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 taking all background contributions into account. At the end of this chapter, you will find examples to guide you through specific cases. In this chapter, for CCD observations (WFC/HRC), one electron is equivalent to one count, unless stated otherwise. For MAMA observations (SBC), the detector is operated in ACCUM mode and one count is equivalent to an event.

9.1.1 The ACS Exposure Time Calculator

The ACS ETC is available to help with proposal preparation. It can calculate 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 available, both circular and square, allowing the user to either select a radius in arcseconds or a size in pixels. The current default apertures for all three channels enclose 80% of the PSF flux. Square and circular apertures are available between 0.1 and 2.0 arcseconds. For spectroscopic calculations with extended sources, the signal-to-noise ratio is based on counts summed over one resolution element of 2 × 2 pixel², 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 ETC, or you can choose from a variety of templates. 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). For the CCDs, a warning will appear if the background of the observation is below the recommended limit of 30 electrons. A higher background mitigates CTE losses. (See Sections 2.5 and 9.6.6 for more on CTE and background levels, including discussion of when post-flash is warranted.)

An ETC user's guide is available for execution and interpretation of results, and further questions can be directed to the HST Help Desk. Alternatively, users can use pysynphot or stsynphot to calculate count rates and the wavelength distribution of detected counts. Users must specify the ETC's evaluation MJD (which can be found in the release notes) in their pysynphot/stsynphot calculations to match ETC results.