Spitzer Documentation & Tools
Spitzer Telescope Handbook

3.2            Observatory Description

Spitzer is a 3-axis stabilized pointing and scanning observatory.  The top-level observatory characteristics are summarized in Table 3.1. Spitzer’s science payload consists of three cryogenically-cooled instruments, which together offer observational capabilities stretching from the near- to the far-infrared.  Table 3.2 gives a summary of the characteristics of each instrument, including limits due to confusion.

Table 3.1: Summary of Spitzer Characteristics.

Aperture (diameter) 85 cm
Orbit Solar (Earth-trailing)
Cryogenic Lifetime 5.5 years (est.); 5.7 years actual
Wavelength Coverage
(passband centers)
3.6 - 160 ΅m (imaging)
Diffraction Limit 5.5 ΅m
Image Size 1.5’’ at 6.5 ΅m
Pointing Stability (1σ, 200s, when using star tracker) <0.1’’
As commanded pointing accuracy (1σ radial) <0.5’’
Pointing reconstruction (required) <1.0’’
Field of View (of imaging arrays) ~ 5’x5’ (each band); at 160 ΅m a single field of view is 0.053’ x 5.33’
Telescope Minimum Temperature  5.6 K (cryo); 27.5 K (post-cryo)
Maximum Tracking Rate  1.0’’/ sec
Time to slew over ~90Ί ~8 minutes


The InfraRed Array Camera (IRAC) – Giovanni G. Fazio, Smithsonian Astrophysical Observatory/Harvard-Smithsonian Center for Astrophysics, PI

IRAC provides images at 3.6, 4.5, 5.8 and 8.0 microns, with two adjacent 5.2’ x 5.2’ fields of view.  One field of view images simultaneously at 3.6 and 5.8 microns and the other at 4.5 and 8.0 microns via dichroic beamsplitters.  All four detector arrays are 256 x 256 pixels with 1.2 arcsecond square pixels. 


The InfraRed Spectrograph (IRS) – James R. Houck, Cornell University, PI

IRS performs both low and high-resolution spectroscopy. Low-resolution, long slit spectra (= 64–128) can be obtained from 5.2 to 38.0 microns. High-resolution spectra ( ~600) in Echelle mode can be obtained from 9.9 to 37.2 microns. The spectrograph consists of four modules, each of which is built around a 128×128 pixel array.  One of the modules incorporates a peak-up function that can be used in locating and positioning sources on any of the four spectrometer slits with sub-arcsecond precision.  The IRS Peak-Up array has 1.8 arcsecond square pixels and a field of view of 1’ x 1.2’.  Two filters are available for use with an IRS peak-up (and imaging), covering 13.5–18.5 microns (blue) and 18.5–26 microns (red), respectively.


The Multiband Imaging Photometer for Spitzer (MIPS) – George H. Rieke, University of Arizona, PI

MIPS is designed to provide photometry and super resolution imaging, as well as efficient mapping capabilities, in three wavelength bands centered near 24, 70 and 160 microns.  The array materials, sizes and pixel scales vary; they are given in Table 3.2. MIPS is also capable of low-resolution spectroscopy ( ~15–25) over the wavelength range 55–95 microns and a Total Power Mode for measuring absolute sky brightness.


Table 3.2: Spitzer Instrumentation Summary (NB: limits include confusion).

λ (microns) Array λ/Dλ Field of Pixel Sensitivity (΅Jy)
IRAC 3.6 InSb 4.7 5.21’x5.21’ 1.2 1.6 (3.4)
4.5 InSb 4.4 5.18’x5.18’ 1.2 3.1 (4.3)
5.8 Si:As(IBC) 4.0 5.21’x5.21’ 1.2 20.8 (21)
8.0 Si:As(IBC) 2.8 5.21’x5.21’ 1.2 26.9 (27)
IRS 5.2–14.7 Si:As(IBC) 64–128 3.7’’x57’’ 1.8 250
13.5–18.5 Si:As(IBC) Peak-Up ~3 54’’x80’’ 1.8 116
9.9–19.5 Si:As(IBC) ~600 4.7’’x11.3’’ 2.3 1.2×10-18 W/m2
14.3–35.1 Si:Sb(IBC) 64–128 10.6’’x168’’ 5.1 1500
18.9–37.0 Si:Sb(IBC) ~600 11.1’’x22.3’’ 4.5 2×10-18 W/m2
MIPS 24 Si:As(IBC) 5 5.4’x5.4’ 2.55 110
70 Ge:Ga 4 2.7’x1.4’ 5.20 14.4 mJy
55-95   Ge:Ga 15–25 0.32’x3.8’ 10.1 57, 100, 307 mJy (@60, 70, 90 ΅m)
160 Ge:Ga 5 0.53’x5.33’ 16×18 29 (40) mJy

Notes for Table 3.2:  The sensitivities given are for point sources, and are only representative; IRAC sensititivity is given for intermediate background – the first number in each case with without confusion, and the second number (in parentheses) includes confusion; IRS sensitivity is given for low background at high ecliptic latitude (note that for IRS, sensitivity is a strong function of wavelength); MIPS sensitivity is given for low background; 70 observations can be confusion limited; Because of a bad readout at one end of the slit, spectral coverage for 4 columns in MIPS SED is reduced to about 65-95 microns; 160 microns is often confusion limited, 29 mJy refers to no confusion and 40mJy refers to the estimated confusion limit.