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
CVZ (
Section 2.2.1),
LOW SKY (
Section 8.16.12 of the Call for Proposals), and
SHADOW (
Section 8.16.12 of the Call for Proposals) are used.
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 2.2.2). This is the visibility time that should be used for planning CVZ observations, unless the proposer knows that he or she 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 or timing requirements (see
Section 2.2.1).
Also included in Table 6.1 are visibilities suitable for use in Large Programs (see
Section 3.2.2 of the Call for Proposals). Proposers submitting Large Programs should consult the Large Program Scheduling User Information Report which can be found on the
HST Documents page (linked from the
Cycle 21 Announcement Web page). This document contains a discussion of the issues surrounding Large Program scheduling.
For Pure Parallel observations (see Section 4.2.2 of the Call for Proposals), proposers may not know the prime target declinations. One of the following two options should be used for planning observations:
