Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

13.2 COS Mechanisms

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The Aperture Mechanism was discussed above. The other three mechanisms are the Optics Select Mechanisms, OSM1 and OSM2, and the external shutter.

13.2.1 Optics Select Mechanism 1 (OSM1)

The function of the OSM1 is to position an optic into the optical beam of the COS instrument. The optics mounted on OSM1 receive the input light beam from the HST OTA through the ApM and direct it to the FUV detector or the NUV channel, depending on which optic is rotated into place. The optic positioned by this mechanism will be the first reflecting surface that the light encounters once it enters the instrument. The mechanism will position any one of four different optics into the beam. The OSM1 contains the G130M, G160M, and G140L gratings, and the NCM1 mirror. The gratings direct light to the FUV detector while the mirror directs light to the NUV channel. The four optics mounted on OSM1 are arranged at 90-degree intervals.

Once an optic is positioned by OSM1, the mechanism must allow for small adjustments in 2 degrees of freedom. Rotational adjustments are required to move the spectra on the FUV detector in the dispersion direction for FP-POS positioning in the FUV channel and for recovering wavelengths that fall on the FUV detector gap. Translational adjustments are required to refocus the instrument on orbit in order to optimize the focus of each of the FUV gratings and the NCM1 mirror, and to accommodate any instrument installation misalignments or any modifications to the location of the HST secondary mirror. The translational motions are in the z-direction (towards or away from the HST secondary).

13.2.2 Optics Select Mechanism 2 (OSM2)

The NUV optics mounted on OSM2 receive light from the NCM2 collimating mirror and direct the spectrum or image to the three camera mirrors (NCM3a,b,c). The OSM2 contains the G185M, G225M, G285M, and G230L gratings, and the TA1 mirror. OSM2 rotates but does not translate. Rotations move the spectrum or image in the dispersion direction on the NUV detector. The gratings are flat and each medium resolution grating must be positioned at ~6 discrete positions in order to achieve full wavelength coverage. Small rotational adjustments will also be used for FP-POS positioning. The five optics on OSM2 are distributed at 72-degree intervals, thus each rotational transition is at least ~72 degrees.

13.2.3 External shutter

The external shutter of COS is a small paddle-shaped device with a shutter blade that is a disk about 38 mm in diameter. It is located at the front of the COS enclosure, in the optical path before the Aperture Mechanism. When closed, the shutter blocks all external light from entering COS and it also prevents any light from within COS (such as the calibration lamps) from leaking out. The external shutter is for protection and is not used to determine exposure durations. To protect COS from exposure to bright objects, the shutter is ordinarily closed and is commanded to open at the start of an exposure. It is then closed at the end of the exposure, with the exception of acquisition sub-exposures.