Overview

In this chapter we explain how to use the sensitivities and throughputs provided in Chapter 13 and Chapter 14 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 STIS observations taking various background contributions into account. At the end of the chapter, in Exposure Time Examples, you will find examples to guide you through specific cases.

The STIS Exposure Time Calculators

Three STIS Exposure-Time Calculators (ETCs) are available to help you with your proposal preparation. The three ETCs-available via the STIS web pages-are the imaging ETC, the spectroscopic ETC, and the target acquisition ETC. These calculators provide count rates for given source and background parameters and calculate signal-to-noise ratios for a given exposure time, or count rates and exposure time for a given signal-to-noise ratio. If you have a calibrated spectrum of your source, you can pass it as input via FTP to the Exposure Time Calculator. The ETC also determines peak per-pixel count rates and total (integrated over the detector) count rates to aid you in your feasibility assessment. The ETC also warns you if your observations exceed the local or global brightness limits for MAMA observations (see Chapter 7). Lastly, in the case of the spectroscopic ETC, the task produces a simulated one-dimensional spectrum for a given STIS configuration and source. A graphical interface allows WWW browsers to plot the output spectra or write them out as ASCII text files to your local disk. The input and output parameters in the target acquisition ETC are specifically designed to facilitate exposure-time estimates for target acquisition purposes (see Chapter 8 for more details on acquisitions). The ETCs have extensive online help which explains how to use them and gives the details of the performed calculations.