Space Telescope Science Institute
ACS Instrument Handbook Cycle 19
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Advanced Camera for Surveys Instrument Handbook for Cycle 19 > Chapter 9: Exposure-Time Calculations > 9.1 Overview

9.1 Overview
In this chapter we explain how to use sensitivities and throughputs to determine the expected count rate from your source and how to calculate exposure times to achieve a given signal-to-noise ratio for your ACS observations taking all background contributions into account. At the end of this chapter you will find examples to guide you through specific cases.
9.1.1
The ACS Exposure-Time Calculator (ETC) is available to help with proposal preparation at:
http://etc.stsci.edu/Webetc/index.jsp
This ETC calculates count rates for given source and background parameters, signal-to-noise ratios for a given exposure time, or count rates and exposure time for a given signal-to-noise ratio for imaging, spectroscopic, and coronagraphic observations. A variety of apertures are now available, both circular and square, allowing the user to either select a radius in arcseconds or a size in pixels. The current default are a 0.2 arcseconds radius for the WFC, and a 0.4 arcseconds radius for SBC observations, which enclose approximately 80% of the PSF flux. Square and circular apertures are available between 0.1 and 2.0 arcseconds. For extended sources the S/N calculation is based on counts summed over one resolution element of 2 x 2 pixels, as the source size is assumed to be larger than the ACS resolution. A calibrated spectrum of your source can be provided directly to the Exposure Time Calculator. The ETC also determines peak per-pixel count rates and total count rates to aid in feasibility assessment. Warnings appear if the source exceeds the local or global brightness limits for SBC observations (see Section 7.2). The ETC has online help for its execution and interpretation of results. Alternatively, users can use synphot1 in STSDAS to calculate count rates and the wavelength distribution of detected counts.

1
Synphot will soon be replaced by the pysynphot package, a significantly improved re-implementation of Synphot written in Python. Please visit the pysynphot Web page at http://stsdas.stsci.edu/pysynphot for updates.


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