Proposing with ACS

General Resources

General Phase I and II Resources

Phase I and II Resources Available for ACS

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

The ACS Team strongly recommends that observers 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 is the case for CR-SPLIT images. Further, the CR-SPLIT option, which keeps the pointing of the telecsope fixed (within error) to the same RA/DEC between repeat exposuresm does not remove hot pixels, permanent cosmetic defects (e.g. bad columns), or the gap between the WFC CCD's. To encourage the use of dithering, the ACS Team has made available a list of predefined dither patterns with their recommend usages decsribed on the page listed below.

See More on Dither

Last Updated: 01/22/2024


HST Help Desk 

Please contact the HST Help Desk with any questions.