HST Call for Proposals and HST Primer for Cycle 24

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4.1	Primary Observations

4.1.1 Continuous Viewing Zone (CVZ) Observations

Restrictions on Using the CVZ

4.1.2 Target-of-Opportunity (ToO) Observations

Turn-Around Time and ToO Limits in Cycle 24

Activation of a ToO

Long-Term ToOs

ToO Programs with COS, STIS/MAMA or ACS/SBC

4.1.3 Special Restrictions on Observations with COS, STIS/MAMA and ACS/SBC

Observations of variable sources

Additional restrictions

4.1.4 Solar System Targets

4.1.5 Observations of Targets That Have Not Yet Been Discovered or Identified

4.1.6 Time-Critical Observations

Limitations Related to Time-Critical Observations

4.1.7 Dithering strategies with ACS and WFC3

Primary observations are those observations that determine the telescope pointing and orientation. GO and 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.

4.1.1 Continuous Viewing Zone (CVZ) Observations

Most targets are occulted by the Earth during a portion of the 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. Please refer to the HST Orbital Viewing and Schedulability webpage for information on determining the number of CVZ opportunities in Cycle 24 and their approximate duration for a given target location. Passages of HST through the South Atlantic Anomaly generally restrict the length of uninterrupted observations to 5 to 6 orbits per day. See Section 2.2.1 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 your orbit request can likely be accommodated. (It is not possible, at present, to determine the exact number of CVZ orbits available during a particular opportunity.) In the Description of the Observations 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 orbital 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 8.16).

Restrictions on Using the CVZ

Observations that require special timing requirements (including telescope orientation constraints; see Section 4.1.6) should not be proposed for execution in the CVZ, and orbit estimates in the Phase I proposal should be based on standard orbital visibilities (see Table 6.1 of the HST Primer). 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 unless the Special Requirement will be relaxed. 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 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 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 8.16). 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 and ToO Limits in Cycle 24

The turn-around time for a ToO observation is defined as the time between STScI receiving a ToO activation and the execution of the observations. The HST observing schedule is updated weekly, and construction of each weekly calendar starts approximately eleven days in advance of the first observations on that calendar. Thus, in the normal course of events, almost three weeks can elapse between Phase II submission of a ToO and execution of the observations. 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. ToOs are therefore classified into two categories: disruptive ToOs that require observations on a rapid timescale and therefore revisions of HST observing schedules that are either active or in preparation; and non-disruptive ToOs that can be incorporated within the standard scheduling process. Disruptive ToOs are defined as those having turn-around times of less than three weeks. Non-disruptive ToOs have turn-around times longer than three weeks.

Disruptive ToOs: The minimum turn-around time for ToO activation 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. Any required bright object screening (COS, STIS/MAMA, or ACS/SBC) must be completed before a ToO can be placed on the schedule. The ability to perform any bright-object check will depend on the quality of the flux information provided by the observer, the complexity of the field, and the availability of suitable expertise at STScI to evaluate that information on a short time scale. Under exceptional circumstances, it may be possible to achieve shorter turn-around times, but only at the expense of significant loss of observing efficiency. Ultra-rapid (<2 day turn-around) ToOs therefore require an extremely strong scientific justification, and may only be requested for instruments that do not require bright object checking (ACS/WFC, WFC3, STIS/CCD, FGS). Because of the significant effect disruptive ToO observations have on the HST schedule, the number of activations will be limited to eight in Cycle 24; this allocation will include no more than one Ultra-rapid ToO.

Non-disruptive ToOs: Observations of transient phenomena that require turn-around times longer than three weeks can be accommodated in the normal HST scheduling process. Non-disruptive ToOs will be incorporated in the HST observing schedule at the earliest opportunity consistent with normal scheduling process. Consequently, there is no limit on non-disruptive ToOs in Cycle 24. However, programs that have been allocated a specific number of non-disruptive ToOs may not subsequently request activation on shorter timescales.

Proposers are encouraged to check the ToO webpage for further information and examples on defining and activating ToO observations.

Activation of a ToO

A Phase II proposal must be submitted before the ToO event occurs. If the observing strategy depends on the nature of the event, then the Phase II proposal 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. Disruptive ToOs require 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.

Long-Term ToOs

Proposers may apply for Long-Term status for ToO Programs only if the target phenomena have a low probability of occurrence during one cycle. The request must be justified in the ‘Special Requirements’ section of the proposal (see Section 9.3) and will be subject to review by the TAC. Long-Term ToO Programs will be extended into the following cycle.

If the triggering event for a standard ToO Program does not occur during Cycle 24, the program will be deactivated at the end of the cycle. Unused ToO time carries over to the following cycle only for Long-Term ToO Programs.

ToO Programs with COS, STIS/MAMA or ACS/SBC

ToO Programs that use COS, the STIS/MAMA detectors, or ACS/SBC must pass bright-object checking before they can be scheduled. Ultra-rapid turn-around programs are not allowed with these instruments. For rapid turn-around programs, 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).

