HST Cycle 26 Proposal Roadmap

Step-by-step guide to preparing a Phase I proposal for the Hubble Space Telescope Cycle 26.

Late Breaking News

The call for HST Cycle 26 GO and AR proposals is now closed.

Investigators may still submit proposals for Cycle 26 Mid-Cycle Time or Director's Discretionary Time.

For the current Call for Proposals, see Hubble Space Telescope Call for Proposals for Cycle 27

 Phase I Proposal Roadmap for Cycle 26

This is a high level step-by-step guide to writing a Phase I Proposal; there is also a Phase II Roadmap. For Observing proposals, the Roadmap assumes that you already have a science program in mind, and now need to determine if it can be done with HST, which instrument(s) are appropriate, and how much time it will take. For Archival proposals, it assumes you already have a science program in mind, and now need to determine if the HST archive will support such a program. If you find you want to proceed with an HST proposal, the details of the submission process are given. Contact the STScI Help Desk at hsthelp.stsci.edu if you need assistance with any aspect of proposing for HST time as well as using the Astronomer's Proposal Tool (APT).

The Hubble Legacy Archive (HLA) is designed to optimize science from HST by providing online, enhanced Hubble products and advanced browsing capabilities. In particular, the HLA includes a footprint service that shows where HST has observed on the sky. This tool may be useful in helping you define your science program.


  1. Go to the Announcement Page

    The Announcement Page will provide basic information about submitting an HST proposal (such as the deadline), as well as links to important documents.

  2. Familiarize yourself with the Call for Proposals (CP) and HST Primer

    The Call for Proposals describes the policies and procedures, and explains how to submit a Phase I proposal. The HST Primer provides a basic introduction to the technical aspects of HST and its instruments, and explains how to calculate the appropriate number of orbits for your Phase I observing time requests. Links to the relevant sections of these documents are included in this roadmap, but a general familiarity with these documents is important. In particular, please review the New and Important Features for the Cycle and the Proposal Selection Procedures.

    In recognition of the unique UV capabilities of Hubble coupled with the finite lifetime of the mission, a UV Initiative was introduced in Cycle 21. This initiative will use orbit allocations to increase the share of primary GO observing time dedicated to UV observations.

    In recognition of the upcoming launch of JWST, STScI is encouraging the community to submit JWST Preparatory proposals for observations that will complement and enhance the scientific impact of future JWST observations.

  3. Determine the proposal category

    There are several types of HST proposals:

    • General Observer (GO) programs request new observations with HST, and can be Medium (35-74 orbits), Large (75 or more than orbits), Calibration (requesting new observations), Long-Term (more than one cycle), and Treasury (Large scale projects). Small proposals (fewer than 35 orbits) will not be accepted in Δ Cycle 26.  For important information on orbital viewing and schedulability issues (particularly important for Large and Treasury programs), see the HST Orbital Viewing and Schedulability web page.
    • Snapshot (SNAP) proposals will not be accepted in Δ Cycle 26.
    • Archive Research (AR) programs request funds to use publically available data in the HST archive for a research project. These programs can be Legacy, to analyze data and create data products and/or tools for the benefit of the community, or to support theoretical research that can benefit current or future observational programs with HST. They may also be Calibration, to augment the standard STScI calibration program.  Regular AR  and Regular AR Theory will not be accepted in Δ Cycle 26.
    • Mid-Cycle programs are available to provide the community with an opportunity to propose for in-cycle observations of recently-discovered, non-transient objects.
    • Director's Discretionary (DD) programs are observational programs that cannot be submitted as part of the normal peer-review process (such as an unexpected transient phenomenon). In all other respects, these are normal GO programs.
    • Multi-Mission (GO) programs require both HST observations and observations with another observatory to fulfill the scientific goals. We support joint programs with ChandraXMMNOAO, and NRAO

  4. Complete template for PDF attachment (this step can be done at any time) 

    Download and complete the GO, AR, or GO/DD template which will be attached to your APT file as part of your submission. This PDF attachment includes:

    Helpful hints on making the PDF file, as well as embedding figures are available. Please note the format and page limit restrictions. 

