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Spitzer Observer’s Manual
4.1 Optical Design
The telescope is a Ritchey-Chrétien design, known from on-orbit measurements
to be diffraction-limited at 5.5 µm over the IRAC field of view, and expected to
be diffraction-limited at 6.5 µm over the entire field of view. The 85-cm diameter
primary mirror and the rest of the telescope structure are fabricated entirely of
beryllium, utilizing advances in optical design, testing, and fabrication to produce
a lightweight telescope that operates at cryogenic temperatures. The optical
design parameters for the telescope are summarized in Table 4.1 below. The
telescope configuration is shown in Figure 4.1.
Note that this chapter addresses the telescope itself; the point spread function
(PSF) for IRAC is discussed in chapter 6.
Table 4.1: Telescope Configuration
Optimal Parameter Description
System Parameters:
Focal Length
Focal Ratio
Back focal length (PM vertex to focus)
Field of View (diameter)
Field curvature radius
Wavelength coverage
Aperture Stop:
Location
Diameter of OD obscuration
Diameter of ID obscuration
Linear obscuration ratio
Primary Mirror (hyperbola)
Radius (concave)
Conic constant
Clear aperture
Focal ratio
Secondary Mirror (hyperbola)
Radius (convex)
Conic constant
Clear aperture (OD)
PM to SM spacing (vertex to vertex)
Value at 5.5 K
10,200 mm
f/12
437 mm
32.0 arcmin
140.5 mm
3 µm–180 µm
Edge of primary mirror
850.00 mm
320.00 mm
0.3765
-2040.00 mm
-1.003546
850.00 mm
f/1.2
-294.343
-1.531149
120.00 mm
887.528 mm