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 five orbits (see Section 2.2.2 of the HST Primer).

4.1.3 Special Restrictions on Observations with COS, STIS/MAMA and ACS/SBC

The COS, STIS/MAMA, and ACS/SBC instruments employ photon counting detectors and are vulnerable to damage through exposure to bright sources. Consequently, there are a number of restrictions on the use of these configurations. All targets and field objects within the appropriate field of view must pass bright-object safety reviews (see Section 5.1 of the Primer). All 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 relevant Instrument Handbook sections and calculations with the appropriate APT and ETC tools.

Observations of variable sources

Proposals to observe variable objects with the COS, STIS/MAMA, or ACS/SBC detectors must pass bright-object checking before they can be scheduled (see Section 5.1 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 bright object limits during outburst. If a violation is possible, the proposer must outline a strategy that will ensure that the target is safe to observe with COS, STIS/MAMA, or ACS/SBC.

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 observation requires extra resources. STScI reserves the right to limit the number of visits requiring quiescence-verification observations within 20 days or less of an HST observation to no more than 12 such visits per Cycle. If you are planning such observations, please contact the Help Desk at help@stsci.edu for more information on the options and requirements for confirming quiescence.

Additional restrictions

•	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 five orbits (see Section 2.2.2 of the HST Primer).

•	Pure Parallel observations with COS, STIS/MAMA, or the ACS/SBC detectors are not permitted.

•	SNAP Programs using the ACS/SBC are not permitted.

•	SNAP Programs using STIS/MAMA imaging modes or the STIS/NUV-MAMA PRISM modes are not allowed. SNAP Programs are allowed to use all other STIS/MAMA spectroscopic modes and all STIS/CCD modes.

•	The total number of targets accepted from all SNAP Programs for COS and STIS/MAMA will be limited to 150.

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In order to preserve SAA-free orbits for MAMA observations, STIS programs that contain both CCD and MAMA science observations (excluding target acquisitions) must normally be split into separate CCD and MAMA visits. Exceptions are allowed if at least one of the following conditions apply: A) There is less than 30 minutes of science observing time (including overheads) using the CCD; B) The target is observed for only one orbit; C) There is a well-justified scientific need for interspersed MAMA and CCD observations.

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By default, STIS spectroscopic exposures are accompanied by separate AUTO-WAVECAL exposures. The observer can insert additional GO-WAVECAL exposures adjacent to any external exposure and, although not recommended without adding an equivalent GO-WAVECAL exposure, can turn off the AUTO-WAVECAL exposures. For additional information see Section 4.5 of the Primer.

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To optimize the science return of COS the following is recommended: the use of TIME-TAG mode and the use of the default wavelength calibration procedures. To minimize the effects of gain sag on the FUV detector it is required that all four FP-POS positions be used for each CENWAVE setting. This is done using the FP-POS=ALL parameter in APT for each CENWAVE, by spreading out the four FP-POS positions over multiple orbits within a visit for each CENWAVE, or over multiple visits of the same target. Observers who wish to employ non-optimal observing techniques must strongly justify their observing strategy in the Description of the Observations section of the PDF attachment. Non-optimal observing techniques should not normally be adopted solely for the purpose of producing a modest reduction of the observational overheads; in such cases the observer should normally just request adequate time to use the recommended optimal strategy. For more details, please see Section 4.4 of the Primer.

4.1.4 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 cannot be observed. Venus is always within the 50-degree Solar pointing exclusion, but at maximum elongation can be over 45 degrees from the Sun. STScI and the HST Project at GSFC have developed (and used) procedures that support observations of Venus when it is slightly within the 50 degree limit. Those 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. Observations of comets can be made while they are farther 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. However, lunar observations are possible under gyro control in three-gyro mode. GO proposals to observe the Moon can be submitted for consideration by the Cycle 24 TAC. These programs must use observing strategies that have been used in previous HST lunar observing programs. The execution of lunar observations will be subject to the availability of resources to carry out the extra work required. Investigators interested in proposing for lunar observations are encouraged to consult the Lunar Observations User Information Report, which contains details on how such observations will be scheduled, the rules pertaining to them, and other useful information.

Pointing constraints are discussed further in Section 2.3 of the HST Primer.

4.1.5 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., targets with unknown coordinates, such as the next supernova in our own Galaxy, or the next gamma-ray burst in the southern hemisphere). 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.

4.1.6 Time-Critical Observations

Proposals may request that HST observations be taken 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 2.4 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 2.2.3 of the HST Primer). Proposals to observe such events can therefore be accepted only conditionally.

4.1.7 Dithering strategies with ACS and WFC3

Experience has shown that ACS and WFC3 imaging observations are best taken as dithered exposures (see Section 5.4 of the HST Primer). Proposers who do not intend to use dithering for primary observations 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 ACS or WFC3 detectors will not be approved without strong justification that such an approach is required for the scientific objectives. Otherwise, hot pixels and other detector artifacts may compromise the archival value of the data.