This page describes all proposal types in detail. Proposals can be General Observer (GO) or Archival (AR) in nature, and can be joint projects with other observatories. STScI also accepts Director's Discretionary time proposals.

Overview of Proposal Categories

HST observations can be requested with a General Observer (GO) or a Snapshot (SNAP) Proposal. A GO Proposal in Cycle 33 can be Very Small, Small, Medium, Large, Calibration, Future-Cycle, or Treasury. Subject to the availability of funds from NASA, STScI will provide financial support for U.S. PIs and Co-Is of approved programs. Funding for projects that do not require new HST observations can be requested with an Archival Research (AR) Proposal. An AR proposal can be a Regular AR, Calibration AR, or Theory Proposal. AR proposals can also take advantage of Cloud Computing for compute-intensive analyses.

Proposals can request observing time on Chandra, JWST, NOIRLab telescopes, NRAO facilities, TESS and XMM-Newton, in conjunction with requests for HST observations. Investigators may also request Director’s Discretionary (DD) time at any time for unanticipated and scientifically compelling astronomical observations.

All proposals are peer reviewed. The panels have separate orbit allocations for Small proposals and Medium proposals, based on the submitted orbit pressure. Archival and Snapshot proposals are ranked with the GO proposals. The Large/Treasury proposals are reviewed separately. Further details on the peer review process are described in HST Proposal Selection Procedures.

General Observer (GO) Proposals

A GO Proposal may be submitted for any amount of observing time, counted in terms of HST Orbits. GO Proposals are classified as Very Small (1-15 orbits), Small (16-34 orbits), Medium (35-74 orbits), and Large (75 or more orbits). The proposal categories are intended to ensure that compelling science programs of different sizes have comparable success rates. The HST Primer's Orbit Calculation Overview page describes how the required number of orbits can be calculated for a particular set of observations. 

Proposals in each of these categories can request observing time in future cycles when this is scientifically justified (Future-Cycle proposals). The additional category of Treasury Proposals is designed to stimulate certain types of ambitious and innovative proposals that may not naturally fit into the Very Small, Small, Medium, or Large Proposal categories. 

Proposers should note that all HST observations are accepted with the understanding that the timescale on which the observations will actually be obtained will depend on scheduling opportunities and demands on HST resources. Experience has shown that programs with scheduling constraints may require execution over an extended period.

In general, proposals are either accepted or rejected in their entirety. Accordingly, you are urged to request the actual number of orbits required to achieve your science goals.

Subject to the availability of funds from NASA, STScI will provide financial support for U.S. PIs and Co-Is of approved programs. For accepted proposals, a Program Management Plan is required with the budget proposal submissions (see HST Grant Funding and Budget Submissions).

Very Small GO Proposals

Very Small GO Proposals are those that request between 1 and 15 orbits. Depending on the subject area, Very Small proposals may be reviewed by External Review or Discussion Review. However, the orbit allocations for these proposals are set such that the success rate is comparable for all proposal sizes.

It is anticipated that up to 1300 orbits will be divided proportionally between the Very Small and Small Proposals in Cycle 33. 

Small GO Proposals

Small GO Proposals are those that request between 16 and 34 orbits. Small proposals are reviewed by Discussion Review. Once again, the orbit allocations for these proposals are set such that the success rate is comparable for all proposal sizes.

Medium GO Proposals

Medium GO Proposals are those that request between 35 and 74 orbits.

It is anticipated that up to 650 orbits in total will be available for the allocation to Medium Proposals in Cycle 33. The orbit allocations are for guidance, and the Executive Committee has the ability to to recommend an increase or decrease in the Medium proposal allotment for scientific balance.

Large GO Proposals

Large GO Proposals are those that request 75 orbits or more.

Large Programs should lead to a clear advance in our understanding in an important area of astronomy. They must use the unique capabilities of HST to address scientific questions in a comprehensive approach that is not possible in smaller time allocations. Selection of a Large Proposal for implementation does not rule out acceptance of Medium Proposals to do similar science, but target duplication and overall program balance will be considered.

Approximately 650 orbits will be available to new Large and Treasury Proposals in Cycle 33. Descriptions of previous programs are available on the Treasury, Archival Legacy and Large (TALL) Programs webpage. Most Large Proposals accepted in previous cycles were allocated between 100 and 150 orbits, although larger orbit requests are welcome if scientifically justified.

Data taken for all Large Programs have no exclusive access period as a default. Proposers may request an exclusive access period, and that request should be justified in the ‘Special Requirements’ section of the proposal. Such a request will be subject to review by the TAC.

Investigators submitting Large Proposals should consult the Large Program Scheduling User Information Report (pdf) and the HST Orbital Viewing and Schedulability page. These documents contain necessary information for developing a Large Program that is feasible with respect to HST orbit scheduling. A target’s orbital visibility depends on its declination and varies with HST's 56-day orbit precession. In Phase I, the minimum visibility period must be used for planning Large programs in order to maximize scheduling flexibility. This minimum visibility (SCHED 100) will be enforced in Phase II as well. Proposers should include additional technical detail on the scheduling aspects of their program in the ‘Description of Observations’ section. The shorter orbital visibility will be enforced in Phase II for each approved Large Program.

Detailed Program Management Plans will be required with the budget submissions (see HST Grant Funding and Budget Submissions), and are not requested with the Phase I submission (see HST Anonymous Proposal Reviews).

Future-Cycle GO Proposals

Very Small, Small, Medium, Large, and Treasury GO Proposals may request HST observing time in more than one cycle if a clear scientific case is made.

Future-Cycle Proposals must be limited to cases where long-baseline, multi-epoch observations are clearly required to optimize the scientific return of the project.

Future-Cycle Proposals require a long time baseline, but not necessarily a large number of HST orbits, in order to achieve their science goals. Examples include astrometric observations or long-term monitoring of variable stars or active galactic nuclei.

You may request time in up to three observing cycles (Cycle 33, Cycle 34, and Cycle 35), or in up to five observing cycles under the Long-Term Monitoring Initiative. Future-Cycle Proposals should describe the entire requested program and provide a cycle-by-cycle breakdown of the number of orbits requested. The Cycle 33 review panels and TAC will only be able to award a limited amount of time in future cycles, so a detailed scientific justification for allocating time beyond Cycle 33 must be presented. Scheduling concerns are not a sufficient justification. The sum of all orbits requested in all cycles determines whether a Future-Cycle Proposal is Small, Medium or Large. Only the orbits requested for Cycle 33 count against the Cycle 33 orbit allocation available to the TAC.

