Cycle 9 Continuous Viewing Zone (CVZ) Information

A detailed description of the CVZ and its use in Cycle 9 are given in the Call for Proposals. A summary of the most relevant information is repeated in Section B below (the full text in the Call for Proposals always takes precedence in any cases of conflict). Section A contains information on the CVZ opportunities in Cycle 9 for targets of given RA and DEC. Please study this information before proposing for CVZ observations.

Section A. CVZ Opportunities for given RA and DEC

Section A.1. Tables

We have prepared tables that list for the following information for the 12-month period June 01, 2000 - June 01, 2001 (2000.183:01:00:00 - 2001.182:22:00:00): Be aware that near the "wings" of the CVZ area (i.e., where there is only one CVZ window), the actual availability of CVZ observing will depend in detail on the geometry of the HST orbit during Cycle 9.

Section A.2. Automatic extraction

Alternatively, you may enter below coordinates of your target and use the `SUBMIT' button to automatically extract the CVZ information from the tables. After getting the answer, use the `BACK' button in your browser to return to the present page. Press the `RESET' button below to reset the RA and DEC fields.

Section B. CVZ: Description and Policies

Section B.1. The Continuous Viewing Zones

As seen from HST, targets in most of the sky are occulted by the Earth for varying lengths of time during each 96-min orbit. Targets lying in the orbital plane are occulted for the longest interval, about 36 min per orbit. These orbital occultations are analogous to the diurnal cycle for ground-based observing and impose the most serious constraint limiting the efficiency of most HST observations.

The length of target occultation decreases with angle from the spacecraft orbital plane. Targets lying within 24 degrees of the orbital poles are not geometrically occulted at all during the HST orbit. However, the size of the resulting "Continuous Viewing Zones" (CVZs) is substantially reduced by the Earth-limb avoidance angles. Note also that scattered Earth light may be significant when HST observes near the bright earth-limb.

Since the HST orbital poles lie 28.5 degrees from the celestial poles, any target located in two declination bands near +/- 61.5 degrees may be in the CVZ at some time during the 56-day HST precessional cycle. Some regions in these declination bands can be unusable during the part of the year when the sun is too close to the region. Depending upon the HST orbit and the target position, there may be one to 10 CVZ window durations ranging from one orbit up to 7 days. However, passages through the South Atlantic Anomaly will restrict the longest uninterrupted observation to about 5-6 orbits. Use Section A above to determine the number and length of the CVZ opportunities in Cycle 9 for targets of given RA and DEC.

Section B.2. Use of the Continuous Viewing Zones

Observations of targets that lie in the CVZ have been shown to be more than twice as efficient as the ensemble of non-CVZ observations; hence observers are encouraged to request the CVZ when possible, in order to maximize the scientific return and efficiency of their observations. The allocation of spacecraft orbits allows proposers to evaluate straightforwardly the efficiency gains realized through observations made in the CVZ. It will often be found that use of the CVZ will allow a significant increase in the possible exposure time during a given number of spacecraft orbits, and hence its exploitation is to the proposer's advantage. Proposers should be aware that it is not possible to use the Shadow time (SHD) and Low-sky (LOW) special requirements in the CVZ, and that special timing requirements are not generally compatible with CVZ observations. Hence, observations requiring low background should not be proposed for execution in the CVZ.

In previous cycles, observations in the CVZ were requested by observers in Phase I and were approved or disallowed by the TAC or panels. Proposers using the CVZ had a margin of risk in the cases in which their programs were unschedulable in the CVZ. We have changed the CVZ policy.

In Cycle 9, proposers should use CVZ visibility in their orbit estimates where possible, and STScI will make every effort to schedule the observations in this optimal way. Because CVZ opportunities are limited, however, it may be necessary to schedule the observations using standard orbit visibility (i.e., more orbits), especially for large numbers of CVZ orbits and/or additional scheduling constraints.

Proposers should be aware that all programs approved in Phase I are conditional upon technical review, which is based on variables unknown at the time of program selection, and that rescheduling CVZ proposals by using standard orbit visibility may prove impossible.

Section B.3. Orbit estimates for CVZ observations

If you can utilize CVZ time for your observations, then the visibility period is 96 minutes per orbit for 5-6 orbits, beyond which time SAA interference will limit the visibility to ~70 minutes per orbit for the next 7-9 orbits. It may be to the proposer's advantage to select CVZ targets if possible, since the long visibility period of 96 minutes per orbit will allow a factor of two competitive advantage in terms of required resource charge (orbits) to perform the same science observations relative to non-CVZ targets. However, in practice the utility of CVZ observations could be reduced because the special requirements SHD and LOW are inconsistent with CVZ observations. While the brightness of the scattered Earthshine background during CVZ observations is not greater than during non-CVZ observations (since the same bright limb avoidance angle is used), the duration of high background can be considerably greater since the line of sight can graze the bright earth-limb avoidance zone during CVZ observations. Also, it may not be possible to schedule observations that require special timing as CVZ targets. Observation sets that will use Phase II Special Requirements: ORIENT, ON HOLD (for targets-of-opportunity), AFTER, BEFORE, BETWEEN, or PHASE restrictions should therefore adopt the non-CVZ target visibility period for resource estimation (see Chapter 16 of the Call for Proposals).

Owner: Brett S. Blacker

Technical Questions: HST Help Desk

Last updated: June 08, 1999 by Roeland van der Marel.