HST Observation Types
This page describes the different observations types available to proposers. Primary observations encompass any observation taken with the primary instrument on an astronomical target. There is also the opportunity for Parallel observations, which are simultaneous observations with instruments other than the primary instrument. Special Calibration observations can be obtained if standard observatory calibrations are insufficient for the science goals of the program.
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.
Phase I proposals must itemize and briefly justify the special requirements that will be implemented in Phase II, using the Phase I section designated for this purpose. All visit-level special requirements and exposure-level special requirements must be justified (see Preparation of the PDF Attachment).
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 30 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 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.)
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.
CVZ observations must be marked in the Observation Summary section of the proposal with APT. In the PDF attachment two areas must contain further details. In the Description of the Observations section, you must include the number of CVZ opportunities available for each target in your proposal for which you are requesting CVZ time as well as a list of which observations require CVZ time and a justification within the Special Requirements section.
Restrictions on Using 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 orbital visibilities (see the Orbital Visibility section 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 Helpdesk.
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.). ToOs are generally not suitable for observations of periodic phenomena (e.g., eclipsing binary stars, transiting planets, etc.). ToO Proposals must present a detailed plan for the observations to be performed if the triggering event occurs.
Requests for Target of Opportunity observations should not be submitted as Mid-Cycle or Director's Discretionary proposals.
Target-of-Opportunity observations must be marked in the Observation Summary section of the proposal with APT. In the PDF attachment, the Special Requirements section 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 30
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-disruptive (<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 30.
No Ultra-disruptive ToO opportunities will be available in Cycle 30.
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 30. 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. STScI will give proposers a formal deadline for submission of the Phase II. Any subsequent request for changes may jeopardize the execution of the program and will be accepted only under highly exceptional circumstances. Disruptive ToOs require the PI or their designee to be reachable by STScI personnel on a 24-hour basis between the ToO activation and the scheduling of the program.
Proposers may apply for Carry-Over status for ToO Programs only if the target phenomena have a low probability of occurrence during one cycle. This is not to be confused with the Long-Term program category of proposal. The request must be justified in the ‘Special Requirements’ section of the proposal and will be subject to review by the TAC. Carry-Over ToO Programs are valid for up to 2 cycles. If a Carry-Over ToO does not trigger in Cycle 30 the program will be extended to Cycle 31. If there is no trigger in Cycle 31, the program will be terminated.
If the triggering event for a standard ToO Program does not occur during Cycle 30, the program will be deactivated at the end of the cycle. Unused ToO time carries over to the following cycle only for Carry-Over ToO Programs.
Long Term GO Proposals with ToO targets
Long term GO proposals can also contain ToO targets, but they are restricted to non-disruptive ToO targets.
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-disruptive 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.
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.
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. All Phase I proposals must include a discussion of the safety of the proposed targets and fields in the Description of the Observations, 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. 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.
Observers interested in proposing for UV observations of cool stars should keep in mind the possibility that low mass stars may undergo extreme enhancements during stochastically occurring flares. Proposers must demonstrate the health and safety of their targets under these extreme conditions (STIS ISR 2017-02; COS ISR 2017-01).
In the case of aperiodic variables that are either known to undergo unpredictable flares or outbursts, or belong to classes of objects that are subject to flaring or 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 to deal with bright object checking for variable sources must be included in the Special Requirements section of the proposal.
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 STScI Helpdesk for more information on the options and requirements for confirming quiescence.
- Due to bright object safety considerations, STIS/MAMA observations cannot be performed under gyro control. This limitation is generally identified at the Phase II level when guide-star control is found to be infeasible (e.g., for fast moving targets such as comets near perihelion). Infeasible programs cannot be executed (see HST Proposal Implementation and Execution under Unschedulable or Infeasible Programs).
- 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.
- Pure Parallel observations with COS, STIS/MAMA, or the ACS/SBC detectors are not permitted.
- 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.
- 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 the Contemporaneous Calibrations Section of the STIS Instrument Handbook.
- 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, the use of all four FP-POS positions is required for each FUV CENWAVE setting unless a strong scientific justification is provided in the Phase I proposal. The exception is the G130M/1291 setting, for which only two FP-POS positions (3 and 4) are allowed (and required). 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 the COS section of the HST Primer.
Solar System Targets
HST can observe most targets within our Solar System, although there are a few exceptions. Mercury is always well within the 54-degree Solar pointing exclusion, and cannot be observed. Venus is always within the 54-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 54 degree limit. GO and DD proposals to observe Venus will not be permitted in Cycle 30. Observations of comets can be made while they are farther than 55 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 30 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.
With the current performance of the pointing control system, the gyro bias drift must be updated periodically, and this is not possible when pointing under gyro control or during slewing (e.g., during moving target tracking). For moving target programs, visits cannot be longer than two contiguous orbits.
Pointing constraints are discussed further in the HST Primer.
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.
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 the HST Primer)
- observations coordinated with another observatory
Any requests for time-critical observations must be listed in the Special Requirements section of the proposal. 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 the Orbital Constraints Section of the HST Primer). Proposals to observe such events can therefore be accepted only conditionally.
Undithered Observations with ACS and WFC3
Experience has shown that ACS and WFC3 imaging observations are best taken as dithered exposures. 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. 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.
Since 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, 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.
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 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 must consult the Parallel Observations User Information Report, which provides further details on how coordinated and pure parallels are defined, implemented and scheduled.
Parallel observations are rarely permitted to interfere significantly with primary observations; this restriction applies both to concurrent and subsequent observations. Specifically,
- A parallel observation cannot dictate how the primary observation will be structured (e.g. it cannot cause the adjustment of primary exposures). This is particularly directed toward pure parallels where the definition of the observations is independent of and subordinate to a primary observation.
