Primary observations are those observations that determine the telescope pointing and orientation. GO or SNAP Programs 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 those bands may be viewed without occultations at some time during the 56-day precessional cycle of the HST orbit. The number and duration of CVZ passages depend on the telescope orbit and target position, and may differ significantly from previous cycles. Use the 'Available Science Time and Orientation Tool' on the Two-Gyro Web Page to determine the number of CVZ opportunities in Cycle 16 and their approximate 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.
CVZ orbits are a limited resource whose use can lead to scheduling conflicts. If CVZ orbits are scientifically necessary for your program, check that sufficient opportunities exist that it is likely that your orbit request can be accommodated. (It is not possible, at present, to determine the exact number of CVZ orbits available during a particular opportunity.) In the observations description section (see Section 9.2), you must include the number of CVZ opportunities available 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). |
The following special requirement is generally incompatible with use of the CVZ.
Observations that require 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 turnaround or shorter) will be limited to approximately 6 in Cycle 16. Requests for rapid turnaround should be strongly justified in the Phase I proposal.
STScI has been endeavoring to reduce the turn-around 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 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 associated with this very fast TOO response.
The number of ultra-rapid TOO activations (less than 2 days turn-around) will be limited to 1-2 in Cycle 16.
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.
TOO proposals that use the ACS/SBC must pass bright object checking before they can be scheduled. Ultra-rapid turn-around programs are not allowed with the ACS/SBC. 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).
The ACS/SBC employs a MAMA detector and is therefore vulnerable to damage through exposure to bright sources. Consequently, there are a number of restrictions on the use of this configuration. All targets and field objects falling on the ACS/SBC detector must pass bright-object safety reviews (see Section 5.1.2 of the Primer). All SBC Phase I proposals must include a discussion of the safety of the proposed targets and fields in the Description of the Observations (see Section 9.2), based on the ACS Instrument Handbook Section 7.2 and calculations with the appropriate APT and ETC tools.
Proposals to observe variable objects with the ACS/SBC must pass bright object checking before they can be scheduled (see Section 5.1.2 of the Primer). Proposers should assume the maximum flux values for targets unless there are specific reasons for adopting other values (for example, time constrained observations of periodic variables at flux minima); the justification for adopting alternative flux values should be given in the 'Special Requirements' section of the proposal (see Section 9.3).
In the case of aperiodic variables that are either known to undergo unpredictable outbursts, or belong to classes of objects that are subject to outbursts, the proposer must determine whether the target will violate the ACS/SBC bright object limits during outburst. If violation is possible, the proposer must outline a strategy that will ensure that the target is safe to observe with the ACS/SBC.
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A description of how you plan to deal with bright object checking for variable sources must be included in the 'Special Requirements' section of the proposal (see Section 9.3). |
The observing strategy might include additional observations, obtained over a timescale appropriate to the particular type of variable object, with either HST or ground-based telescopes. Proposers should be aware that this type of SBC observation requires real-time interactions with HST, with the associated additional operational overheads (see Section 4.1.7). If you are planning such observations, please contact help@stsci.edu for more information on the options and requirements for confirming quiescence.
HST can observe most targets within our Solar System, although there are exceptions. Both Mercury and Venus are always well within the (two-gyro) 60 degree Solar pointing exclusion, and cannot be observed. Observations of comets can be made while they are further than 60 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. Observations of the Moon are not possible in Two-Gyro Mode.
Pointing constraints are discussed further in Section 2.4 of the HST Primer.
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 generally not 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:
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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, particularly in two-gyro mode, 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).
Guide star acquisitions in two-gyro mode are more complicated than in three-gyro mode. The time required for target acquisition at the beginning of an orbit in the two modes is similar (~6 minutes). However, it is generally not possible to observe multiple targets within a single HST orbit in two-gyro mode, unless the separation of targets is small enough (<2') that the telescope can be repositioned with a small angle maneuver that does not break guide star tracking. Maneuvers that require a full guide star acquisition, which include observations of the same target with multiple HST instruments, will be forced into a new orbit.
Experience has shown that ACS imaging observations are best taken as dithered exposures (see Section 5.4 of the HST Primer). Proposers who do not intend to use dithering must provide a justification for their choice of strategy in the Description of Observations section of the pdf attachment ( Section 9.2). In general, undithered observations with the ACS CCD detectors will not be approved without strong justification that such is required for the scientific objectives. Otherwise, hot pixels and other detector artefacts may compromise the archival value of the data.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