    Also note that beginning in Cycle 26, proposals should be anonymized in accordance with the guidelines specified in HST Cycle 26 Anonymous Proposal Reviews.


  5. Review the Observatory constraints

    To determine if your observations are feasible, review the observatory constraints.

  6. Determine what instrument to use

    There are 5 functioning instruments on HST. The Instrument Handbooks for ACSSTISCOSWFC3, and the FGS provide all the relevant information on those instruments. There is also an Instrument Comparison page (CamerasSpectrographs), which can assist you in determining the best instrument for your program.

  7. Determine the observation type

    Primary observations are those observations that determine the telescope pointing and orientation. HST can support Target of Opportunity observations, which are linked to the occurrence of an event that may occur at an unknown time.

    Parallel observations are those observations whose pointings are determined by a Primary observation (e.g. with ACS pointing at an object, take an image with WFC3/UVIS of whatever region of the sky is in the WFC3/UVIS field of view). Parallel observations can be Coordinated (where the primary observation is in the same proposal) or Pure (where the primary observation is in a different proposal). See Parallel Observations with HST for more information on Parallel programs. Be sure to note the restrictions on parallel observations.

  8. Determine the instrument configurations and exposure times

    Determine which instrument configurations (detector and spectral elements) are needed for each target. To get the exposure times, Exposure Time Calculators (ETCs) are available for ACS, COS, STIS, and WFC3. For the FGS, exposure time calculations for POS Mode and TRANS Mode are described in the Instrument Handbook.

  9. Perform Duplication Checking

    Observers have exclusive access to their science data during a proprietary period (normally 6 months). Special policies apply to cases in which a proposed observation would duplicate another observation already obtained with HST, or currently in the pool of accepted HST programs. To check for duplications, use one of the following options:

  10. Determine any observation constraints that could impact orbital visibility

    While the HST orbital visibility (the amount of time the target is visible in an orbit) is generally a function of the target declination (see the next step), there are some observation constraints that impact this visibility:

    • Continuous Viewing Zone (CVZ) targets do not suffer from Earth occultations, and can therefore be observed for the entire 96 minute orbit.

    • Shadow observations are those that need to be obtained when HST in in Earth shadow, as the normal geocoronal Lyman-alpha background emission would adversely affect their scientific utility; this requirement greatly reduces the orbital visibility.

    • Low Sky observations are those that need to be obtained with minimal sky background, as the normal scattered light (e.g. zodical light and earthshine) would adversely affect their scientific utility; this requirement greatly reduces the orbital visibility and number of scheduling opportunities.

  11. Determine any observation constraints that could impact scheduling

    The following constraints can have a significant impact on scheduling HST observations (and could make the observations infeasible). You should use the broadest possible ranges (within scientific constraints) to maximize schedulability.

    • ORIENT means that the exposures for a given target MUST be observed at the specific orientation range.

    • BETWEEN means that the exposures for a given target MUST be observed in the specific range of dates.

    • AFTER OBSERVATION BY means that the exposures in a given observation MUST occur after another observation by the specified time range.

    • PERIOD means that the exposures must be made in a given Phase range (for periodically variable targets).

    • For a target in a single observation block (see step BELOW) which will be observed at multiple times (visits), SAME ORIENT means that the exposures in all visits MUST be observed at the exact same telescope orientation.

    • Also note that the Continuous Viewing Zone (CVZ) constraint mentioned in the previous step can greatly impact schedulability.

  12. Determine the target visibility and scheduling feasibility of your observations 

    Use the Orbital Visibility Table in the HST Primer to determine the available orbital visibility time for your observation based on the target declination.

    Even though the schedulability of observations has increased with the return to 3-gyro operations, it is still useful to verify that the constraints you have chosen for your observations allow a reasonable number of scheduling opportunities for your program. This is useful for all constrained proposals (i.e. any proposal with ORIENT, BETWEEN, or AFTER OBSERVATION BY), but particularly important for Large programs. The Verifying scheduling feasibility section of the HST Orbital Viewing and Schedulability web page provides a Decision Tree and instructions on how to use it and APT to evaluate scheduling feasibility.