Certain Target-of-Opportunity (ToO) Observations are eligible to be in Future-Cycle Proposals.  Specifically, non-disruptive ToO requests are allowed across all requested cycles within a Future-Cycle Proposal.  Disruptive ToO requests, however, are allowed only in the current cycle within a Future-Cycle Program (i.e., Cycle 33).  Future-Cycle Proposals may not request disruptive ToO observations within its subsequent cycles (i.e., in Cycle 34 or Cycle 35).  Finally, Carry-Over ToO requests are not permitted within Future-Cycle Proposals at all.  Details about these different types of ToOs can be found on the HST Observation Types page.

In APT, the cycle-by-cycle breakdown needs to be entered in the APT Proposal Information form using the "Future Cycles" menu. If this field is not marked and filled out, the future-cycle request might not be met, even if it is described in the proposal. The APT Observations folder should only contain the observations corresponding to the current cycle (i.e., Cycle 33). APT files for time awarded in the subsequent cycles (i.e., Cycle 34 and Cycle 35) will be requested at a later time.

GOs with approved Future-Cycle Proposals need not submit continuation proposals in the subsequent cycles (and hence, GOs who had Cycle 33 time approved in Cycle 32 or Cycle 31 do not have to submit a Phase I continuation proposal, although a new Phase II and budget submission will be required for each cycle).

Budget requests submitted for the first cycle of a Future-Cycle Proposal should include costs only for the effort to reduce and analyze the data obtained in the first cycle. Separate budget proposals are required in each subsequent cycle; see Grant Funding and Budget Submissions.

Treasury GO Proposals

Treasury Proposals are those designed to create datasets of lasting value to the HST project that should be obtained before HST ceases operations. A Treasury Program is defined by the following characteristics:

• The program should focus on the potential to solve multiple scientific problems with a single, coherent dataset. It should enable a variety of compelling scientific investigations.

• Enhanced data products are desirable to add value to the data. Examples are reduced images, object catalogs, or collaborative observations on other facilities (for which funding can be provided). Funding for the proposed data products will depend on their timely availability, as negotiated with the STScI Director. They should be delivered to STScI in suitable digital formats for further dissemination via the HST Data Archive or related channels.

• Data taken under a Treasury Program will usually have no exclusive access period (see Data Rights), although brief exclusive access periods may be requested if that will enhance the public data value. Such requests are subject to TAC approval.

The following additional characteristics are particularly encouraged:

• Development of new techniques for observing or data reduction.

• Creation and dissemination of tools (software, web interfaces, models, etc.) for the scientific community to work with the data products.

The emphasis in Cycle 33 remains on observations whose value is maximal if taken soon. However, Treasury Proposals may request observing time to be distributed in future cycles if scientifically required (similar to the situation for Medium and Large Future-Cycle GO Proposals). In this cycle approximately 650 orbits of HST time will be available for new Large and Treasury Proposals. Descriptions of previous Treasury Programs are available on the HST Treasury, Archival Legacy and Large Programs webpage.

Treasury Programs will be selected by the TAC as part of the normal peer review process. Successful proposals will be reviewed by STScI to ensure observing efficiency. STScI resources may be made available to approved Treasury Programs by decision of the STScI Director. In particular, some programs require substantial pipeline processing of their data to generate the final products. Examples are large mosaics for surveys, or co-additions of many exposures in deep fields.

Investigators submitting Treasury Proposals must select the Treasury Program flag on the APT Proposal Information form, use an orbital visibility that enhances schedulability (use the Increase Schedule Flexibility flag in APT), and include additional technical details on the scheduling aspects of their program in the “Description of the Observations” section of the "Scientific Justification". Note that a proposal can be, but does not need to be, both Large and Treasury. Submitters of Large Treasury Proposals should consult the Large Program User Information Report, which can be found on the HST Documents webpage (linked from the Cycle 33 Announcement webpage.) This document contains a discussion of the issues surrounding Large Program scheduling.

The Scientific Justification section of the proposal (see HST Preparation of the PDF Attachment) should include a description of the scientific investigations that will be enabled by the final data products, and their importance. The Description of the Observations section of the proposal should not only describe the proposed observations and plans for data analysis, but should also describe the data products that will be made available to STScI and the community, the method of dissemination, and a realistic time line.

Detailed Program Management Plans should be submitted with the budget submissions (see HST Grant Funding and Budget Submissions).

Calibration GO Proposals

HST is a complex observatory, with many possible combinations of observing modes and spectral elements on each instrument. Calibrations and calibration software are maintained by STScI for the most important and most used configurations. However, STScI does not have the resources to calibrate fully all potential capabilities of all instruments. On the other hand, the astronomical community has expressed interest in receiving support to perform calibrations for certain uncalibrated or poorly calibrated modes, or to develop specialized software for certain HST calibration and data reduction tasks. In recognition of this, STScI is encouraging outside users to submit Calibration Proposals, which aim to fill in some of the gaps in our coverage of the calibration of HST and its instruments.

Calibration Proposals should be identified by checking the Calibration box on the Proposal Information form. 

Calibration Proposals should not be linked explicitly to a specific science program, but should provide a calibration or calibration software that can be used by the community for existing or future programs. A specific science program that has special calibration requirements is not a Calibration Proposal; such a proposal should be submitted as a normal GO Proposal and the necessary calibration observations should be included in the science program.

Users submitting Calibration Proposals must contact the appropriate instrument group via the STScI Helpdesk to discuss their program prior to submission at least two weeks prior to the proposal deadline (preferably sooner). This step is required to determine any overlaps with the proposed program and current or upcoming STScI-led calibration programs. Failure to contact the instrument team via the STScI Helpdesk at least two weeks prior to the proposal deadline will result in automatic rejection of the proposal. In addition to the normal peer review process, Calibration Proposals will be also reviewed internally at STScI by the Instruments Division. The internal review will provide the TAC with an assessment of the feasibility of the proposal and how the proposal complements or extends the existing calibration program.

Successful proposers will be required to deliver documentation, data products, and software products (depending on the case) to STScI to support future observing programs or archival research. Proposers should summarize the relevance and overall scientific utility of the calibration techniques and products described in their proposal. Funding is available to support Calibration Proposals in the same manner as for normal science programs, with the exception that scientists affiliated with STScI are not eligible for any funding to support their role (as PI or Co-I) in a Calibration Proposal.

