Primary observations are those observations that determine the telescope pointing and orientation. GO or SNAP proposals with external targets are normally scheduled as primary. Primary observations can use a variety of special requirements and observation types, as described in the following subsections. There is also the opportunity for parallel observations, described in Section 4.2, which are simultaneous observations with instruments other than the primary instrument.
Most targets are geometrically occulted in part of every HST orbit. However, this is not true for targets that lie close to the orbital poles. This gives rise to so-called Continuous Viewing Zones (CVZ) in two declination bands near +/- 61.5 degrees. Targets in these bands may be viewed without occultations at some time during the 56-day precessional cycle of the HST orbit. Depending upon the telescope orbit and the target position, there may be up to 7 CVZ intervals with durations ranging from 1 to 105 orbits (7 days). Check the CVZ Tables on the Web to determine the number of CVZ opportunities in Cycle 13 and their duration for a given target location. Passages of HST through the South Atlantic Anomaly restrict the longest uninterrupted observations to about 5-6 orbits. See Section 2.3.1 of the HST Primer for technical details about the CVZ.
Observations of targets in the CVZ are nearly twice as efficient as non-CVZ observations. Therefore, proposers should use CVZ visibility in their orbit estimates where possible. In the observations description section (see Section 9.2), you must include the number of CVZ opportunities available (using the CVZ Tables on the Web) for each target in your proposal for which you are requesting CVZ time.
STScI will make every effort to schedule the observations in this optimal way. However, because the number of CVZ opportunities are limited and unpredictable conflicts may occur between the proposed CVZ observations and other observations, a particular target's CVZ times may be oversubscribed. Therefore, it may be necessary to schedule the requested CVZ observations using standard orbit visibilities (i.e., using a larger number of total orbits). This will be done at no penalty to the observer.
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Continuous Viewing Zone observations must be marked in the 'Observation Summary' section of the proposal (see Section 8.15). |
Note that it is to the proposer's advantage to request CVZ observations (where possible). It allows a given set of observations to be done in a smaller number of orbits, which gives the proposal a competitive advantage over non-CVZ proposals during peer review.
The following special requirements are generally incompatible with use of the CVZ:
Hence, observations that require low background or special timing requirements should not be proposed for execution in the CVZ, and orbit estimates in the Phase I proposal should be based on standard orbit visibility (see Table 6.1 of the HST Primer). Please note that because of the extra scattered earthshine that enters the telescope on the day side of the orbit, sky-background limited observations through broadband optical or infrared filters do not gain significant observing efficiency from CVZ observations. If it is determined during the Phase II proposal implementation that an observation is unschedulable because of conflicts between the CVZ requirement and any other Special Requirements (e.g., SHD, LOW, timing, etc.), then the observing time may be revoked. Proposers who are in doubt about whether or not to request CVZ observations should contact the STScI Help Desk (see Section 1.5).
A target for HST observations is called a 'Target-of-Opportunity' (TOO) if the observations are linked to the occurrence of an event that may occur at an unknown time. TOO targets include objects that can be identified in advance but which undergo unpredictable changes (e.g., specific dwarf novae), as well as objects that can only be identified in advance as a class (e.g., novae, supernovae, gamma ray bursts, newly discovered comets, etc.). TOO proposals must present a detailed plan for the observations that are to be performed if the triggering event occurs.
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Target-of-Opportunity observations must be marked in the 'Observation Summary' section of the proposal (see Section 8.15). In the 'Special Requirements' section of the proposal (see Section 9.3) you must provide an estimate of the probability of occurrence of the TOO during the observing cycle, and describe the required turn-around time. |
The turn-around time for a TOO observation is defined as the time between an observer's request for TOO activation and the execution of the observations. The HST observing schedule is constructed eleven days in advance of the actual observations. Therefore, any short-notice interruptions to the schedule place extra demands on the scheduling system, and may lead to a decrease in overall efficiency of the observatory. For this reason, the minimum turn-around time for TOO activation, while depending on the particular circumstances, is normally 2-5 days; this can be achieved only if all details of the proposal (except possibly the precise target position) are available in advance. Because of the significant effect TOO observations have on the short and medium-term HST schedule, the number of rapid TOO activations (i.e., 2 weeks turn-around or shorter) will be limited to approximately 6 in Cycle 13. Requests for rapid turn-around should be strongly justified in the Phase I proposal.
STScI has been endeavoring to reduce the turnaround time for activated TOO proposals that require fast response and are of the highest scientific importance. This has made it possible to reduce the nominal minimum 2-5 day interval to as short as 1 day but only for those detectors that do not require bright object checking (i.e., ACS/WFC, ACS/HRC, NICMOS, STIS/CCD and WFPC2). We therefore encourage the community to identify exciting HST science that would be enabled by a 2 day (or less) TOO turnaround time, and to submit proposals accordingly.