  13. Determine Orbit Allocation

    Note that if you were required to use the 'Increase Scheduling Flexibility' flag for any of your observations, please see the special instructions for determining your orbit request on the  HST Orbital Viewing and Schedulability web page.


  14. Download and install the APT proposal submission software

    APT is available for WindowsLinux, and Mac OS X. The remaining steps are all performed within APT.

  15. Get familiar with APT

    Here are some links to good introductory documentation:

  16. Start APT and load a proposal

    Start up APT (as described on the download page for your platform), and either load in a previous cycle's proposal (under the File Menu, select Open, and give the name of the old file) or open a new proposal (click on the New Document/New HST Proposal on the toolbar or under the File Menu, select New/New HST Proposal).

  17. Fill in the Proposal Information

    Complete the Proposal Information  section. Be sure to attach your Science Justification via the Proposal PDF Attachment (use the Browse button on the far right to select the file that contains your PDF Scientific Justification). 

    For additional information on parameters, use the Context-Sensitive Help in APT. Place your cursor on the field name in the Form Editor and click when you see the  symbol. This will get you information on the field of interest from the appropriate document.

    APT will not compromise the anonymous status of the proposal. It will keep investigator and institutional information, as well as the separate Team Expertise and Background section, from the TAC and Panels until they are requested by an authorized person to be utilized in a last sensibility check.

  18. Fill in the Principal Investigator (PI) information

    While online, complete the PI information section. (If you are not connected to the internet, then a pop-up will be displayed and you will not be able to input investigators.)

    • Enter your Last Name, hit return and APT will search the STScI Address database. If you locate your name, click on your record and hit Select.

    • If APT is unable to find your information (i.e. you are not in the STScI address database), you will be given the option to add a new address or add a new investigator either of which will take you to a new entry page in the MyST web page.

    • If you are updating a previous cycle's proposal, you should review all of your PI (and Co-I) information from our database. Note that the address information will be automatically updated to reflect the current information in our database (for those investigators already in the database).

    • If you need to revise your address information, click on the Update This Address. This button will take you directly to your information in the MyST web tool for updating.

    • APT will not reveal the name(s) of the principal investigator(s) to the TAC or TAC panels.

  19. Fill in the Co-Investigator (Co-I) information

    If you have one or more Co-Is, complete the Co-I information section.

    • For more Co-I pages, click on the New Co-I button at the bottom of the form or at the top of the tool bar.

    • If there are no Co-Is in your program, then click on the Unnamed Co-I in the tree view on the left and delete it (via the Delete command in the APT Edit Menu). If you wish to have a Co-PI (an investigator who will share the scientific responsibility for the proposal), select the Co-Principal Investigator box.

    • If you are a foreign PI and have at least one US Co-Investigator (Co-I), then return to the PI address view and select the US Admin PI, after you have entered your Co-I information.

    • APT will not reveal the list of investigators until requested by an authorized person to be used at the end of the review.

  20. Fill in the Target information (GO only)

    Open the targets folder  and complete the Targets section. You can specify your target as a Fixed , Solar System , or a Generic  Target. APT also has a target ingest capability to read in an ascii file with target positions. See the Movie (3 minutes) or Text Equivalent.

    We have also included a Target Selector Tool that can be used to search for your targets from the SIMBAD, NED, GSC2, 2MASS, and GALEX databases. Enter the target name, click on the Search button, select the target you want from the list returned, and click on the Select Object as Target button to ingest the information into APT.

  21. Fill in the Observation Summary information (GO only)

    Complete the Observation Summary section.

    • Add a new Observation.

    • Select your target from the pull-down list.

    • Select your instrument from the pull-down list.

    • Click on the Add button to get the instrument setup box.

    • Specify your instrument setup.

    • Use the Add button to add additional configurations, the Remove button to delete configurations, and the Edit button to revise configurations.