Proposed science programs that have special calibration requirements should not be submitted as Calibration Proposals, and should instead be submitted according to its size categorization (Very Small, Small, Medium, or Large). Such a proposal should describe the necessary calibration observations in the PDF attachment (see HST Preparation of the PDF Attachment). 

Investigators interested in the submission of a Calibration Proposal are encouraged to study the Instrument Handbooks to determine the level at which STScI provides calibration and characterization. Examples of the kinds of topics that have been addressed by previous Calibration Programs include:

• Calibration of faint photometric standards for ACS and WFC3.
• ACS photometric zero point verification.
• Calibration of the ACS emission line filters

See the HST Scientific Instruments webpage for a complete description of the instrument calibration plans and accuracies, and for other potential topics.

The data obtained for a GO Calibration Proposal will nominally be non-exclusive access, as is the case for regular calibration observations. Proposers may request an exclusive access period (which should be explained in the ‘Special Requirements’ section of the proposal), but such a request will be subject to panel and TAC review and will be granted only in exceptional circumstances. Calibration Proposals may also be submitted as Snapshot Proposals or Archival Proposals. AR Proposals are appropriate in cases where the necessary data have already been taken, or for programs that do not require specific data but aim to develop specialized software for certain HST calibration and data reduction tasks.

Combined GO-Archival Programs

In past cycles, we required separate GO and AR proposals for programs that included new observations and substantial analysis of HST archival data so that both could be funded at an appropriate level. We are now offering the Combined GO-Archival option for research programs where substantial effort (>10%) will be devoted to analyzing HST archival data.  Combined GO-Archival proposals should include an Analysis Plan for the archival data.  Both the GO and the Archival science must be clearly described and justified.

Proposers should select the GO-Archival flag on the APT Proposal Information form to identify the combined nature of the proposal.

The GO categories Very Small, Small, Medium, Large, Calibration, Future-Cycle, or Treasury are all permitted; proposals sizes will be determined by the GO orbit request. The chosen GO category determines the page limit for the PDF attachment; see Page Limits for PDF Attachment for details.

Proposers will also be able to select appropriate AR flags for the proposal, as detailed in Archival Proposals below.

Snapshot (SNAP) Proposals

Snapshot (SNAP) Programs consist of separate, relatively short observations that are required to have a duration of no more than 45 minutes per visit including all  overheads except for the final data buffer dump which can be pushed into occultation or be done during slews. During the process of optimizing the HST observing schedule, the scheduling algorithm occasionally finds short time intervals where it is impossible to schedule any exposures from the pool of accepted GO Programs. In order to make the HST schedule more efficient, STScI has developed the capability to insert Snapshot exposures of objects selected from a large list of available candidates. In Cycle 33, up to 1000 SNAP observations may be accepted to provide a sufficiently large pool of candidates.

Subject to the availability of funds from NASA, STScI will provide financial support for U.S. PIs and Co-Is of approved programs.

Characteristics of SNAPs

Accepted SNAP Programs are allocated a specific number of targets. However, there is no guarantee that any individual target will be observed, because SNAPs are placed on the schedule only after the observing sequence has been determined for the higher-priority GO targets. The number of observations actually executed depends on the availability of appropriate schedule gaps. In general, only a fraction of the allocated targets will be observed. Unlike GO Programs, SNAP Programs cannot request observing time in future cycles. However, un-executed SNAPs remain active at decreased priority for a second cycle.

There is no commitment on the part of STScI to obtain any specific completion factor for SNAP Programs.

The average expected completion rate for SNAP Programs is ~33%. However, the actual completion rates for individual programs vary, depending on several factors including the number of targets and the average duration and sky distribution of the observations. In general, shorter-duration and well-distributed SNAP observations have a higher number of scheduling opportunities and a higher chance of being executed than longer duration and/or spatially clustered SNAP observations.

Investigators interested in proposing for SNAPs are encouraged to consult the SNAP User Information Report, which contains details on how SNAPs are scheduled, the rules pertaining to them, and other useful information.

Budget proposals for SNAPs should be submitted, and will be reviewed, based on the average completion rate. Subject to availability, supplemental funding may be requested for SNAPs that execute at a higher rate.

Calibration SNAP Proposals

Calibration Proposals may also be submitted as SNAP Proposals. As with GO Calibration Programs, all data obtained will be non-exclusive access unless proposers specifically request an exclusive access period. Successful proposers will be required to deliver documentation, and data products, and software (when applicable) to STScI to support future observing or archival programs.

Users submitting Calibration Proposals must contact the appropriate instrument group via the STScI Helpdesk to discuss their program prior to submission at least two weeks prior to the proposal deadline (preferably sooner). This step is required to determine any overlaps with the proposed program and current or upcoming STScI-led calibration programs. Failure to contact the instrument team via the STScI Helpdesk at least two weeks prior to the proposal deadline will result in automatic rejection of the proposal. In addition to the normal peer review process, Calibration Proposals will be also reviewed internally at STScI by the Instruments Division. The internal review will provide the TAC with an assessment of the feasibility of the proposal and how the proposal complements or extends the existing calibration program.

Calibration Proposals should be identified by checking the Calibration box on the Proposal Information form. 

Guidelines for SNAP Proposals

Please consider the following when developing your SNAP Proposal:

