Figure II.C.3 Cross-sectional view of optical subsystem.
The optical subsystem (Fig. II.C.3) imaged the
infrared and visible light onto the focal plane. The two-mirror
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.
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).
Primary mirror diameter
Unvignetted field of view
System focal length(design)
Back focal length
Primary mirror vertex radius
Secondary mirror vertex radius
Entrance pupil diameter
Central obscuration diameter
Effective collecting area
System focal length (measured)
Plate scale at focal plane (measured)
Diameter of 80% encircled energy
Infrared surface reflectivity (all bands)
* 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
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.
Out-of-field radiation was absorbed by aluminum baffle structures
which were coated with Martin Optical Black.
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
Further discussion of the out-of-field performance is included in
Harned, Breault, and Melugin (1980).