Many issues may arise during the preparation of observing programs. In order to help forestall problems where non-optimal program design is proposed and/or implemented, this documents some issues and standard references which users should consider when proposing (Phase I) and planning accepted observations (Phase II) using the Advanced Camera for Surveys (ACS). Although the Phase I documents (Call for Proposals etc.) are the definitive source for the list of such items, this document also especially highlights some limited-resource issues you must consider for Phase I proposals and the related requirements you must specify in your Phase I proposal if you wish to be allowed to use them in a TAC-approved Phase II observing program. (Failure to list such requirements in Phase I may prevent their use in Phase II since very strong scientific and technical justification to the Telescope Time Review Board - TTRB - is then required, and such requests are often not granted.) You may see the companion document ACS ISR 2019-07 from which this web page was derived at https://www.stsci.edu/files/live/sites/www/files/home/hst/instrumentation/acs/documentation/instrument-science-reports-isrs/_documents/isr1907.pdf.

New items of note within:

• There is a revised/corrected small-scale WFC-DITHER-BOX pattern. Please see Section V "Dither Optimization" of this document for any further links and/or discussion. This is a Phase II issue and it has already been updated on the relevant ACS dithering web pages and Phase II Proposal Instructions and in APT, etc., so this is mostly transparent to the user, and is not a critical issue in Phase I.
• As of 05 September, 2023, a new ISR, ACS ISR 2023-04 (Anderson and Grogin, 2023) has been published, and details optimized dithering strategies for using both ACS/WFC and either of the WFC3 cameras, UVIS or IR, in parallel with each other. If you are using both ACS/WFC and either of the WFC3 cameras in parallel, you are strongly encouraged to read this before preparing and submitting your Phase II program. A bit more about this is also listed in Section V (Dither Optimization) of the Phase II portion of this document. It involves optimization of small 4-point dither-box patterns. If needing to optimize parallel chip-gap dithers and 2- or 3-point parallel dithers, please contact the HST Help Desk. 
• A new operating mode, Imaging Spectropolarimetry, is offered as of Cycle 31. Please see "Special Calibrations" section under "Phase I Proposing Issues - Overview" of this document and Section X "Ramp Filters; Prisms; Polarizers" of this document for further information and links. Also see the related item on Imaging Spectropolarimetry in the 04 April, 2023 ACS STAN for more. There is also a newly-published March 2024 "Research Note" RNAAS article on the preliminary calibration results for HST/ACS imaging spectropolarimetry which may be of help to those wishing to propose for such observations. Please read these published materials and contact the HST Help Desk for more info if considering using this mode.
• A new version of HSTaXe for grism (slitless) spectroscopy has been released. See the article on HSTaXe in the 04 April, 2023 ACS STAN.
• Unlike the much more uniform WFC3/UVIS postflash, given the large (factor of ~2) radial variability in the effectiveness and level of the ACS/WFC postflash from the center of the 2-chip ACS/WFC FOV outwards, we currently usually limit our recommendations for the use of the ACS/WFC postflash to situations where mere detection of sources is the primary goal, as opposed to doing relative photometry both across the FOV as well as in comparison to other unflashed observations at other epochs. There may also be some limited subset of cases where it may be useful to have both flashed and unflashed observations at the same epoch. However, no matter the strategy, if using the ACS/WFC postflash, as opposed to the 20 e^- level we have recommended in the past, based on newer, more recent work (ACS ISR 2024-02) we now recommend an increased postflash level of 30 e^- going forward. NOTE: If use of the postflash is needed/desired, the extra time required for using it to the desired electron level must be accounted for in Phase I proposals so that the necessary orbital allocation is available in Phase II.
• Visits of more than 6 consecutive orbits are considered "shared risk" between STScI and the observer.  Any request for Visits of more than 6 consecutive orbits must be described and justified in the Special Requirements section of the Phase I proposal, and the impact to science goals must be explained there if the observations cannot be scheduled as a consecutive set of more than 6 orbits, whether initially, or as a repeat in the event of failure. 
• ACS ISR 2025-02 demonstrated that extended sources on size scales of 10s-100s of pixels can suffer from artificially low brightness measurements due to degraded CTE. Users are encouraged to keep backgrounds above 30 e-/pixel and place targets close to the serial registers if spatially resolved analyses need to be conducted and if the targets are of small angular size and the environment around the target is not of concern. (The WFC1-CTE aperture location and locations close to the serial registers are near the edge of the chip.) Note, however, that the ACS/WFC postflash is currently only recommended where mere detecton of faint sources is the primary goal. If highly accurate photometry across the 2-chip ACS/WFC mosaic at a single epoch, or highly accurate comparisons of photometry with that of other epochs, or other cameras, or other telescopes is the goal, the ACS/WFC postflash is not recommended for raising the background level to minimize CTE effects. For further details see Phase II Program Development Design Issues.
• Analysis of historical ACS/SBC data has shown that airglow is usually significantly fainter than what was previously predicted by the ETC, meaning far shorter integration times are necessary for background-limited observations in F115LP, F122M, F125LP, and F140LP. Specifically, the average backgrounds in these filters are lower than standard ETC predictions by factors of 4.04, 4.27, 167, and 5.94, for F115LP, F122M, F125LP, and F140LP, respectively. ETC version 34.1 provides the option to use an empirical background model that incorporates this change; see ACS ISR 2025-04 and the ETC users guide backgrounds page (https://etc.stsci.edu/etcstatic/users_guide/1_ref_9_background.html) for details.
• Also regarding ACS/SBC backgrounds, for filters affected by geocoronal emission, observing with SHADOW reduces backgrounds by roughly a factor of 10. Observing with LOW-SKY can further reduce backgrounds, by avoiding low Earth limb angle observations that would otherwise increase backgrounds by 10–20%. These special requirements need to be explicitly justified and included in Phase I proposals in order to use them in Phase II observing programs. See section 9.4 of the ACS Instrument Handbook, and ACS ISR 2025-04.

