Proposing

Every year, typically between mid-December to mid-January, the HST call for observing proposals is announced. An observing proposal consists of two distinct phases: Phase I and Phase II. In Phase I, observers describe the scientific justifications of their program and why Hubble is essential to achieve their scientific goals. Phase I also provides a list of targets with their coordinates, the instrument(s) to be used and their modes, and an estimate of how many orbits it would take.

The deadline to submit the Phase I proposal is typically about four months after the initial call. The Telescope Allocation Committee (TAC) is then organized to review and evaluate the submitted Phase I proposals. Upon favorable TAC review and approval by STScI's Director, the Phase II proposal must then be submitted. Phase II details the observations of the targets and converts the observing programs into machine-readable format that can used by schedulers to implement and schedule the observations. 

This page provides information and links about general proposing with Hubble, as well as those specific to ACS.

A number of things relating to the field of view should be considered when planning ACS observations. Figures showing the location of the amplifiers, amplifier quadrants, and CCD chip readout directions are linked below and should be kept in mind when thinking about CTE effects, even if not using the special CTE-minimizing aperture WFC1-CTE for small targets. The location of the ACS/WFC Amplifier D quadrant is also featured in the figures below, which can cause ghosting across the two CCD chips if a bright star is allowed to be placed in that quadrant.

By default, ACS images are required to be dithered, and if any observer wishes not to dither for some reason, that choice requires review and approval by STScI with a strong justification by the observer. The former use of CR-SPLIT, for example, is no longer recommended due to possible drift roll in 1-Gyro Mode when using a single guide star, in which case, the potential drift in roll is greater the further the guide star is from the chosen ACS reference aperture. Dithering provides several benefits; It not only can improve resolution beyond the native pixel scale if non-integer pixel dithers are used, but it also helps mitigate effects of detector defects or features in the field of view such as ACS/WFC CCD bad columns, hot pixels, and the ACS/SBC repeller wire.

Depending on the nature and purpose of the observing program design, it may help to define mosaics for larger dithers, and may cover the ACS/WFC chip gap for intermediate-sized dithers. Small non-integer-pixel dithers of various sizes are best for addressing the problems with detector defects and CCD hot pixels, and for improving the resolution. Integrity of the dither pattern will be somewhat compromised farther away from the location of the chosen aperture for larger dithers. Hybrid dither patterns may also be defined, which use both larger chip-gap dithers and smaller non-integer pixel dithers, both of which address detector defects and improve resolution. A number of multi-point DITHER-LINE and DITHER-BOX patterns are linked below in the "Resources Available for ACS" section, where the observer may choose a particular pattern or create those which best serve their purposes.

Note: Any Special Requirements affecting scheduling, e.g., timing, ORIENT-related (target geometry, bright-star scattered light and ghost avoidance etc.), sky background-related, CVZ, must be specified in Phase I in order to use them in Phase II.

Helpful Quick Links

• ACS Location in the HST Focal Plane
• ACS/WFC CCD Readout Format
• ACS/WFC and HRC Apertures and Amp Locations Shown in the V2/V3 Reference Frame
• ACS/WFC-Specific Apertures Shown in the V2/V3 Reference Frame

Exposure Time Calculator (ETC)

• The ETC is a web-based application developed at STScI to assists Hubble astronomers and engineers in preparing Phase I and Phase II observing proposals.
• The ETC’s sole purpose is to calculate the Signal-To-Noise (SNR) Ratio for a given exposure time, or, calculate the exposure time for a given SNR Ratio using user specified configurations (instrument, filter, source, etc..).
• Each year the ACS Team submits data updates (dark current, read noise, throughput tables, encircled energies, etc...) as necessary to ensure the calculations provided by the ETC are as accurate as possible.

ETC Application

ACS Dither Patterns

Users are expected to dither their observations to remove hot pixels, cosmetic defects, and cosmic rays in their combined images. Dithering allows improved sampling of the point spread function (PSF) that is otherwise undersampled, as was the case for CR-SPLIT images. Further, the CR-SPLIT option, which kept the pointing of the telecsope fixed (within error) to the same RA/DEC between repeat exposures, could not remove hot pixels, permanent cosmetic defects (e.g. bad columns), or the gap between the WFC CCDs. To support the use of dithering, the ACS Team has made available a list of predefined dither patterns with their recommend usages, described on the page linked below.

See More on Dither