Science Operations

Overheads

Exposure Time Overheads

A summary of the readout times for the standard observing readout modes is summarized below. The measurements listed include all overheads associated with image acquisition: pre-exposure erase cycles, post-exposure readout time, disk-write time, and instrument/telescope telemetry queries prior to start of the exposure. Overheads should be treated as guidance-only for observing planning: variations of up to as much as 2 seconds are not unusual, mostly due to unpredictable communication latencies in the system.

 ROI Mode  Blue
1×1 (sec)
 Blue
2×2 (sec)
 Red
1×1 (sec)
 Red
2×2 (sec)
 Notes
 MODS1  8Kx3K  104  57  99  52  Grating Spectroscopy (Full Frame)
 4Kx3K  76  50  71  45  Prism Spectroscopy
 3Kx3K  69  48  64  43  Direct Imaging
 1Kx1K  43  40  38  34  Acquisition
 MODS2  8Kx3K  105  50  96  48  Grating Spectroscopy (Full Frame)
 4Kx3K  71  41  67  41  Prism Spectroscopy
 3Kx3K  63  39  60  39  Direct Imaging
 1Kx1K  34  30  34  30  Acquisition

Telescope Overheads

 Source of Overhead  Action Type  Typical Time [sec]  Maximum Time [sec]
 Telescope  Preset  75  300
 Pointing Correction  150  300
 Dithering  5  300
 AGw Collimation – Imaging   100  300
 AGw Collimation – Spectroscopy 0 0

For Imaging Scripts, the script is paused during collimation. Once the wavefront sensor error WFE is below ~800 nm, the user can continue the script. This usually takes about 3 wavefront sensor cycles but is highly dependent on the thermal state of the telescope and weather. For spectroscopic scripts, MODS1 & 2 takes a slit image during collimation essentially removing any overhead due to collimation.

Other Overheads

There are a number of typical overheads one should consider when setting up an observing program including things like filter change times which are about 2 to 8sec, or the overheads associated with each dither is 5-10sec.    Overheads associated with dithering are dominated by waiting for the guide probe to move and the GCS to re-lock on the guide star.

 Source of Overhead  Action Type  Typical Time [sec]  Maximum Time [sec]
 Instrument  Filter Change  2  8
 Shutter lag  1.55  1.68
Calibration mode to Observing mode and vice versa  30  40
 Other  10  1200

Other overheads include moving from calibration mode to observing mode and vice versa. To view the calibration system the AGw guide stage has to be retracted to its home position, the instrument dark hatch closed, and the calibration optics inserted into the beam, a process that requires about 30 seconds (up to 40 seconds if the guide probe is near the science field center). This is not typically done during night time observing, but does highlight the importance of properly following the start up procedure as waiting for the calibration unit or guide probe can be a source of delay if the procedure is not followed.