Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

6.2 Configurations and Optical Elements

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COS has a simple NUV imaging mode with a minimal number of options available for the observer. No FUV imaging is possible.

To obtain an image, OSM1 and OSM2 are set to their mirror positions. These are referred to as NCM1 and TA1, respectively. TA stands for target acquisition, and it is anticipated that the greatest use of imaging mode will be for target acquisition (see Chapter 7 on page 83), but science exposures may be obtained as well. When OSM1 and OSM2 are in the mirror positions, a circular image of the sky, 2.5 arcsec in diameter is formed on the NUV MAMA detector.

The NUV imaging mode requires the observed to make only two optical element selections. First, either the PSA or BOA is selected. The BOA provides an attenuation factor of approximately 170 compared to the PSA (which is completely open and provides maximum transmission through the aperture). The second selection required is MIRRORA or MIRRORB. MIRRORA refers to the usual position of the mirror on OSM2. MIRRORB refers to the arrangement in which OSM2 rotates the position of this mirror slightly so that the front surface of the order sorter filter on this mirror is used. This provides an attenuation factor of approximately 25 compared to MIRRORA.