COS Instrument Handbook for Cycle 24

help@stsci.edu

9.2	Generic Observatory Overheads

The first time that you acquire an object, you must include a 6-minute overhead for the HST guide-star acquisition. In all subsequent orbits of the same visit, you must include the 4-minute overhead for the guide-star reacquisition. If you are observing an object in the Continuous Viewing Zone (CVZ), then no guide-star re-aquisitions are required.

You must allocate additional time for each deliberate movement of the telescope; e.g., if you are performing a target-acquisition exposure on a nearby object and then offsetting to your target, or if you are taking a series of exposures in which you move the target on the detector (POS-TARG), then you must allow time for telescope motion. The time varies depending on size of the slew; see Table 9.2.

Table 9.2: Generic Observatory Overhead Times

Action	Overhead type	Time needed
Guide-star acquisition	Initial acquisition	6 min
	Re-acquisition	4 min per orbit
Spacecraft movements	10 arcsec < Offset < 1.5 arcmin1	60 s
	1.25 arcsec ≤ Offset ≤ 10 arcsec	30 s
	Offset < 1.25 arcsec	20 s

1

Spacecraft motions larger than ~ 1.5 arcmin are likely to result in the loss of guide stars.