Spitzer Documentation & Tools
Spitzer Telescope Handbook


3.8            Optical Design

The telescope is a Ritchey-Chrétien design, known from on-orbit measurements to be diffraction-limited at 5.5 microns over the IRAC field of view, and expected to be diffraction-limited at 6.5 microns 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 3.3 below.  The telescope configuration is shown in Figure 3.11.

Table 3.3. Telescope Configuration.

Optimal Parameter Description Value at 5.5 K
System Parameters:
Focal Length
Focal Ratio
Back focal length (PM vertex to focus)
Field of View (diameter)
Field curvature radius
Wavelength coverage
10,200 mm
437 mm
32.0 arcmin
140.5 mm
3 µm–180 µm 
Aperture Stop:
Diameter of OD obscuration
Diameter of ID obscuration
Linear obscuration ratio
Edge of primary mirror
850.00 mm
320.00 mm
Primary Mirror (hyperbola)
Radius (concave)
Conic constant
Clear aperture
Focal ratio
-2040.00 mm
850.00 mm
Secondary Mirror (hyperbola)
Radius (convex)
Conic constant
Clear aperture (OD)
120.00 mm
PM to SM spacing (vertex to vertex) 887.528 mm



Figure 3.11: Spitzer Telescope Assembly.


The telescope employs a single arch primary mirror, which reduces mass.  The primary mirror is supported on three bipod flexures relatively close to its axis.  The spider blades, primary mirror baffle, and secondary mirror baffle are integrated into a one-piece, relatively small diameter metering tower extending through the central hole in the primary mirror.