IRAS Explanatory Supplement
II. Satellite Description
C. Telescope System Overview
C.3 Optics

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Figure II.C.3 Cross-sectional view of optical subsystem.
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The optical subsystem (Fig. II.C.3) imaged the infrared and visible light onto the focal plane. The two-mirror Ritchey-Chretien telescope was made of beryllium to reduce mass and minimize thermal distortion upon cooling to cryogenic temperatures. The secondary mirror was coated with aluminum to enhance its reflection at visual wavelengths.

Table II.C.2 Telescope Optical Characteristics
Primary mirror diameter 60 cm
Unvignetted field of view 63.6' dia.
System focal length(design) 550 cm
Back focal length 18.35 cm
Primary mirror vertex radius -180.0 cm
Secondary mirror vertex radius -36.48 cm
Primary eccentricity 1.00569
Secondary eccentricity 1.43206
Primary-secondary spacing 74.74 cm
Entrance pupil diameter 57 cm
Central obscuration diameter 24 cm
Effective collecting area 2019 cm2
System focal length (measured) 545 cm
System F/number 9.56
Plate scale at focal plane (measured) 1.585 mm/'
Diameter of 80% encircled energy
12 µm
25 µm
25" *
60 µm
60" *
100 µm
100" *
Infrared surface reflectivity (all bands) 96%
* diffraction limited

The telescope optical parameters and performance are given in Table II.C.2. The design goal for the image quality was that it be diffraction limited in all infrared bands This goal was met except at 12 µm. Since the telescope was intended to be a survey instrument rather than a high resolution imaging instrument, the poor image quality at 12 µm did not interfere with the mission. For further discussion of the optical system, see Harned, Harned and Melugin (1981).
Figure II.C.4 Internal reference source assembly showing radiation path from source to focal plane and details of thermal source design
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An assembly mounted behind the secondary mirror contained ten thermal calibration sources, hereafter called "internal reference sources", several of which were used to provide stable pulses of infrared radiation for use as a reference during the mission and for ground testing prior to launch. Figure II.C.4 shows the location of the internal reference source assembly, the way in which a source illuminates the focal plane through a small hole in the center of the secondary mirror, and a cutaway view of an individual thermal source. The thermal source consisted of a 1 mm square diamond substrate coated with nichrome film and suspended by 0.051 mm diameter brass wires. During the mission an applied voltage ohmically heated the substrate to ~200 K in 13/16 sec. Two optical sources were included in the calibration assembly and used for ground testing of the star sensors.
Figure II.C.5 Calculated out-of-field rejection performance of telescope system compared to the total flux photometric reference, or TFPR (Section VI.B), for 12, 25, 60 and 100 µm bands. The sunshade temperature was taken to be 95 K; the Earth was assumed to radiate as a 280 K blackbody; the moon, Sun and Jupiter were taken as 370, 50O0 and 133 K black-bodies with angular diameters of 31, 31, and 0.75', respectively.
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Out-of-field radiation was absorbed by aluminum baffle structures which were coated with Martin Optical Black. Figure II.C.5 shows the calculated out-of-field performance in the four wavelength bands. The survey strategy (Section III.C) limited the angle between the boresight and the Moon, Earth, Sun and Jupiter to greater than 24°, 88°, 60° and 5°, respectively. At these angles the out-of-field radiation from these sources is thought to be negligible (see, however, Section III.B.5 and IV.C for a discussion of lunar "glints"). Further discussion of the out-of-field performance is included in Harned, Breault, and Melugin (1980).

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