Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

6.2 Configurations and Imaging Quality

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COS includes an NUV imaging mode; no FUV imaging is possible. It is anticipated that the greatest use of this imaging capability will be for target acquisition (see Chapter 7), but science exposures may be obtained as well. With OSM1 set to mirror NCM1 (which occurs automatically when any NUV mode is selected), and with OSM2 set to MIRRORA, an image of the sky is formed on the NUV MAMA detector. The plate scale on the detector is 23.6 mas per pixel. Because the entrance aperture is out of focus at the detector, the image receives light out to a radius of about 2 arcsec. However, as can be seen from Figure 7.1, once a point source is more than about 0.5 arcsec from the aperture center its light is diminished.

The NUV imaging mode requires the observer to make only two optical element selections. First, either the PSA or BOA is selected as the aperture. The BOA (see Figure 3.4) provides an attenuation factor of approximately 200 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 TA1 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. Because of the finite thickness of the order-sorting filter, MIRRORB produces a doubled peak in the image that may impede its use for imaging; see Section 7.5.3 for details. Ground testing shows that the secondary peak will contain ~1/2 the flux of the primary image (see Figure 7.4). The secondary peak is located ~20 pixels in the -y direction on the MAMA detector from the primary image. This is easily separable for an isolated point source but may present difficulties for extended sources or crowded fields. Similarly, the BOA includes a neutral density filter and the optical properties of that filter degrade the image compared to what is possible with the PSA (see Figure 7.5, Figure 7.6, and Figure 7.7).

To record an image, Config = COS/NUV is used, together with Mode = ACCUM or TIME-TAG. If TIME-TAG mode is selected, the minimum allowable BUFFER-TIME is 80 seconds, which may be longer than the expected exposure time. ACCUM mode is recommended for short exposures.

All COS exposure times must be an integer multiple of 0.1 seconds. If the observer specifies an exposure time that is not a multiple of 0.1 sec, its value is rounded down to the next lower integral multiple of 0.1 sec, (or set to 0.1 seconds if a smaller value is specified). The minimum COS exposure time duration is 0.1 seconds. The maximum COS exposure time is 6,500 seconds. Bear in mind that exposure time values much larger than 3,000 seconds are normally appropriate only for visits with the CVZ Special Requirement.