• Your willingness to waive part or all of the exclusive access data-rights period. This willingness is included in the selection criteria.
• You need not give a complete list of all targets and their coordinates in your Phase I proposal. However, you must specify the number of targets, and unambiguously identify the targets (e.g., reference to target lists in papers, or give a detailed description of the target characteristics). SNAP exposures may not be used for targets of opportunity observations. 
• In the ‘Observation Summary’ section of the proposal you should provide a typical example of a SNAP exposure.
• SNAP Programs cannot request observation times longer than 45 minutes, including guide star acquisition and target acquisition. In general, shorter duration SNAP observations have more scheduling opportunities than longer ones.
• All SNAP targets must be submitted by the Phase II deadline, and follow the same procedures for target changes as GO programs.
• SNAP observations should not include any special scheduling constraints (e.g., CVZ or telescope orientation requirements). However, the special requirement BETWEEN may be used in the Phase II Program in some circumstances; for details see the SNAP User information Report.
• A SNAP observation must not have any links to other SNAPs (e.g., relative timing or orientation constraints), even if the SNAPs are of the same source.
• SNAP Programs may not contain identical observations of the same source in different visits, unless there is a scientific motivation for obtaining observations of the same source at different times (e.g., monitoring or follow-up observations). In the latter case, multiple identical visits of the same source may be requested; they should be counted as multiple targets (e.g., 10 different SNAP visits of the same galaxy count as 10 targets). Due to the nature of SNAPs, repeated observations are not guaranteed.
• Moving-target SNAP Programs are acceptable only if the timing requirements are of at least one month duration. Solar system targets interior to the orbit of Jupiter are not permitted. Timing constraints will reduce the chance of a target being scheduled. Due to the amount of effort required in implementing moving target SNAP Programs, these observations ordinarily cannot be revised during the observing cycle, once the initial processing has been completed.
• SNAP Programs with the ACS/SBC are not allowed.
• The total number of proposed spectroscopic COS and STIS/MAMA SNAP targets (other than those using the NUV-PRISM) is limited in a given Cycle to 150 across all programs per instrument. If multiple SNAPs are accepted the targets will be distributed as equitably as possible so that program science goals can still be met. Variable STIS/MAMA and COS SNAP targets must have well-defined MAXIMUM UV fluxes, which will be used for the bright-object checking. There are no restrictions on the numbers or variability of proposed STIS/CCD SNAP targets, which do not require bright-object checking and have a higher expected completion rate since they are not restricted to SAA-free orbits. Thus, use of the CCD NUV configurations should be considered instead of the MAMA NUV when possible.
• STIS/CCD SNAPs are allowed for both imaging and spectroscopic modes.
• STIS/MAMA SNAP Proposals should be limited to one or a few straightforward configurations. Specifically, use of the NDQ filters is not allowed. Use of the 0.2x0.2 echelle aperture is recommended for first-order programs without a scientific long-slit requirement, in order to expedite the field-screening process. Excessively complex STIS/MAMA SNAP targets, fields, or instrumental configurations may not be implemented in Phase II because of the limited resources available for bright-object checking, combined with the relatively low expected completion rate; if you are in doubt on this issue, contact the STScI Helpdesk. 
• Programs that require both GO orbits and SNAP targets should be submitted as two separate proposals. This allows you to ensure that some essential targets are observed (the GO Program) with the rest of the targets being sampled statistically (the SNAP Program). The proposals will be reviewed independently and therefore should ​not refer to each other.
• Because SNAP targets are added to the observing schedule at a late stage of the schedule building process, moving-target SNAP Programs may not use any detector that requires bright object screening (e.g. STIS/MAMA or COS). It is simply not practical to screen the field for any background objects that might violate bright-object screening limits.

Archival Research (AR) Proposals

Observations that are no longer exclusive access are available for analysis by interested scientists through direct retrieval from the HST Data Archive or from the Hubble Legacy Archive (HLA). The retrieval is free and does not involve financial support. The HST Archival Research (AR) Program can, however, provide financial support for the analysis of such data sets. AR Phase I proposals must outline an Analysis Plan for the program. Detailed budgets are due in Phase II only (as is the case for GO Proposals). Proposals for AR funding are considered at the same time, and by the same reviewers, as proposals for observing time, on the basis of scientific merit.

Subject to the availability of funds from NASA, AR Programs by default, will be awarded financial support within 30 days of receipt of the PI Notification Letters (See HST Grant Funding and Budget Submissions), unless the program has explicitly requested a delay in funding in the Phase I Analysis Plan and the request is approved by the STScI Director.

Regular AR Proposals

The general goal of a Regular AR Proposal is to analyze a subset of data from the HST Archive to address a specific scientific issue. The analysis must improve on the previous use(s) of the data, or the scientific questions addressed must differ from those tackled by the original programs that obtained the data.

There is no limit to the amount of funding that may be requested in a Regular AR Proposal. Future funding levels for AR programs will depend on the overall grants allocation. Budget details are not required in the Phase I submission. Detailed Program Management Plans are required with the budget submissions; funded work must be described in the work plan scoped by the Phase I (see HST Grant Funding and Budget Submissions).

An AR Proposal will be considered to be a Regular AR Proposal, unless it is identified with the appropriate flag in the Proposal Information form in APT as a Theory, AR Cloud-Computing, or Calibration AR Proposal. Multiple flags can be selected.

Calibration AR Proposals

Calibration Proposals may be submitted as AR Proposals. AR Proposals are appropriate in cases where the necessary data have already been taken, or for programs that do not require specific data but aim to develop specialized software for certain HST calibration and data reduction tasks. Examples of topics that have been addressed by Calibration Programs of the type discussed here are:

• Calibration of Lyman-alpha flat fields
• Creation of a coronagraphic PSF library for STIS/CCD
• Characterization of the spectroscopic PSF for STIS/CCD

For a complete description of the instrument calibration plans/accuracies, and for other potential topics, please see the Scientific Instruments webpage.

Users submitting Calibration Proposals must contact the appropriate instrument group via the STScI Helpdesk to discuss their program prior to submission at least two weeks prior to the proposal deadline (preferably sooner).

AR Theory Proposals

The opportunity exists under the HST Archival Research Program to obtain financial support for theoretical research. Research that is primarily theoretical can have a lasting benefit for current or future observational programs with HST, and it is appropriate to propose theory programs relevant to the HST mission. 

A Theory Proposal should address a topic that is of direct relevance to HST observational programs, and this relevance should be explained in the proposal. Funding of mission-specific research under the HST Theory Program will be favored over research that is appropriate for a general theory program (e.g., the NASA Science Mission Directorate Astrophysics Theory Program; ATP). The primary criterion for a Theory Proposal is that the results should enhance the value of HST observational programs through their broad interpretation (in the context of new models or theories) or by refining the knowledge needed to interpret specific observational results (a calculation of atomic cross sections may fall under the latter category). The results of the theoretical investigation should be made available to the community in a timely fashion. 

Theory proposals should describe the impact on observational investigations with HST. Review panels will consist of observational and theoretical astronomers with a broad range of scientific expertise. They will not necessarily have specialists in all areas of astrophysics, particularly theory, so the proposals must be written for general audiences of scientists. The "Analysis Plan" section of the proposal should discuss the types of HST data that will benefit from the proposed investigation, and references to specific data sets in the HST Data Archive should be given where possible. This section should also describe how the results of the theoretical investigation will be made available to the astronomical community, and on what time-scale the results are expected.

As with the other AR Proposals, there is no limit to the funding that may be requested in Theory Proposals. Commensurate with the expected scope, Theory Proposals are allowed to be multi-year projects, although this is not a requirement. Multi-year projects will be funded on a yearly basis, with continued funding beyond the first year subject to a performance review. Descriptions of past programs are available on the HST Treasury, Archival Legacy and Large Programs webpage.

