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There are three main sources of bHROS observing overheads: target acquisitions, instrument setup and detector readout. For Phase I observing proposals that do not require detailed timing, you can use the acquisition times below, plus 2 minutes per exposure to cover readout. Any reconfiguration of the instrument (changing the grating settings) will require an additional 45s per change as well as require additional calibrations to be taken. For more exact calculation of the overheads, the detailed listings on this page may be used. This information is also useful for the Phase II planning of your observing program.

Telescope Acquisition Time

The peak-up times below are for the acquisition of the target on the desired fiber. These times do not include telescope slew time and guiding acquisition. See the bHROS acquisition page for details.

Target Magnitude Acquisition Overhead (minutes)
8 10
12 20
16 30

Any placement of the target on a new fiber (i.e. switching between Object/Sky fibers) will require a reacquisition of the target on the fiber. Hence it is strongly recommended that nodding between the two fibers is not employed. Improvements to the GMOS-S OIWFS probe mapping and other instrument and telescope developments may significantly reduce the need for peaking-up and acquisition time, however the values used above should be used at present.

bHROS Configuration Times

Below are listed approximate configuration times for various components with bHROS. Exact times depend on the amount by which a particular component must be moved. bHROS can be reconfigured while slewing to a new target, but not during the spiral acquisition procedure or detector readout.

bHROS component Configuration Time
Filter Change 20s
Grating Change 45s
Fiber mode change
(also requires reacquisition)

Readout Times

The table below contains the readout times for several configurations of the bHROS detector (full frame readout).

Spatial Binning Read Speed Readout Time (s)
1 slow 125
1 fast 40s
4 slow 46s

Configuration for Calibrations

A portion of the overhead for taking calibrations is the time required to move the science fold mirror to send the beam from GCAL into bHROS. This move cannot be done while reading out the detector. This move of the science fold requires about 20 seconds. Therefore the total overhead to move to and from the calibration position is about 40s. This is in addition to the time to make the calibration observations (see bHROS Calibrations page for more details).


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Last update September 6, 2005; Steven Margheim