRACES pick-off mirror. Normal GMOS imaging setup time is 6 min. If we add the time necessary to move the GRACES cassette in the beam (70 sec), and the time required by the observer to measure the position of the target on the GMOS-N CCD, it comes to a total average setup time of 10 min. This value will be adjusted once we get a real average statistics on GRACES acquisition time.

Because of ESPaDOnS' detector esthetic and cosmic ray contamination, the maximum single exposure time is 1 h. GRACES' cassette flexions are forcast to be small. But before we know how GRACES will behave in real life, we ask PIs to allow for one re-acquisition for approximately every hour observed (including overheads and calibrations). We estimate each re-acquisition will take about 5 min. Shorter observations may also be split in order to accommodate them in the queue planning. 

Please note that if acquiring particularly faint targets requiring long exposure times (> 2 min) the actual acquisition times will be longer. PIs should take this into account when filling out their Phase I proposals and Phase II programs. 

 

Readout time

Below are listed the readout times, including the overhead added by the sequence executor software, which is used to execute sequences. It is unfortunately not possible to dither the telescope position or begin setup on the next target during readout. 

 Readout mode   Readout time 
 Fast  31.5 s
 Normal  35.5 s
 Slow  60.0 s

 

Telescope offsetting time

The time to offset the telescope as part of a dither sequence is currently approximately 10s (from the time of turning off guiding at one position to guiding at the next position). At present it is not possible to offset the telescope while reading out the detector when these operations are part of a sequence. Please note that offsets greater than 20 arcsec are less accurate than 0.1 arsec, and if they are done blindly (i.e. without re-acquistion), the accuracy on the GRACES' fiber centering on the target cannot be guaranteed to be at an appropriate level.