Overheads

Obtaining astronomical observations at any telescope will typically face four different sources of overhead: from the telescope, instrument, detector and any operation that requires human interaction. The Table below summarizes these overheads.

Source of Overhead	Action Type	Typical Time [sec]	Maximum Time [sec]
Telescope	Preset	75	300
	Pointing Correction	150	300
	AGw Collimation	100	300
	AO setup	600	1200
	ARGOS setup	240	1200
	DIther(SL)	14	20
	Dither(AO)	10	20
	Dither(ARGOS)	20	60
Instrument	Camera, Filter changes	15	30
	Grating Tilt	20	60
	Mask: Storage to FPU	180	200
	Mask: Turnout to FPU	72	72
Detector	LIR Readout	2.51
	MER Readout	6.26
	SUR Readout	1.26
	Save LIR/MER FITS image	7
	Save SUR FITS image	TBD
Human Element	Field/Source Identification	5	1800
	MOS Alignment	60	1800
	Other	10	1200

Presetting the telescope to a new target is an obviously unavoidable overhead at the beginning of each program. Near-IR observations also typically involve regular dithering (offsets) to allow for better measurement and removal of the variable sky background and instrumental signature. Each dither has an associated overhead. Once on target, the AGw will auto-acquire the supplied guide star and begin guiding and collimating. The speed of collimation often depends on the thermal state of the telescope and weather. For AO observations, once collimated by the AGw, the AO system must acquire the reference star on the pyramid and optimize the gains (~3min). For ARGOS, once collimated by the AGw, the AO system must acquire a reference star for the tip-tilt signal as well as its lasers on its ground-layer wavefront sensor (~4min).

Overheads for moving mechanical parts in LUCI like the filter or camera wheels are short, just a few tens of seconds at most. Instrument and telescope configurations are also done in parallel so most instrument changes are done while the telescope is slewing to a new field. The longest overheads for instrument reconfigurations in LUCI occur when moving a mask in or out of the FPU.

Each of the three detector readout modes offered for science and their associated overheads (LIR, MER, SUR) are described in more detail here.

There are three main sources of overheads we see at the LBT that arise from the human element: difficulties identifying sources from the finding charts, problems aligning MOS masks on sky, and unfamiliarity with the instrument or documentation causing delays.  Observers and PI’s with a strong familiarity with recent documentation, and PI’s with well planned programs containing good finder charts help to minimize the human element overhead.