8.1.2 Instrument Design

Most of the visual light passes SOFIA’s tertiary beam splitter (M3-1) before it is reflected into the Nasmyth tube by the fully-reflective tertiary (M3-2). A significant amount of visual light is not transmitted, but rather absorbed or reflected along with the longer, infrared wavelengths. However, in the range between 480 nm to 800 nm, where the visual-light CCD cameras are most sensitive, more than 50% of the light is transmitted to the FPI+. The visual light continues through a set of four silver-coated folding mirrors inside the so called "delay line assembly" of the telescope. This setup allows focusing the FPI+ independently from the instrument at the telescope science instrument flange. A pair of windows is installed between the Nasmyth tube and the delay line that create the boundary between the stratospheric conditions in the telescope cavity and cabin conditions inside the delay line assembly. Two "eyepiece" lenses are used to collimate the telescope beam. Close to the camera is a pellicle beam splitter made of a nitrocellulose membrane with 85% transmission. The beam splitter can be used to reflect a reticle into the light path for camera alignment purposes. The last optical element in front of the camera is an industrial ZEISS 1.4/85 mm Planar T* IR photo lens.

A double-carousel, filter wheel with six positions on each carousel is installed between the reticle beam splitter and the ZEISS lens.

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