Some other issues to be aware of and beware of:

• As implied by the statements at the top of this page, if you wish to use ORIENT or relative ORIENT constraints in Phase II, you must specify the actual ORIENT ranges (i.e. with ORIENT range values) or relative ORIENT constraints in your Phase I proposal in the manner required in detail as per the instructions in the latest HST Call for Proposals and related documents. Similar detail is required in Phase I for timing and some other Special Requirement constraints such as CVZ, SHADOW, or LOW-SKY etc. which will affect scheduling in Phase II. Users are reminded that they must follow the instructions in detail regarding these and related requirements as specified and required in the Phase I Call for Proposals and related documents in order to use these detailed requirements in Phase II. Failure to do so will result in being required to submit a request for such to the TTRB, and since it has been publicly required in the Call for Proposals etc., such requests are often denied in Phase II if the required details were not specified in Phase I.
• HRC is not operational.
• SBC has been experiencing some Dark Rate irregularities recently. See the SBC Dark Rate Monitoring item in the 05 June, 2023 ACS STAN for more.
• Observations using F892N and some other filters on the HRC filter wheel will be highly vignetted resulting in a small FOV and/or small-area subarray readout that cannot be overridden if used with ACS/WFC, and the same is also true especially with all polarizer filters and some ramp filters amd ramps + polarizers when used with ACS/WFC, etc. Please see the ACS Instrument Handbook Footnote "a" of Table 3.3 and also ACS Instrument Handbook Section 7.7 on ACS Apertures with related tables and figures in its various subsections, and especially Section 7.7.2 on Ramp-Filter Apertures, Section 7.7.3 on Small Filter Apertures, and Section 7.7.4 on Polarizer Apertures for more. Please contact the HST Help Desk first for more details if trying to consider their use for wider-FOV observations.
• Observations simultaneously using the CLEAR filters in both ACS/WFC filter wheels are NOT currently calibrated. Please contact the HST Help Desk first for more details if trying to consider their use.