The Scientific Justification section of the proposal should include a description of the scientific investigations that will be enabled by the final data products, and their importance. The Analysis Plan section should describe the plans for data analysis, the data products that will be made available to STScI and the community, the method of dissemination, and a realistic timeline.

Detailed Program Management Plans are required with the budget submissions (see HST Grant Funding and Budget Submissions).

AR Cloud Computing Studies

All non-exclusive access data for current Hubble instruments (ACS, COS, STIS, WFC3, FGS), have been made available as part of the Amazon Web Services (AWS) public dataset program (aws.amazon.com/public-datasets/). Providing these data in close proximity to AWS faciliates new types of compute-intensive analyses that may have not previously been possible due to individual researcher or research group compute resources. Proposals to make use of this dataset should select the Cloud Computing check box next to the AR category in APT, and be prepared to include a line item in their budget for AWS costs (limit $10,000).

Example use cases for leveraging this data could include: Large scale (re)analyses of data to measure photometric properties or proper motions, computationally-intensive tasks such as training machine learning classifiers, and live community-facing services.

Further reading:

• Link to HST data on AWS: https://registry.opendata.aws/hst/
• AWS machine learning services: aws.amazon.com/machine-learning/
• AWS spot computing: aws.amazon.com/ec2/spot/spot-and-science/
• Educational & research use cases: aws.amazon.com/government-education/research-and-technical-computing/

AR Data Science Software Proposals

Proposers have an opportunity under the AR Program to obtain financial support for the development of  software products that will be made available to the community for the purposes of analyzing HST data.  Introductory descriptions of the data products created by the HST calibration pipeline and related software tools and links to more details are available on the HST Primer: Data Processing and the HST Data Archive page. Examples of additional products include, but are not restricted to,

• scripts to mitigate artifacts from specific detectors,
• tools to identify and extract fluxes/magnitudes from multiple sources within a field,
• utility software for working with HST data products, 
• or codes to produce background-subtracted spectra or software to interact with HST archive services.

The primary criterion for a Community Data Science Proposal is that the results should broadly enhance the value of HST observational products for anyone in the astronomical community. The results of the data science software development should be made available to the community in a timely fashion through an appropriate distribution platform. Open source software using a standard license (https://opensource.org/licenses) is encouraged. The software should have thorough internal documentation at a level consistent with software best practices, and, if computationally intensive, should be compatible with a cloud computing service. 

There is no limit to the amount of funding that may be requested, but it is expected that the amounts will be at a similar level to those in the Regular AR category. The effort detailed in the Management Plan section of the proposal should be commensurate with the level of funding requested. 

A Community Data Science Software Proposal may be submitted by a non-U.S. PI if there are one or more U.S. Co-Is who request funding. 

The ‘Scientific Justification’ section of the proposal should describe the proposed software plan and also its impact on observational investigations with HST. Review panels will consist of observational and theoretical astronomers with a broad range of scientific expertise. They will not necessarily have specialists in all areas of astrophysics, particularly software development, so the proposals must be written for general audiences of scientists. The ‘Analysis Plan’ section of the proposal should discuss the types of HST data that will benefit from the proposed investigation, and references to specific data sets in MAST should be given where possible. This section should also describe how the results of the investigation will be made available to the astronomical community, and on what time-scale the results are expected.

Guidelines for AR Proposals

Please consider the following when developing your AR Proposal:

• In general, any HST data that you wish to analyze must reside (or be expected to reside) in the Archive, and be released from exclusive access rights by the start of Cycle 33 (November 1, 2025). 
• Users should consult the Large Searches and Requests webpage for information on searching for and downloading large datasets.
• Programs that require funding for Archival Research alongside new observations should be submitted as a single GO Proposal, regardless of the relative size of the AR component. See Combined GO-Archival Proposals for more information.
• Investigators are allowed to submit an AR Proposal to analyze data that was obtained in a previous GO Program on which they were themselves PI or Co-I, but only if the goals of the AR Proposal differ significantly from those for which GO funding was awarded previously.
• STScI encourages the submission of AR Proposals that combine HST data with data from other space-missions or ground-based observatories, especially those data contained in the Mikulski Archive for Space Telescopes (MAST). STScI is an active partner of the Virtual Observatory (VO), and MAST is implementing VO technology to make its data holdings available. In particular, the MAST Data Discovery Portal is available at http://mast.stsci.edu/explore. The Discovery Portal is a one-stop Web interface to access data from all of the MAST supported missions, including HST (in particular the Hubble Legacy Archive- HLA, and Hubble Source Catalog- HSC), TESS, Kepler, GALEX, FUSE, IUE, EUVE, and Swift-UVOT.

Suggestions for AR Proposals

STScI would like to point out the following sources for Archival Research:

• The data being obtained from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program.
• The data being obtained for the Frontier Fields Program.
• The data obtained by the HST Pure Parallel Program.
• The data obtained for the Hubble Deep Field (HDF), the Hubble Deep Field-South (HDF-S) and the Hubble Ultra Deep Field (UDF).
• The data obtained by the HST Treasury Programs, which are described on the HST Treasury, Archival Legacy and Large Programs webpage. Community-contributed high-level science products from imaging and spectroscopic surveys (including GOODS, GRAPES, and GEMS) are available from the MAST High Level Science Product webpage.
• Projects that utilize the Hubble Source Catalog. A few potential examples are listed in the HST Primer.

Joint Observing Programs

STScI has reached agreements with several other observing facilities (Chandra, JWST, NOIRLab, NRAO, TESS and XMM-Newton) to award time for joint programs in which HST science is the prime science, but multi-wavelength observations from another ancillary observatory are vital for the science goals of the proposal. The only criterion above and beyond the usual review criteria is that both sets of data of the same target(s) are required to meet the primary science goals. Joint programs may be for any amount of HST time. Historically, joint proposals have the same success rate as all other proposal types. A description of past HST joint programs is available on the HST Joint Programs webpage.

Proposers are responsible for fulfilling all necessary requirements requested by the partner observatories. This includes noting what information needs to be submitted, meeting the deadlines for submitting that information, and ensuring that contact details are up to date.

