The information in the next two chapters is gleaned from the Spitzer Observer’s Manual, the primary source of technical information for planning Spitzer observations during the mission.
3.1 Overview
Spitzer was launched from Cape Canaveral, Florida into an Earth-trailing heliocentric orbit on 25 August 2003. The observatory was launched with the telescope at ambient temperature; only the focal plane instruments were cooled to cryogenic temperatures. The telescope gradually cooled to ~30 K over a period of ~45 days. Liquid helium was used to further cool the telescope to 5K.
Following launch, Spitzer entered a 63-day In-Orbit Checkout (IOC) phase, followed by a 35-day Science Verification (SV) phase, during which the planned capabilities of the telescope were verified, the detailed performance characterized, and the three science instruments and their operational modes commissioned. Following the completion of IOC/SV, Spitzer was commissioned for routine cryogenic science operations on 1 December 2003. Only one science instrument could be on at any given time so observations were done in instrument campaigns ranging from ~ 7 – 21 days in length. The order of the campaigns during the cryogenic mission was IRAC – MIPS – IRS. This order permitted the optimal cryogen usage by matching telescope temperature to the needs of the instrument in use, as discussed in Werner, M. (2012, SPIE, 51, 011008).
The cryogenic mission continued until the exhaustion of the onboard cryogen used to cool the telescope and science instruments. The Spitzer cryogenic lifetime requirement was 2.5 years of normal operations, which was passed on 26 April 2006. Due to the excellent performance of the cryogenic system after launch, the utilization strategy described above, and the implementation of warm and cold MIPS campaigns begun at the start of Cycle-3 (August 2006) to further extend the prime mission the actual cryogenic lifetime was ~5.5 years. The cryogen was depleted on 15 May 2009.
The first science observations during the prime mission included the First Look Survey (FLS), which was conducted by the SSC on behalf of the Spitzer observer community. The FLS was a ~110-hour survey using Director’s Discretionary Time. The goals of the FLS were to provide a characteristic first look at the mid-infrared sky at Spitzer sensitivity levels and to rapidly process the data and place it in the public domain in time to impact early Spitzer investigations (specifically, Cycle-1 planning). The first science also included Guaranteed Time Observer (GTO) and Legacy Science observations. About 6 months after IOC/SV was completed Cycle-1 General Observer (GO) observing commenced (June 2004). The majority of the original Legacy Science observations (the data of which were made public immediately) were completed within the first 18 months of the prime mission. In Cycles 2 and up, GO, GTO, DDT and Legacy Science were executed concurrently.
The Spitzer Science Archive opened in May 2004, and initially contained FLS data, early release observations, and any Legacy data taken and reprocessed with the most current pipelines at that time. The start and end dates for each cycle of proposals were:
Cryogenic Mission
Original GTO and Legacy Science: Dec 1, 2003 - April 2006
Cycle-1: July 2004 - May 2005
Cycle-2: June 2005 - May 2006
Cycle-3: June 2006 - June 2007
Cycle-4: July 2007 - June 2008
Cycle-5: July 2008 – May 15, 2009
Warm and Beyond Missions
IRAC Warm Instrument Characterization (IWIC): May - July 2009
Cycle 6 Exploration Science: Aug 2009 - Aug 2011
Cycle 6 GO: Aug 2009 - July 2010
Cycle 7: Aug 2010 - July 2011
Cycle 8: Aug 2011 - Dec 2012
Cycle 9: Nov 2012 - Dec 2
Cycle 10: Dec 2013 - Feb 2015
Cycle 11 Exploration Science and Large Programs: Feb 2015 - Sep 2016
Cycle 11 GO: Feb 2015 - Dec 2015
Cycle 12: Dec 2015 - Sep 2016
Cycle 13: Oct 2016 - Oct 2018
Cycle 14: Nov 2018 - Jan 29, 2020
During the 2016 NASA Senior Review process, the agency made a decision to close out the mission in 2018 in anticipation of the launch of the James Webb Space Telescope, which will also conduct infrared science. Since Cycle 13 was planned to be a two-year cycle, two smaller DDT proposal calls occurred in February and September of 2017 to provide opportunities for science that could not be proposed in Cycle 13. When Webb’s launch was postponed, the Spitzer mission was granted its fifth and final extension until January of 2020. Cycle 14 was selected to fill this time, with two more DDT proposal calls in March and May of 2019. DDT proposals continued to be accepted on an ad hoc basis until the end of 2019.
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 consisted of three cryogenically-cooled instruments, which together offer observational capabilities stretching from the near- to the far-infrared.
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 µm (imaging)
5.3 - 40 µm (spectroscopy)
55 - 95 µm (spectral energy distribution)
Diffraction Limit
5.5 µm
Image Size
1.5’’ at 6.5 µm
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) except for:
70 µm: 2.5’x5’
160 µm: 0.53’ x 5.33’
Telescope Minimum Temperature
5.6 K (cryo); 27.5 K (warm)
Maximum Tracking Rate
1.0’’/ sec
Time to slew over ~90ş
~8 minutes
The InfraRed Array Camera (IRAC) – Giovanni G. Fazio, Smithsonian Astrophysical Observatory/Harvard-Smithsonian Center for Astrophysics, PI
IRAC provided 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. During the warm mission, only the 3.6 and 4.5 micron arrays remained operational.
The InfraRed Spectrograph (IRS) – James R. Houck, Cornell University, PI
IRS performed both low and high-resolution spectroscopy. Low-resolution, long slit spectra (λ/Δλ = 64–128) could be obtained from 5.2 to 38.0 microns. High-resolution spectra (λ/Δλ ~600) in Echelle mode could 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 incorporated two peak-up windows that could be used in locating and positioning sources on any of the four spectrometer slits with sub-arcsecond precision. Each IRS Peak-Up window has 1.8 arcsecond square pixels and a field of view of 1’ x 1.2’. One of the windows covered 13.5–18.5 microns (blue) and the other 18.5–26 microns (red). Due to the higher operating temperatures IRS was not operational during the warm mission.
The Multiband Imaging Photometer for Spitzer (MIPS) – George H. Rieke, University of Arizona, PI
MIPS was 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 was also capable of low-resolution spectroscopy (λ/Δλ ~15–25) over the wavelength range 55–95 microns and a Total Power Mode for measuring absolute sky brightness. Due to the higher operating temperatures MIPS was not operational during the warm mission.
Table 3.2: Spitzer Instrumentation Summary (NB: limits include confusion).
λ (microns)
Array
Type
λ/Δλ
Field of
View
Pixel
Size
(arcsec)
Sensitivity (µJy)
(5σ in 500 sec,
incl. confusion)
IRAC
3.6
InSb
4.7
5.21’x5.21’
1.2
1.6 (3.4) / 2.3 (3.8)
4.5
InSb
4.4
5.18’x5.18’
1.2
3.1 (4.3) / 3.2 (4.4)
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
18.5–26
Si:As(IBC) Peak-Up
~3
54’’x80’’
1.8
116 80
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
MI PS
24
Si:As(IBC)
5
5.4’x5.4’
2.55
110
70
Ge:Ga
4
2.7’x1.4’
5.2’x2.6’
5.20
9.98
14.4 mJy
7.2 mJy
55-95
Ge:Ga
15–25
0.32’x3.8’
10.1
57, 100, 307 mJy (@60, 70, 90 µm)
160
Ge:Ga
(Stressed)
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 sensitivity is given for intermediate background – the first number in each case without confusion, and the second number (in parentheses) includes confusion. For IRAC the numbers before the slash ("/") sign are for the cryogenic mission and the numbers after the slash sign are for the warm mission. 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 micron 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.
