Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

10.1 The COS Exposure Time Calculators

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Three COS Exposure-Time Calculators (ETCs) are available on the COS web pages to help with proposal preparation. These 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 the exposure time needed 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 count rates per pixel, to compare against the local count rate limits. The spectroscopic ETC reports the total count rates, integrated over detector segments A and B (in the case of FUV observations) or integrated over the entire MAMA detector (in the case of NUV observations) to aid you in your feasibility assessment. The ETC also warns you if your observations exceed the local or global brightness limits (see Chapter 2xx). Lastly, in the case of the spectroscopic ETC, also displayed are the input spectrum, a simulated one-dimensional output spectrum, and S/N and number of counts per resolution element for the selected COS configuration and source. These outputs can also be downloaded by the user in ascii format. The ETCs have extensive online help which explains how to use them and provides the details of the performed calculations.

The imaging ETC is simple because COS has only a single imaging mode. However, this NUV mode does allow a variety of attenuations by selection of either the primary science aperture or bright object aperture, and selection of either MIRRORA or MIRRORB on OSM2. The ETC reports count rate in the brightest pixel, total counts in the detector, and S/N per resolution element.

The target acquisition ETC returns the acquisition time for both imaging and spectroscopic acquisitions. Target acquisition is described in Chapter 7xx.