Hubble Space Telescope Call for Proposals for Cycle 13

Chapter 4:
Observation Types and Special Requirements

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4.1 Primary Observations
    4.1.1 Continuous Viewing Zone (CVZ) Observations
    4.1.2 Target-of-Opportunity (TOO) Observations
    4.1.3 Solar System Targets
    4.1.4 Observations of Targets that have not yet been discovered or identified
    4.1.5 Time-Critical Observations
    4.1.6 Real-Time Interactions
4.2 Parallel Observations
    4.2.1 Coordinated Parallel Observations
    4.2.2 Pure Parallel Observations
    4.2.3 Restrictions and Limitations on Parallel Observations
4.3 Special Calibration Observations

4.1 Primary Observations

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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.

4.1.1 Continuous Viewing Zone (CVZ) Observations

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.

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.

Restrictions on Using the CVZ

The following special requirements are generally incompatible with use of the CVZ:

• The Shadow time (SHD) and Low-sky (LOW) special requirements (see Section 5.5 of the HST Primer)
• Special timing requirements (including telescope orientation constraints; see Section 4.1.5)

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).

4.1.2 Target-of-Opportunity (TOO) Observations

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.

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.

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.

Ultra-Rapid Turn-Around Time

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:

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.

Activation of a TOO

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 Programs with STIS/MAMA or ACS/SBC

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).

4.1.3 Solar System Targets

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.

4.1.4 Observations of Targets that have not yet been discovered or identified

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.

4.1.5 Time-Critical Observations

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:

• astrometric observations,
• observing specific phases of variable stars,
• monitoring programs,
• imaging surface features on solar-system bodies,
• observations requiring a particular telescope orientation (since the orientation is fixed by the date of the observations; see Section 2.5 of the HST Primer),
• observations coordinated with observations by another observatory.

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.

Limitations Related to Time-Critical Observations

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.

4.1.6 Real-Time Interactions

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.

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).

4.2 Parallel Observations

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Since all of the scientific instruments are located at fixed positions in the telescope focal plane, it is possible to increase the productivity of HST by observing simultaneously with one or more instruments in addition to the primary instrument. Those additional observations are called parallel observations.

Since each instrument samples a different portion of the HST focal plane (see Section 2.2 of the HST Primer), an instrument used in parallel mode will normally be pointing at a "random" area of sky several minutes of arc away from the primary target. Thus parallel observations are usually of a survey nature. However, many HST targets lie within extended objects such as star clusters or galaxies, making it possible to conduct parallel observations of nearby portions of, or even specific targets within, these objects.

Parallel observations are never permitted to interfere significantly with primary observations; this restriction applies both to concurrent and subsequent observations. Some examples of this policy are the following:

• A parallel observation will not be made if its inclusion would shorten the primary observation.
• Parallel observations will not be made if the stored command capacity or data volume limits would be exceeded.

Depending on whether a parallel observation is related to any specific primary observation, it is defined either as a coordinated parallel or pure parallel. Coordinated parallel observations are observations related to a particular primary observation in the same proposal. Pure Parallel observations are unrelated to any particular primary observation (i.e., the primary observation is in another program). Investigators interested in proposing for parallels are encouraged to consult the Parallel Observations User Information Report, which contains details on how parallels are scheduled, completion rates for recent cycles, and other useful information.

4.2.1 Coordinated Parallel Observations

Coordinated Parallel observations must be marked in the `Observation Summary' section of the proposal (see Section 8.15).

Coordinated Parallels use one or more instruments, in addition to and simultaneously with the primary instrument in the same proposal, e.g., to observe several adjacent targets or regions within an extended object. Proposals that include coordinated parallel observations should provide a scientific justification for and description of the parallel observations. It should be clearly indicated whether the parallel observations are essential to the interpretation of the primary observations or the science program as a whole, or whether they address partly or completely unrelated issues. The parallel observations are subject to scientific review, and can be rejected even if the primary observations are approved.

Proposers are not allowed to add coordinated parallel observations in Phase II that were not explicitly included and approved in Phase I.

4.2.2 Pure Parallel Observations

Pure Parallel observations must be marked in the `Observation Summary' section of the proposal (see Section 8.15).

Proposals for pure parallel observations may specify either specific or generic targets, although the latter are more common. Appropriate scheduling opportunities will be identified by STScI.

Experience with pure parallel observations over the last several years indicates that parallel observing programs should be kept simple, in order for them to schedule on top of the typical prime observing programs. Parallel programs requiring multiple successive orbits are less likely to schedule than those which can be executed in a single orbit (over the past year, of the parallel visits scheduled, 91% were a single orbit in duration, 6% were two orbits, 2% were three orbits, 1% were four orbits, and less than 1% were longer than 4 orbits). Due to resource constraints at STScI, only a limited number of pure parallel programs can be accepted in each cycle.

For Cycles 11 and 12, the Advanced Camera for Surveys (ACS) was the most heavily subscribed instrument for primary science observations. This is expected to be true for Cycle 13 as well. Therefore, proposers should be aware that opportunities to use the ACS for pure parallel observations will be more limited than for other instruments, and propose accordingly.