In order to reflect the true cost of such ultra-rapid TOOs, and to enable the TAC to weigh science and resources appropriately, note that:
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Proposers requesting a less than 2-day turnaround should add a fixed overhead of 15 orbits per activation to the total orbit request in their proposal. |
This is the average expected telescope down-time due to this very fast TOO response.
The number of ultra-rapid TOO activations (less than 2 days turn-around) will be limited to 3 in Cycle 13.
A Phase II program must be submitted before the TOO event occurs. If the observing strategy depends on the nature of the event, then the Phase II program should include several contingencies from which the observer will make a selection. The PI is responsible for informing STScI of the occurrence of the event and must provide an accurate target position. Implementation of a TOO observation after notification of the event requires approval by the STScI Director and is not guaranteed (e.g., high-priority GO observations, critical calibrations, and engineering tests may take precedence over TOO programs). If approval is granted, then the HST observing schedule is replanned to include the new observations. A turn-around time of less than 1 month requires the PI or his/her designee to be reachable by STScI personnel on a 24 hour basis between the TOO activation and the scheduling of the program.
If the triggering event for an approved TOO program does not occur during the observing cycle, the program will be deactivated at the end of the cycle. Unused TOO time does not carry over into the following cycle.
TOO proposals that use the STIS/MAMA detectors or ACS/SBC must pass bright object checking before they can be scheduled. For rapid turn-around proposals, where the target may be varying in intensity, a strategy must be outlined to ensure that the TOO will be safe to observe. A description of how you plan to deal with this issue should be provided in the 'Special Requirements' section of the proposal (see Section 9.3).
Note also that STIS/MAMA and ACS/SBC observations cannot be scheduled in orbits affected by passages of HST through the South Atlantic Anomaly (SAA), which limits the duration of a MAMA visit to 5 orbits (see Section 2.3.2 of the HST Primer).
HST can observe most targets within our Solar System, although there are a few exceptions. Mercury is always well within the 50 degree Solar pointing exclusion, and hence cannot be observed. Venus is always within the 50 degree Solar pointing exclusion, but at maximum elongation can be greater than 45 degrees from the Sun. The STScI and the HST Project at GSFC have developed (and used) procedures which support observations of Venus when it is slightly within the 50 degree limit. These procedures require extra planning and implementation steps. Venus observations may be proposed, but execution of these observations is subject to the availability of resources to carry out the extra work required for them. Observations of comets can be made while they are further than 50 degrees from the Sun. The HST pointing control system and the HST scheduling systems were not designed to support observations of objects as close as the Moon. The effort required to carry these out is well beyond the available resources, so observations of the Moon should not be proposed.
There are a variety of plausible scenarios in which investigators may wish to propose for HST observations of targets that have not yet been discovered or identified (i.e., for which the coordinates are unknown; e.g., the next supernova in our own Galaxy, the next gamma-ray burst on the southern hemisphere, etc.). In general, such proposals are allowed only if there is a certain time-criticality to the observations; i.e., proposing for the same observations in the next regular review cycle (after the target has been discovered) would be impossible or would make the observations more difficult (e.g., the object fades rapidly, or its temporal behavior is important), or would lead to diminished scientific returns. These criteria are generally satisfied for GO observations of TOO targets, and there may also be other circumstances in which proposals for such targets are justified. However, in the absence of demonstrated time-criticality, observations will not generally be approved for targets that have not yet been discovered or identified.
Proposals may request that HST observations be made at a specific date and time, or within a range of specific dates, when scientifically justified. Some examples of such cases are:
Any requests for time critical observations must be listed in the 'Special Requirements' section of the proposal (see Section 9.3).
Time-critical observations impose constraints on the HST scheduling system, and should therefore be accompanied by an adequate scientific justification in the proposal.
Time-critical events that occur over short time intervals compared to the orbital period of HST (such as eclipses of very short-period binary stars) introduce a complication because it will not be known to sufficient accuracy, until a few weeks in advance, where HST will be in its orbit at the time of the event, and hence whether the event will occur above or below the spacecraft's horizon (see Section 2.3.3 of the HST Primer). Proposals to observe such events can therefore be accepted only conditionally.
Communications with HST in "real-time" are a limited resource, which require additional operational overheads, reduce observing efficiency, and greatly increase the scheduling complexities. However, in exceptional circumstances, some science programs may require such interactions. These observations will generally require the presence of the PI (or his/her designee) at STScI during such exposures, and STScI personnel will be present to assist the PI and send the command requests.
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Any requests for real-time interactions must be listed in the 'Special Requirements' section of the proposal (see Section 9.3). |
In such cases the scientific and operational justification for this should be presented clearly in the observing proposal. Typically, two-way real-time interactions for position updates should only be used when early-acquisition or reuse target offset techniques to refine the telescope pointing cannot be used (see Section 5.2 of the HST Primer).
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |