9.2 Generic Observatory Overheads

The first time that you acquire an object, you must include a 6.5-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 re-acquisition. If you are observing an object in the Continuous Viewing Zone (CVZ), then no guide-star re-acquisitions 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.

ActionOverhead typeTime needed

Guide-star acquisition

Initial acquisition

6.5 min


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.