Spitzer Documentation & Tools
Spitzer Telescope Handbook
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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 were 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.



Figure 3.11: Spitzer Telescope Assembly.


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


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.

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