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SOFIA IHB-0.0
CHAPTER 2: SOFIA Science Instrument ICDs
Table 2-1. Telescope Assembly Optical First Order
First Order Parameter
FPI angular magnification
FoV at FPI exit pupil
Value
62.6
±4.2 arcmin
2.3.1 - Secondary-Mirror Buttons
The secondary-mirror button defines the central aperture stop for the telescope and prevents science instruments from imaging themselves. Specifically, the button ensures that the primary-mirror hole and the edges of the tertiary mirror are not visible in the science instrument focal plane.
The details of the button design depend on wavelength and other science-instrument specific considerations. For example, some buttons are reflectors (“scatter-cones”), deflecting cold sky emission into the focal plane, while others are flat, high-emissivity (black) absorbers. In some cases,
buttons may have optical surfaces that are asymmetrical with respect to the secondary-mirror axis,
as the central obscuration of the telescope can be minimized if the buttons optical surface is decentered (see SOFIA Technical Note SER-ASD-023).
Science instrument teams can select a suitable secondary mirror button from several provided by
USRA or they can design and build their own. The details of the secondary mirror design requirements are found in SOFIA Document TA_SSMO_09.
Two possible secondary-mirror button designs are illustrated in Figure 2-7. The dimensions given
in these figures are only approximate. Final dimensions depend upon the material selected for the
secondary-mirror and button. Both designs show exposed screw heads on the button surface facing the primary mirror; these represent x% of the button surface and, if painted black to suppress
star-light, produce a negligible increase in telescope emissivity (< 0.1 %). “Clip-on” secondarymirror buttons designs are also possible, provided they meet all in-flight loading specifications.
SOFIA Telescope Optical Prescription
2-7