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For the Phase I observing proposals, that are not using Nod-and-Shuffle and do not require very detailed timing, you can use the telescope setup times given below, plus 2 min per exposure to cover readout, filter changes etc. For Nod-and-Shuffle additional overheads apply. These are described below. These overheads should be added to the overheads from the telescope setup and the readout etc., whenever Nod-and-Shuffle is used.

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. The various overheads can be broken into the following categories:

Telescope setup (acquisition) time

The setup times below include slewing to a new target, starting guiding, and accurately centering objects on slits (if appropriate).

GMOS modeSetup time
Imaging6 min
Long-slit16 min
IFU18 min
MOS18 min

Applicants for GMOS time should allow for one acquisition for approximately every 3 hours observed (including overheads and calibrations). Shorter observations may also be split in order to accommodate them in the queue planning. 

The acquisition times listed above are based on average statistics from the last two years of GMOS observations.  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.

GMOS configuration times

Below are listed approximate configuration times for various components within GMOS (the exact times depend on the details of the positions between which a particular component is being moved). It is possible (and usual) to reconfigure GMOS while slewing to a new target, but it is not currently possible to reconfigure GMOS while reading out the detector. This is a limitation of the current software that is used to execute sequences.

GMOS componentConfig Time
Filter change 20 sec
Grating change 90 sec
Mask move in or out 60 sec

Readout times

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 reconfigure GMOS, dither the telescope position or begin setup on the next target during readout. The difference in overhead between GMOS-N and GMOS-S is due to the number of amplifiers normally used during read out.

ROI*BinningRead speedReadout Time (s)
GMOS-N (6amp)GMOS-S (3amp)
Full frame1x1slow76130
Full frame1x1fast3671
Full frame1x2slow4672
Full frame2x1slow4980
Full frame2x2slow3155
Full frame2x2fast2035
   Central spectrum**1x1slow2546
Central spectrum1x2slow1831
Central spectrum2x1slow1833
Central spectrum2x2slow1424
Central spectrum4x4slow1119
Central spectrum4x4fast1118

* For more details on Regions of Interest (ROIs) see the GMOS OT component page.
** The "Central spectrum" ROI corresponds to the central 1024 (unbinned) rows of the detector array in the spatial direction and the full width of the array in the spectral direction.

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.

Configuration for Calibrations

A portion of the overhead for taking calibrations is the time it takes to move the science fold mirror, which sends the beam either from the sky or from GCAL into GMOS. Again, when using the sequence executor to run sequences, this move cannot be done while reading out the detector array.

With GMOS on side port 5, the relevant science fold moves take about 20 seconds each. Therefore the total overhead to move to and from the calibration position is about 40s. (This does not include the time to actually take the calibrations).

Additional Observing Overheads with Nod & Shuffle

When observing with Nod & Shuffle the same GMOS observing overheads as for classical long-slit and MOS spectroscopy target acquisition and detector readout are still applicable. However there are additional Nod & Shuffle overheads which are substantial and must be considered in the Phase I observing proposal. The "rule of thumb" given below can be used for a large fraction of Nod & Shuffle programs. Details are also given, in case the "rule of thumb" does not apply to your program.


  • Rule of thumb:If you are nodding along the slit and use nods of less than 10arcsec but greater than 2arcsec, the approximate time required to get 1800sec open shutter time (A-position plus B-position) is 2200sec, plus time for acquisition and readout. For nods with a total distance of 2arcsec or less one can elect to use electronic offsetting of the OIWFS, in which case this time is reduced to 2000 sec.

  • GMOS Nod & Shuffle overheads are defined as the time per cycle that the shutter is closed. This excludes readout and target acquisition. This also assumes nodding along the slit, meaning all open-shutter time is spent collecting data on the target.


    • Example 1: Exposure time A = 60sec, Overhead = 24sec, nodding along the slit.
      Effective overhead (time spent not observing target) per cycle is 24sec, or 24/(60+60) = 20%

    • Example 2: Exposure time A = 30sec, Overhead = 24sec, nodding along the slit.
      Effective overhead (time spent not observing target) per cycle is 24sec, or 24/(30+30) = 40%

    • Example 3: Exposure time A = 60sec, Overhead = 24sec, nodding off to sky.
      Effective overhead (time spent not observing target) per cycle is 84sec, or (24+60)/60 = 140%


  • Additional overheads above the 24 sec depend on the size and the direction of the telescope nod, and appear to be limited by the speed at which the GMOS OIWFS moves into position. Nods in the q-direction appear to be much more efficient (less overhead) than nods in the p-direction (perpendicular to the slit).

  • Nods in the q-direction can be approximated as adding another additional 1sec of overhead per cycle for every 15 arcsec of distance, eg. a nod of q=30 arcsec gives an overhead of 26 sec per cycle.

  • Nods in the p-direction can be approximated as adding another additional 0.6sec of overhead per cycle for every 1 arcsec of distance, eg. a nod of p=30 arcsec gives an overhead of 42 sec per cycle. It is strongly recommended to nod in the q-direction (parallel to the slit).

  • Very small absolute nod sizes (< 2.0arcsec) can take advantage of electronic offsetting of the OIWFS. In this case the GMOS OIWFS does not actually move, only the position of the spots on the wavefront sensor is electronically shifted. The effective overhead per cycle is reduced to 13.5 seconds, so observations which can tolerate such small nods are encouraged to use to use this feature.