Overheads

Detector Overheads

A summary of the detector overheads 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. We are currently experiencing very long, 100-sec, delays between the close-of-shutter and the start of readout for about 10-15% of the MODS1 Red and MODS2 Blue images (Jan 2023).

	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.

Instrument 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	instconfig: dual imaging -> dual grating	19.2	19.2
	instconfig: dual grating -> red-imaging (for target acquisition)	19.2	19.2
	instconfig: dual imaging -> dual prism	34.6	34.6
	IMCS Lock (the red channel is the slowest, and in dual mode, the limiting factor).	90	120
	Slitmask select	3	10
	Slitmask insert/retract to/from focal plane	10	10
	Filter Change	2	8
	Shutter lag	1.55	1.68
	Calibration mode to Observing mode and vice versa (usually done only before and after the night)	30	40

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.