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The Stratospheric Observatory for Infrared Astronomy (SOFIA) is now being designed and developed, with first light expected in late 2002. Flying at 41,000 feet or higher for 6 hours or more during 120 nights each year, SOFIA will be used for high-resolution observations of celestial objects in the infrared and submillimeter regions, spanning a factor of 1000 in wavelength. In many respects, building SOFIA is a greater challenge than an orbiting observatory would be, but advantages of economy and continuous access make it worthwhile. For work in the infrared, where everything at temperatures above absolute zero can be a source of background interference, the telescope and associated infrared sensors must be carefully designed and constructed to minimize such background. The far-infrared properties of the telescope surfaces, surrounding cavity walls, and surfaces within focal-plane instruments can be significant contributors to background noise. Infrared radiation from sources well off axis, such as the Earth, moon, or aircraft engines, may be multiply scattered by dust on the optics, the cavity walls, and/or surface facets of a complex telescope structure. This report briefly describes recent efforts at Ames Research Center to evaluate some of the infrared properties of the SOFIA telescope surfaces, and also some of the surface treatments that may be used in focal-plane infrared sensors.
In support of progress in the design and development of the SOFIA telescope, the nonspecular reflectometer (NSR) at Ames was reactivated and upgraded, enabling the NSR to be used to measure infrared reflectance properties for samples of planned SOFIA telescope system structural materials and associated surface treatments. Measurements of specular reflectance and bidirectional reflectance distribution functions (BRDFs) were made at wavelengths from 2.2 to 640 microns, at two angles of incidence, and at scattering angles as far as
85 degrees from normal. Samples of planned telescope system materials included carbon fiber reinforced plastic (CFRP), insulating foams, and Nomex fabric. Samples of candidate surface treatments for focal-plane instruments included two commercial surface treatments and several samples prepared at Ames with black paints and other components. The commercial surface treatments investigated were "Optoblack," a paint-like surface treatment from Labsphere, Inc. (North Sutton, New Hampshire), and "Vel-Black," a carbon fiber applique from Energy Science Laboratories, Inc. (San Diego, California). In general, the samples of telescope structural materials appear to have acceptable far-infrared reflectance and scattering properties, even compared to surface treatments expressly developed to minimize such effects. Figure 1 shows specular reflectance results for the telescope samples, compared to infrared-
optimized black paints. The commercial surface treatments appear to have excellent characteristics for use in the far infrared. Samples prepared at Ames performed well when silicon carbide grit was mixed in. These Ames-prepared samples approached but did not equal the performance of more carefully developed infrared black paints such as Ames 24E2 and Ball Infrared Black (BIRB).
These empirical results can now be incorporated into a software model of the SOFIA telescope, which would provide predictions of likely infrared background noise levels. However, it appears already possible to conclude from the results of the work described that the surfaces evaluated will probably not contribute significant infrared stray light.
Point of Contact: A. Meyer
(650) 604-1612
ameyer@mail.arc.nasa.gov
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Fig. 1. Surface treatment samples measured by the
NASA Ames nonspecular reflectometer, including
infrared reflectance spectra for samples of Rohacell
white foam, roughened CFRP, Nomex over melamine
foam, and, for comparison, previously published data
for the black paints Ames 24E2 and BIRB.
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