Overheads

Detector Overheads

Detector overheads for the standard observing readout modes are summarized in the table below. These numbers include all overheads associated with image acquisition: pre-exposure reset and erase cycles, shutter delay, post-exposure readout time, and disk-write time. With the Archon controllers, detector overheads are significantly reduced, e.g. for an unbinned full frame spectrum, they are now 25 instead of 105 seconds. They are also the same for the all 4 detectors: MODS1/MODS2/Blue/Red.

ROI mode	binning	overhead (tover) [sec]
8192 x 3088	1×1	25
	2×2	11
	1×2	20
3088 x 3088	1×1	14
4096 x 3088	1×1	16

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 – Imaging1	100	300
	AGw Collimation – Spectroscopy2	0	0

Table Notes:

(1) 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.

(2) MODS spectroscopic acquisition scripts take first a slit image and then a field image. Collimation occurs during the slit image and is usually complete before the field image is started, although observers should keep an eye on this with the upgraded MODS, since the acquisition overheads are now so much faster (14 instead of 45 seconds).

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.