Joint HST-Chandra Observing Proposals

If your science project requires observations from both HST and the Chandra X-ray Observatory, you can submit a single proposal to request time on both observatories to either the HST Cycle 33 or the Chandra Cycle 27 review. This avoids the “double jeopardy” of having to submit proposals to two separate reviews. A description of past HST joint programs with Chandra is available on the HST Joint Programs webpage.

By agreement with the Chandra X-ray Center (CXC), STScI will be able to award up to 600 kiloseconds of Chandra observing time. Similarly the CXC will be able to award up to 150 orbits of HST time to highly rated proposals awarded Chandra time in its TAC process. The only criterion above and beyond the usual review criteria is that the project must be fundamentally of a multi-wavelength nature, and that both sets of data are required to meet the science goals. Time will only be awarded to joint proposals if both data sets are required for the proposed science. It is not essential that the project requires simultaneous Chandra and HST observations. Chandra time will only be awarded in conjunction with new HST observations (and should not be proposed for in conjunction with an AR or Theory Proposal). Proposers should take special care in justifying both the scientific and technical reasons for requesting time on both missions.

Of the Chandra observing time that can be awarded in the HST review, only approximately 15% of the observations may be time-constrained. In addition, only one rapid ToO can be awarded (less than 20 days turn-around time). A Chandra ToO is defined as an interruption of a command load, which may include several predictable observations within that one-week load. HST Cycle 33 proposers should keep their Chandra requests within these limits.

Proposals for combined HST and Chandra observations should be submitted to the observatory that represents the prime science (not to both observatories). Large HST-Chandra proposals (or Very Large Programs, as CXC defines them) may be submitted to either, but not both, observatories. STScI reserves the right to disallow HST observations that duplicate those approved via any joint program unless the duplications are justified in the original proposals. While there is multi-wavelength expertise in the review panels for both observatories, typically the HST panels will be stronger in IR/optical/UV science and the Chandra panels in X-ray science.

Establishing the technical feasibility of the Chandra observations is the responsibility of the PI, who should review the Chandra documentation or consult with the CXC. A description of the technical information that should be included in the proposal is given in Joint HST-Chandra Observations. For proposals that are approved by HST, the CXC will perform detailed feasibility checks in Chandra Cycle 25. The CXC reserves the right to reject any previously HST-approved observation that proves infeasible, impossible to schedule, and/or dangerous to the Chandra instruments. Any Chandra observations that prove infeasible or impossible could jeopardize the overall science program and may cause revocation of the corresponding HST observations. Duplicate Chandra observations may also be rejected by the CXC.

The following list on considerations will help a proposer determine the feasibility of their Chandra observations:

1. Chandra estimates the difficulty of observing a given target as Resource Cost (RC). Every non-TOO proposal requesting joint Chandra time should use the RC calculator (https://cxc.harvard.edu/toolkit/rccalc.jsp) to determine the RC value for their observation configuration, which includes target coordinates, instrument set-up, and any constraints required to achieve the science, including coordination with joint facilities. The proposer should provide the total RC for their requested observations in the body of the science justification. Nominal RC values are 1.6 RC units per kilosecond, factors well above this should be carefully justified. Please read the “Resource Cost” section in the Chandra Call for Proposals (CfP) for detailed information.
2. The amount of Chandra exposure time available for High Ecliptic Latitude (HEL) targets with |b_Ecliptic| > 55deg is limited. Very long exposures for HEL targets will incur a higher RC and targets may be rejected for exceeding Cycle-wide HEL time limits.  When proposing HEL targets, a proposer must explicitly note the requested amount of Chandra HEL time in the body of your science justification. Refer to section on HEL targets in the Chandra Call for Proposals (CfP) for detailed information.
3. Observations with the High Resolution Camera (HRC) will be split based on operational limits that restrict observing to 14.5 ks segments with a minimum 30 ks buffer between subsequent HRC observations. Refer to section on HRC in the Chandra Call for Proposals (CfP) for detailed information.
4. The proposers must verify that Chandra will be able to acquire suitable star fields for a given target using the Star Checker tool (https://cxc.cfa.harvard.edu/toolkit/starchecker.jsp).

Joint HST-Chandra Proposals must be identified in the "Coordinated Telescopes" section of the Proposal Information form in APT. Also, you must include technical information about the Chandra observations in the ‘Coordinated Observations’ section of the proposal.

Joint HST-JWST Observing Programs

If a science project requires observations with both the Hubble Space Telescope (HST) and JWST, then a single proposal may be submitted to request time on both observatories to the HST Announcement of Opportunity, so that it is unnecessary to submit proposals to two separate reviews. The proposal should be submitted to the observatory that requires the larger time allocation (where 1 HST orbit is equivalent to 1 JWST hour). Since STScI operates both HST and JWST, the amount of time for HST-JWST Joint Proposals could be revised upwards if the demand is high.

By agreement with the JWST Project, the HST TAC may nominally award 150 hours of JWST observing time. Similarly, the JWST TAC may nominally award 300 orbits of HST time. The time will be awarded only for highly ranked proposals that require use of both observatories and shall not apply to Archival or Theory Proposals. The only criterion above and beyond the usual review criteria is that both sets of data of the same target(s) are required to meet the primary science goals. Proposers should take special care in justifying both the scientific and technical reasons for requesting observing time on both missions. It is not essential that the project requires simultaneous HST and JWST observations.

Joint proposals requesting JWST time may request standard JWST observing modes. Establishing the technical feasibility of the JWST observations is the responsibility of the PI, who should carefully review the JWST Call for Proposals, JWST Documentation, and/or contact the JWST Helpdesk at jwsthelp.stsci.edu. The JWST and HST Helpdesks offer features to search the documentation and to send your question directly to the appropriate team of experts. 

For proposals that are approved by the HST TAC, STScI will perform final detailed feasibility checks. STScI reserves the right to reject any HST-approved Joint Proposal observation that proves infeasible, impossible to schedule, dangerous to the JWST instruments, and/or requires resources beyond those initially approved. Any JWST observations that prove infeasible or impossible could jeopardize the overall science program and may cause revocation of the corresponding HST-observations. Duplicate JWST observations may also be rejected by the STScI.

Regarding Joint Target-of-Opportunity (ToO) proposals, ToO proposals must state explicitly whether the JWST observations require a disruptive ToO (observations within 14 days of notification).  No more than one (1) disruptive JWST ToO via a joint program will be performed per JWST Cycle. Furthermore, Ultra-rapid JWST ToO requests (reaction time 2 days or less) will not be accepted for this program; proposals asking for Ultra-rapid JWST ToO observations must be submitted in response to the JWST Call for Proposals, with JWST as the primary observatory. It is mandatory that the PI informs both observatories immediately if the trigger criterion is fulfilled.  For this solicitation, no JWST time will be allocated without the need for HST time on the same target to complete the proposed investigation.