    • Enter the orbit request for the Observation.

    • Select any special requirements flags. (Only check the Coordinated Parallel flag for parallel observations and not for the corresponding prime observations.)

    • Select any scheduling requirements. If there are no requirements, or if the Observation is a coordinated parallel observation, select the No Scheduling Constraints box.

  22. Run the Visit Planner (GO only)

    This tool is used to check the schedulability of your observations, and should be run if you have specified any scheduling constraints or if you are submitting a Large program. This check can only be done for fixed targets. However, if you have a specific (short) time period for your moving target observations, you could determine the coordinates of the object at the time you want to observe it, create a fixed target with those coordinates, and process the observation to check the schedulability. See the Movie (6 minutes) or Text Equivalent for more details.

    Please note that the Visit Planner will only check schedulability for one cycle. If you need to check schedulability for future cycle observations, contact the help desk at https://hsthelp.stsci.edu.

    • Select the Visit Planner  from the toolbar with the observation of interest selected in the Tree Editor. Note that if you link observations with the AFTER OBSERVATION BY, then selecting any one of those observations will cause the Visit Planner to process all linked observations together (since all observations impact the total schedulability). Also, selecting the Observations container will select ALL observations for processing.

    • Press the Update Display button to process the observation.

    • Review the Total Visibility for the Observation from the graphical output.

    • If the Observation is not schedulable, then:

      • Examine the details to determine which scheduling requirements are inconsistent (e.g. the absolute ORIENT conflicts with the target availability). 

      • Use the Roll Range reports to get a summary of the available ORIENTs for your target, and the number of days each ORIENT is schedulable.

      • Revise the scheduling requirements to make the observation schedulable, or in some rare cases, you may need to select a different target (if possible).

      • Press the Update Display button to get an updated timeline.

  23. Review the Proposal

    When you have a schedulable program with no errors, review your completed proposal by selecting the PDF Preview  tool. This view will merge the information provided in APT along with the PDF attachment, and is what the TAC will review.

  24. Submit the Proposal

    When your proposal is complete, select the Submit  tool to begin the submission process.

    • If this is the initial submission of a proposal, click on the Submit button. 

    • If you are resubmitting a corrected version of a proposal, click on the Resubmit Phase I button.

    • If you are creating a new proposal from a similar proposal already submitted, click on the Clear ID# button prior to submission.

    • If you are submitting a proposal with warnings, you will get a popup asking for an explanation for why you are submitting with an outstanding warning

    In the Submission Log window you will see a message giving the time of the submission, the assigned proposal ID (if a new proposal), and the submission status.

  25. Submit the "Team Expertise and Background" section

    Please remember to compile this information and submit separately to STScI at hubblereview@stsci.edu. See HST Cycle 26 Anonymous Proposal Reviews for further details.

  26. Post-submission activities

    After you submit your proposal, a verification of a successful submission will appear in the Submission Log on the Submission Screen in the APT within about a minute. Also, the PI and all Co-Is will receive an automatic email acknowledgment that the merged PDF submission was received successfully. After the Phase I deadline has passed, and all submissions are in their final form, you will receive final notification that your submission has been successfully processed; this email will mark the completion of the submission. If you do not receive an acknowledgement within 48 hours of the deadline, please contact the STScI Help Desk as your submission was NOT RECEIVED and the Telescope Allocation Committee WILL NOT see your proposal; please provide the submission ID information from the APT Submission Log window. If there are any problems associated with your PDF attachment, you will be contacted by email separately.

    Notification of your proposal's status (approved or rejected) generally occurs within 2-3 weeks after the Telescope Allocation Committee meeting. Budgets are not requested in Phase I, but are required in Phase II from successful U.S. proposers.

  27. Give us feedback

    This roadmap was created in response to feedback from several users. If you found it helpful, or have suggestions for making it better, please let us know. You can also provide feedback on other aspects of APT or its documentation. You can send your comments to the STScI Help Desk or simply click on the "Feedback" button in APT.

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HST Proposal Opportunities and Science Policies

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