6.3 The Visibility Period

The visibility period is the amount of unocculted time per orbit during which observations of a given target can be made. Table 6.1 gives the visibility period for fixed targets of given declination, for moving targets (assumed to be near the ecliptic plane), and for cases in which the special requirements LOW (see Section 5.5.1), SHD (see Section 5.5.2), or CVZ (see Section 2.3.1) are used.

For SHD observations you have only 25 minutes for the observations, regardless of target declination. However, guide-star acquisitions and re-acquisitions, as well as end-of-orbit overheads, may be done outside the narrower shadow time window, depending on the HST ephemeris at the time of execution.

The listed visibility time for the CVZ (96 minutes, i.e., the entire HST orbit) assumes that there are no SAA intersections in these orbits (see Section 2.3.2). This is the visibility time that you should use if you are planning CVZ observations, unless you know that you may have to observe in orbits that are SAA-impacted. In the latter case the visibility time is approximately 70 minutes per orbit. Note that CVZ orbital visibility should not be requested if there are special background emission (SHD, LOW) or timing requirements (see Section 2.3.1).

Visibility Period for Pure Parallel Observations

If you are proposing for Pure Parallel observations (see Section 4.2.2 of the Call for Proposals), then you may not know the prime target declinations. You should then use one of the following two options when planning your observations:

• Use the minimum allowed visibility period given your target selection criteria; e.g., if your requirement calls for fields around M31 (at a declination of 41 degrees), then use 54 minutes.
• Map out the exposures (plus overheads) you wish to obtain in an orbit for any legal visibility period (52-60 minutes). If you select this method, note that longer total exposure times typically have fewer opportunities to schedule.