- Parallel observations will not be made if the stored command capacity or data volume limits would be exceeded.
- Pure Parallel observations may not explicitly constrain the scheduling of the primary observations, that is, they may not specify orientation or timing constraints.
- Coordinated Parallel observations may include orientation or timing constraints as requested and justified in the accepted HST Phase I proposal.
- Pure Parallel observations are subject to the availability of parallel observing opportunities as identified by STScI.
Coordinated Parallel Observations
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 generally not allowed to add Coordinated Parallel observations in Phase II that were not explicitly included and approved in Phase I. Any such requests will be adjudicated by the Telescope Time Review Board (TTRB). Coordinated Parallel Observations will ordinarily be given the same exclusive access period as their associated primary observations.
Coordinated Parallel observations must be marked in the Observation Summary section of the proposal.
Pure Parallel Observations
The Pure Parallel observing process is designed to take advantage of the full complement of instruments installed in SM4. Similar to primary science planning, the parallels process provides a reliable estimate, in advance of observations, of the number of orbits that will be executed on accepted parallel programs during the cycle. The Parallel Observing User Information Report provides a complete description of this observing mode and is required reading if you are considering submitting a Pure Parallel Proposal. It is anticipated that up to 500 orbits of Pure Parallel observations will be available in Cycle 30.
Pure Parallel observations are currently restricted to orbits where COS and STIS are the primary instruments. Consequently, parallel opportunities will be limited by the actual number of orbits allocated to these instruments and to the corresponding regions of sky being observed. Past experience shows that the final allocation of Pure Parallel orbits also depends on the science goals of the parallel programs (e.g. desired targets may not be available and multiple Pure Parallel Programs can compete for the same primary opportunities.) STScI continues to investigate ways to expand the number of Pure Parallel observing opportunities.
For the purpose of Pure Parallel orbit allocation, an orbit is defined as having visibility of at least 2500 seconds. The number and types of parallel observing opportunities will vary depending on the mix of primary GO Programs each cycle. Additionally, the total number of Pure Parallel orbits actually executed could be less than planned due to changes to the Primary Programs or on-board execution failures. PIs with accepted Pure Parallel Programs will be given a list of parallel science opportunities that STScI has identified as being suitable for their program. The PI then selects and submits a final list of opportunity matches to STScI in the Phase II Pure Parallel Program submission.
The process of matching Pure Parallel observations to Primary Programs will occur during the planning and implementation phase (Phase II) so that it can be known in advance when and how the parallel observations can be executed. Proposals for Pure Parallel observations may specify either particular or generic targets, although the latter are more common and provide more flexibility for matching parallel observations to actual opportunities.
Review and Execution
The review panels and the TAC will select the programs based on the proposed science. The TAC will consider all accepted programs and produce a ranked list as an aid for resolving potential conflicts. The exclusive access period for a GO Pure Parallel Program will depend on the number of orbits requested, as is the case for Primary GO Programs. Medium (35-74 orbits) Pure Parallel Programs will have a default exclusive access period of 6 months; Large (75 orbits or more) Pure Parallel Programs will have no exclusive access period by default. Pure Parallel observations are assigned to specific primary observations, and the parallel observations will be carried over to subsequent cycles if the primary observations are not executed in Cycle 30.
Restrictions and Limitations on Parallel Observations
Parallel Observations with ACS
- The ACS/SBC may not be used for either Pure or Coordinated Parallel observations in any mode.
- The ACS/WFC detector may be used for Coordinated Parallel observations with any other instrument as primary.
- The ACS/WFC may be used for Pure Parallel observations with the COS and STIS instruments as primary.
Parallel Observations with COS
- The COS/FUV MCP detector may be used for Coordinated Parallel observations with any other instrument as primary, provided that the telescope orientation is specified exactly and the parallel field passes bright-object checking.
- The COS/NUV MAMA detector may be used for Coordinated Parallel observations with any other instrument as primary, provided that the telescope orientation is specified exactly and the parallel field passes bright-object checking.
- COS may not be used for Pure Parallel observations in any detector mode.
Parallel Observations with FGS
- The FGS cannot be used for either Pure or Coordinated Parallel observations.
Parallel Observations with STIS
- The STIS/CCD detector may be used for Coordinated Parallel observations with any other instrument as primary.
- Neither the STIS/NUV-MAMA PRISM mode nor any STIS/MAMA imaging mode can be used for Coordinated Parallel observations.
- STIS/MAMA spectroscopic modes (other than the NUV/PRISM) may be used for Coordinated Parallel observations, but only if an exact ORIENT is specified, and the field passes bright object checking.
- STIS may not be used for Pure Parallel observations in any detector mode.
- When STIS is the primary instrument and another instrument is used for a Coordinated Parallel, STIS auto-wavecals will never be done during an occultation. Instead these calibration exposures have to be scheduled when the external target is visible, leading to a slight reduction in the observing efficiency.
Parallel Observations with WFC3
- WFC3 may be used for Coordinated Parallel observations with any other instrument as primary. WFC3 may only be used for Pure Parallel observations with COS or STIS 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.
Special Calibration Observations
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.
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.
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 justified explicitly. During the Phase II process, proposals to calibrate Available-but-Unsupported modes must be pre-approved by the appropriate instrument team. For details please consult the relevant Instrument Handbook.
Proposers can estimate the time required for any special calibration observations from the information provided in the Instrument Handbooks. Also, the STScI Help Desk 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.
Data flagged as having been obtained for calibration purposes will normally be made non-exclusive access.