Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.1 Configurations and Optical Elements

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To obtain a spectrum, an exposure must use either the FUV or NUV detector configuration. A spectral element, central wavelength, and FP-POS must be chosen for each exposure as well.

Both detectors may be used efficiently within the same visit and the same orbital visibility period as there is very little overhead involved in switching between detectors for consecutive exposures. To do this it is necessary to move the Optical Selection Mechanism to a different grating, but the time needed to do that is the same for any grating, meaning that there is no penalty for switching from the NUV to the FUV or vice versa. For the times needed for these motions, see Chapter 9 on page 111.

Detector memory and buffer-time considerations are the same for both the NUV and FUV when using TIME-TAG mode (see below). However, only two FUV ACCUM images can fit in the on-board memory, whereas up to nine NUV ACCUM images can be accommodated. This means that more time may be needed for detector readout with the FUV than with the NUV in ACCUM mode observing.