STScI will contact successful PIs of joint programs. US-based Investigators of HST-JWST Programs are eligible for funding and will be eligible to submit budgets to JWST Grants.

Joint HST-JWST Proposals must be specified in the ‘Coordinated Telescopes’ section of the Proposal Information form of APT with the necessary JWST Hours request. Joint HST-JWST Proposals require both scientific and technical justification of the requested JWST observations, which are detailed on the Coordinated Observations page.

Exclusive Access Periods for HST data and JWST data will be set independently following the policies for each observatory according to proposal size and type.

Joint HST-NOIRLab Observing Proposals

By agreement with the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory (NOIRLab), STScI will be able to award time on NOIRLab facilities to highly ranked proposals that request time on both HST and NOIRLab telescopes. The award of time on NOIRLab facilities will be subject to approval by the NOIRLab Director, after nominal review by the NOIRLab TAC to avoid duplication of programs. Joint HST/NOIRLab Proposals should be submitted to the observatory that represents the prime science facility (but not both). The important additional criterion for the award of NOIRLab time is that both the HST and the ground-based data are required to meet the science goals of the project. Time will only be awarded to joint proposals if both data sets are required for the proposed science. It is not essential that the project requires simultaneous NOIRLab and HST observations. Under this agreement, NOIRLab time will only be awarded in conjunction with new HST observations (and should not be proposed for in conjunction with an AR or Theory Proposal). Major results from these programs would be credited to NOIRLab and HST. A description of past HST joint programs is available on the HST Joint Programs webpage.

NOIRLab has offered up to 5% of its available time to proposals meeting the stated criteria. NOIRLab observing time will be implemented during the two 2025 NOIRLab observing semesters (2025B for August 2025 to January 2026, and 2026A for February to July 2026). Time cannot be requested for the preceding semester, 2025A. Time may be requested only for those facilities listed on the NOIRLab/NASA Collaboration webpage. In addition, time on heavily-subscribed resources may be limited by the NOIRLab Director.

Establishing the technical feasibility of the proposed NOIRLab observations is the responsibility of the PI, who should review the NOIRLab documentation or consult with NOIRLab directly. A description of the technical information that should be included in the proposal is given in Joint HST-NoirLab Observations. If recommended to receive NOIRLab time, the PI must submit the technical description through the standard NOIRLab process by the nominal deadline for semester 2025B. For Gemini proposals, a Gemini PIT proposal must be submitted. Detailed information for Gemini and other telescopes can be found in the NOIRLab Call for Proposals. Proposals not received by the April 1, 2025 deadline for semester 2025B may not be scheduled for NOIRLab time.

NOIRLab will perform feasibility checks, and reserves the right to reject any approved observation determined to be infeasible, impossible to schedule, and/or dangerous to the telescopes or instruments. Any NOIRLab observations that prove infeasible or impossible could jeopardize the overall science program and may cause revocation of the corresponding HST time allocation.

Joint HST-NOIRLab Proposals must be identified in the "Coordinated Telescopes" section of the Proposal Information form in APT. Also, you must include technical information about the NOIRLab observations in the ‘Coordinated Observations’ section of the proposal.

Joint HST-NRAO Observing Proposals

By agreement with the National Radio Astronomy Observatory (NRAO), STScI will be able to award time on NRAO facilities to highly ranked proposals that request time on both HST and NRAO telescopes. NRAO has offered up to 3% of the available time on its North American facilities, namely the Robert C. Byrd Green Bank Telescope (GBT), the Very Large Array (VLA), and the Very Long Baseline Array (VLBA), for allocation by the HST TAC, subject to a maximum of 5% of the available time in any given array configuration. In return, STScI has offered 30 orbits of HST time for allocation by the NRAO TAC to proposals submitted on or before either of the two NRAO semester deadlines. These are the closest dates to February 1 and August 1 that fall on a Wednesday each year. Joint HST/NRAO Proposals should be submitted to the observatory that represents the prime science facility (not to both observatories). STScI reserves the right to disallow HST observations that duplicate those approved via any joint program unless the duplications are justified in the original proposals. A description of past HST joint programs is available on the HST Joint Programs webpage.

NRAO observing time awarded through the HST Cycle 33 review will be implemented during the 2025B and 2026A observing semesters. The award of time on NRAO facilities will be subject to approval by the NRAO Director, after nominal review by the NRAO TAC to avoid duplication of programs. The important additional criterion for the award of NRAO time is that both the HST and the radio data are required to meet the science goals of the project. Time will only be awarded to joint proposals if both data sets are required for the proposed science. It is not essential that the project requires simultaneous NRAO and HST observations. Under this agreement, NRAO time will only be awarded in conjunction with new HST observations (and should not be proposed for in conjunction with an AR or Theory Proposal). Major results from these programs would be credited to NRAO and HST.

Establishing the technical feasibility of the proposed radio observations is the responsibility of the PI, who should review the NRAO documentation or consult with NRAO directly. If approved for NRAO time, the PI must submit detailed observing information appropriate to the relevant NRAO facility. A description of the technical information that should be included in the proposal is given in Joint HST-NRAO Observations.

GBT MUSTANG-2 is a shared risk instrument and proposals using it should include the instrument team. For more information please refer to the MUSTANG-2 Requirements.

NRAO will perform a technical review of proposals approved by the HST TAC, and reserves the right to reject any approved observation determined to be infeasible, impossible to schedule, and/or dangerous to the telescopes or instruments. Any NRAO observations that prove infeasible or impossible could jeopardize the overall science program and may cause revocation of the corresponding HST time allocation. We therefore urge proposers to discuss technical concerns with appropriate staff at both observatories. Discussions with NRAO staff should occur via the NRAO helpdesk.

Proposers must always check whether appropriate archival data exist, and provide clear scientific and technical justification for any new observations of previously observed targets. Observations awarded time that duplicate observations already approved by HST or NRAO for the same time period may be canceled, or data sharing and cooperation among different groups may be necessary, as determined by the two observatories. This includes ToOs with similar trigger criteria, with or without previously known coordinates.

