7.1 The COS Exposure Time Calculators

To help with proposal preparation, four COS Exposure Time Calculators (ETCs) are available on the STScI ETC webpage:

These calculators model spectroscopic and imaging exposures for both target acquisitions and scientific observations. They estimate count rates for given source and background parameters, and calculate either the signal-to-noise ratio for a given exposure time or the exposure time needed to achieve a desired signal-to-noise ratio. If you have a calibrated spectrum of your source you can upload it to the Exposure Time Calculators. The ETCs warn you if your observations exceed local or global brightness limits (see Table 10.1). The ETCs offer extensive on-line help that provides instructions and explains their calculations.

A unique exposure ID is assigned to each calculation performed by the ETCs, allowing results from previous calculations to be retrieved easily. This number must be included in the appropriate box in your Phase II proposal to document your work and to facilitate Phase II review. Proposers are urged to check the COS ETC webpage for any updates or issues related to the COS ETCs before performing ETC simulations.

The spectroscopic ETC can display the input spectrum, a simulated one-dimensional output spectrum, the S/N as a function of wavelength, and the number of counts per resolution element for the selected instrument configuration and source. These outputs can be downloaded in ASCII format. The ETC also computes the BUFFER-TIME, which is required for TIME-TAG observations. Scale this value by 2/3 and, following the discussion in Section 5.4, enter it in the Astronomer’s Proposal Tool.

The imaging ETC allows for the selection of either the PSA or BOA and either MIRRORA or MIRRORB. It reports the count rate in the brightest pixel, total counts in the detector, and S/N per resolution element.

The target-acquisition ETCs return the acquisition exposure time to be entered in APT for both imaging and spectroscopic acquisitions. Target acquisition is described in Chapter 8.