A "Default" HST Archival Pure Parallel Program has been in place since Cycle 7. This program consists of observation types using each of the possible pure parallel instruments and is used to obtain uniform, non-proprietary data sets for the HST Data Archive during parallel observing opportunities not used as coordinated or pure parallel GO observations. A plan for the Cycle 13 default programs was developed by the Parallel Observing with Space Telescope (POST) team (see the Parallel Observations User Information Report for details). The scientific rationale for the observations, a description of the observations, and additional information can be found on the HST Archival Pure Parallel Program Web Page.

STScI encourages submission of:

• Archival Research proposals to analyze data obtained as part of the HST Archival Pure Parallel Program in previous cycles.
• GO pure parallel proposals for observations that differ from the Cycle 13 Default HST Archival Pure Parallel Program.
• GO pure parallel proposals to carry out, for each of the instruments, either selected pieces or the entire Cycle 13 Default HST Archival Pure Parallel Program. The advantage of such proposals for GO investigators, if approved, is that they become eligible for funding (U.S. Investigators only) and will have responsibility for the Phase II implementation of the proposed part of the program. In addition, investigators in this category will be asked to participate in a reconstituted Parallel Working Group to refine the observing strategy of the Default Parallel Program for each scientific instrument, and to monitor the observations to assure that high quality data are taken. This coordination will be important for the final definition of the program but will not have to be extended to the data analysis phase.

The review panels and TAC will select the best science and prioritize the parallel programs, including the Default Program. GO Programs may well replace the Default Program in order to keep the total number of parallel programs at a supportable level.

As in previous Cycles, all data taken in the context of the Default HST Archival Pure Parallel Program will be non-proprietary. In addition, all GO pure parallel programs will have zero proprietary period. Pure parallel programs will not be carried over to the next cycle.

4.2.3 Restrictions and Limitations on Parallel Observations

Parallel Observations with ACS

When ACS is the primary instrument, it permits WFC parallel observations with HRC in prime and vice versa (so called "auto-parallels"). The filter choice for auto-parallels is predetermined by the filter used in the prime observation (see the ACS Instrument Handbook for details). For this reason, auto-parallels are created by software and execute automatically as pure parallels. Even though the observer does not specify auto-parallels in his/her proposal, the parallel data are proprietary, exactly in the same way as the prime data. It depends on the primary exposures whether or not auto-parallels can be created.

The ACS/WFC and ACS/HRC may be used for either pure or coordinated parallel observations with any other instrument as primary.

The ACS/SBC may be used for coordinated parallel observations with any other instrument as primary, but only if the telescope orientation is exactly specified and the field passes bright-object checking. The ACS/SBC may not be used for pure parallel observations.

Parallel Observations with FGS

The FGS cannot be used as a parallel instrument.

Parallel Observations with NICMOS

NICMOS may be used for either pure or coordinated parallel observations with any other instrument as primary. Note that observations with different NICMOS cameras at the same time are not considered parallel observations.

Parallel Observations with STIS

The STIS/CCD detector may be used for either pure or coordinated parallel observations with any other instrument as primary.

The STIS/MAMA detectors may be used for coordinated parallel observations with any other instrument as primary, but only if the telescope orientation is exactly specified and the field passes bright-object checking. The STIS/MAMA detectors may not be used for pure parallel observations.

Parallel Observations with WFPC2

The WFPC2 may generally be used for either pure or coordinated parallel observations with any other instrument as primary.

Pointing Accuracy for Parallel Observations

The spacecraft computers automatically correct the telescope pointing of the primary observing aperture for the effect of differential velocity aberration. This means that image shifts at the parallel aperture of 10 to 20 mas can occur during parallel exposures.

4.3 Special Calibration Observations

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Data from HST observations are normally provided to the GO after application of full calibrations. Details of the standard calibrations are provided in the Instrument Handbooks (see Section 1.4.3).

In order to obtain quality calibrations for a broad range of observing modes, yet not exceed the time available on HST for calibration observations, only a restricted set, the so-called `Supported' modes, may be calibrated. Other modes may be available but are not supported. Use of these `Available-but-Unsupported' modes is allowed to enable potentially unique and important science observations, but is discouraged except when driven by scientific need. Observations taken using Available-but-Unsupported modes that fail due to the use of the unsupported mode will not be repeated. Use of these modes must be justified prior to the Phase II submission. For details consult the Instrument Handbooks (see Section 1.4.3).

Projects may need to include special calibration observations if either:

• a Supported mode is used, but the calibration requirements of the project are not addressed by the standard STScI calibration program, or
• an Available-but-Unsupported mode is used.

Any special calibration observations required in these cases must be included in the total request for observing time and in the Observation Summary of the proposal, and must be explicitly justified. Proposers can estimate the time required for any special calibration observations from the information provided in the Instrument Handbooks (see Section 1.4.3). Also, the STScI Help Desk (see Section 1.5) can assist you on this estimate, but such requests must be made at least 14 days before the submission deadline.

The data reduction of special calibration observations is the responsibility of the observer.

All data flagged as having been obtained for calibration purposes will normally be made non-proprietary.

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