Be aware that some HST targets might not require new NRAO observations because the joint science goals can be met using non-exclusive access archival data from the VLA, VLBA, or GBT that are available at http://science.nrao.edu/facilities/vla/archive. Also note that VLA continuum images from sky surveys at a wavelength of 20cm and at a FWHM resolution of 45 arc seconds (see http://www.cv.nrao.edu/nvss/) or 5 arc seconds (see http://sundog.stsci.edu/top.html) are available.

All scientific data from NRAO telescopes have an exclusive access period where the data are reserved for the exclusive use of the observing team. The data archive policy and exclusive access periods are given at https://science.nrao.edu/observing/policies/docs/manuals/users-policy/preparation-and-execution-of-observations/data-delivery-and-data-rights. This policy applies to NRAO data taken through the joint HST-NRAO program.

Joint HST-NRAO Proposals must be identified in the "Coordinated Telescopes" section of the Proposal Information form in APT. Also, you must include technical information about the NRAO observations in the ‘Coordinated Observations’ section of the proposal.

Joint HST-TESS Observing Proposals

By agreement, STScI will be able to award a limited number of short cadence target slots from the Transiting Exoplanet Survey Satellite (TESS) mission. Currently in its second extended mission, TESS will be observing fields in both the northern and southern ecliptic hemispheres in addition to parts of the ecliptic plane which have not been observed with TESS to date. TESS operates by staring at one part of the sky for a duration of ~27 days in length (known as a Sector), collecting 200 second Full Frame Images (FFIs) which cover a 24 x 96 degree field of view, and 120-second or 20-second cadence Target Pixel Files (TPF) - postage stamps for selected targets of interest. After this ~27 day coverage, TESS slews to observe a different Sector of sky for another ~27 days. All TESS observations are collected in a single, broadband, red- optical bandpass.

Information about the specific TESS pointings for upcoming sectors is available on the TESS General Investigator (GI) Program Office website and the MIT TESS website.

TESS 120-second and 20-second targets selected for postage stamps are pipeline processed for background subtraction, and are delivered to MAST as both pixel products and time-series light curve products with no proprietary period. Calibrated TESS FFI’s are observed at a 200 second cadence, provide no target apertures, and have not had scattered or background light subtracted. TESS FFIs are available to the community via MAST with no proprietary period. Programs requiring finer time sampling than the FFI data and/or pipeline processed time-series products should propose to the joint HST-TESS program and request specific TESS targets. Proposals relying solely on the TESS FFI data do not need to be proposed for through the HST-TESS joint program.

The joint HST-TESS program can allocate 120-second cadence observations for up to 1,000 targets and 20-second cadence observations for up to 50 targets. Proposers should identify which targets should be considered for TESS observations and provide an explicit justification that a 120-second or 20-second cadence is sufficient to achieve their science goals. TESS observations will only be obtained for approved HST targets. There is no guarantee that the HST and TESS observations will be simultaneous. Joint program targets will be observed by TESS no earlier than the beginning of Cycle 8 of TESS observations. Accepted targets will be passed to the TESS GI Program Office by STScI. Additional information about TESS can be found on the TESS GI Program Office website.

Joint HST/XMM-Newton Observing Proposals

If your science project requires observations from both HST and the XMM-Newton Observatory, you can submit a single proposal to request time on both observatories to either the HST Cycle 33 or the XMM-Newton Cycle AO-25 review. Joint HST/XMM-Newton Proposals should be submitted to the observatory that represents the prime science facility (not to both observatories). A description of past HST joint programs with XMM-Newton is available on the HST Joint Programs webpage.

By agreement with the XMM-Newton Observatory, the HST TACs will be able to award up to 150 kiloseconds of XMM-Newton observing time. Similarly the XMM-Newton TACs will be able to award up to 30 orbits of HST time. The only criterion above and beyond the usual review criteria is that the project must be fundamentally of a multi-wavelength nature, and that both sets of data are required to meet the science goals. Time will only be awarded to joint proposals if both data sets are required for the proposed science. XMM-Newton time will only be awarded in conjunction with new HST observations (and should not be proposed for in conjunction with an AR or Theory Proposal). Proposers should take special care in justifying both the scientific and technical reasons for requesting time on both missions.

It is not essential that the project requires simultaneous XMM-Newton and HST observations. No XMM-Newton observations with a reaction time of less than five working days from the trigger date will be considered. Target of Opportunity (ToO) Proposals must state explicitly whether the HST observations require a disruptive ToO. No more than one disruptive ToO will be allocated per proposal. It is the responsibility of the PI to inform both observatories immediately if the trigger criterion is fulfilled.

Proposals for combined HST and XMM observations should be submitted to the observatory that represents the prime science (not to both observatories). STScI reserves the right to disallow HST observations that duplicate those approved via any joint program unless the duplications are justified in the original proposals. The XMM-Newton AO-25 deadline is nominally in early October 2025. While there is multi-wavelength expertise in the review panels for both observatories, typically the HST panels will be stronger in IR/optical/UV science and the XMM panels in X-ray science.

Establishing the technical feasibility of the XMM-Newton observations is the responsibility of the PI, who should review the XMM-Newton Instrument Handbooks. A description of the technical information that should be included in the proposal is given in Joint HST-XMM Observations. All standard observing restrictions for both observatories apply to joint proposals. For proposals that are approved, both projects will perform detailed feasibility checks. Both projects reserve the right to reject any approved observation that is in conflict with safety or schedule constraints, or is otherwise deemed to be non-feasible.

Joint HST/XMM-Newton Proposals must be identified in the "Coordinated Telescopes" section of the Proposal Information form in APT. Also, you must include technical information about the XMM-Newton observations in the ‘Coordinated Observations’ section of the proposal.

Director's Discretionary (DD) Time Proposals

Director's Discretionary (DD) programs provide an opportunity to obtain observations that could not have been proposed for at the last main cycle and cannot wait until the next main cycle, either because there are time constraints or there is exceptional scientific urgency. They are limited in both scope and in time request, and there is typically no exclusive access period. Scientists wishing to request DD time can do so at any time during the year. Please pay careful attention to the guidelines and policies for Director's Discretionary Time Submission before submitting.

DD observations should not generally be requested if any of the following is true:

• The observations could plausibly have been proposed in the most recent regular proposal cycle, possibly as a Target-of-Opportunity Proposal. Requests to reserve ToO targets that have not been discovered would not be appropriate.
• The observations were proposed in a recent regular proposal cycle, and were rejected.
• The proposal observations do not have compelling scientific urgency, and would therefore be more appropriately evaluated by the TAC review process during a standard observing cycle.

Next: HST Special Initiatives