The Spitzer Space Telescope and IRAC


Spitzer is the last of NASA “Great Space Observatories” (the others are the Hubble Space Telescope, the Chandra X-Ray and the Compton Gamma-Ray Observatories). Launched into space by a Delta rocket from Cape Canaveral, Florida, on 25 August 2003, it is currently orbiting the Sun in a “Earth Trailing Orbit”. With its 85 cm primary mirror, cryogenically cooled by a reservoir of LHe, Spitzer is the largest infrared telescope ever launched into space.


Infrared light, or heat energy, allows  studying the properties of cool astronomical objects through their thermal emissions, like cool stars, brown dwarfs and extrasolar planets. In addition, infrared radiation can penetrate the screen of dust pervading our galaxy: this means that in the infrared we can study regions of space that we cannot see in visible light, such as the dusty center of our galaxy or newly formed stars still embedded in their dusty cocoons. These observations are severely limited from the ground, because water vapour in Earth's atmosphere absorbes much of the infrared radiation. Spitzer, on the other end, takes advantage of the exceptionally stable and cool environment of deep space, which results in unprecedented sensitivity and image quality.


Spitzer has three scientific instruments:


  1. ‣the InfraRed Array Camera (IRAC), a four channels imager operating at 3.6, 4.5, 5.8 and 8.0 μm. Each channel has a spectral resolution between ~3 and 5, a field of view of ~5 arcmin and a spatial resolution of ~2 arcsec.

  2. ‣the InfraRed Spectrograph (IRS), capable to collect spectra between 5.2 and 38 μm with a spectral resolution R~60-600. IRS has also two smal imaging subarrays operating at 16 and 22 μm.

  3. ‣the Multiband Imaging Photometer for Spitzer (MIPS), producing images at 24, 70 and 160 μm and Spectral Energy Distributions (SEDs) between 55 and 95 μm with a spectral resolution R~10.



The Infrared Array Camera (IRAC)


IRAC was built by the NASA Goddard Space

Flight Center (GSFC) with management and scientific leadership by the Smithsonian Astrophysical Observatory (SAO) at CfA under principal investigator Giovanni Fazio [1]. The camera  provides simultaneous 5.2’x5.2’ images in four bands at 3.6, 4.5, 5.8 and 8.0 μm. Two adjacent fields of view are imaged in pairs (3.6 and 5.8 μm; 4.5 and 8.0 μm) using dichroic beamsplitters.  All four detector arrays in the camera are 256 × 256 pixels in size, with a pixel size of ~1.2′′ × 1.2′′.  The two short
wavelength channels use InSb detector arrays and the two longer wavelength channels use Si:As detectors. IRAC was designed with sensitivity in mind: among its most spectacular accomplishments in the first 5 years of its mission are the first direct detection of light coming from an extrasolar planet (Charbonneau et al. 2005), the first mapping of the surface temperature of another planet (Knutson et al. 2007), the light that may come from the first objects in the universe (Kashlinsky et al. 2005), and fundamental contribution to understand star formation, the interstellar medium, stellar populations including brown dwarfs, galaxy evolution and dynamics, and cosmology. After the exhaustion of the Spitzer cryogenic fluid predicted by April 2009, IRAC will still be able to operate without loss of performance at 3.6 and 4.5 μm, opening the possibility of “warm” mission after the end of the nominal Spitzer operations.


Calibration of the IRAC extended PRF

As part of the IRAC science team at SAO I have participated in the calibration, test and scientific verification of the camera during the In-Orbit Checkout (IOC) and Science Verification. Since then, I have worked on the characterization of IRAC long lived latents, and the IRAC High Dynamic range Point Response Function (PRF).

The PRF is the response function of the telescope and camera system (optical train and electronics) to the observation of a point source (a source whose size is below the angular resolution power of the telescope). Due to the physics of optical systems, a point source observed through a telescope of finite aperture does not appear like a “point”, but rather like a pattern of concentric rings and radial structures determined by the diffraction of light. This pattern is called Point Spread Function (PSF). When the PSF is recorded by the camera detector arrays, it is sampled on a discrete grid of pixels each characterized by a specific response function. The PRF is the image of the PSF as recorded by the camera detectors. In the deep space environment in which Spitzer operates, the IRAC PRF is exceptionally stable. By combining the repeated observations of 6 nearby stars with different brightness (Vega, Sirius, Fomalhaut, ε Eridani, ε Indi and BD +68 1022), I have built a “High Dynamic Range” PRF for IRAC [2,3]. This PRF has a S/N as high as
and shows structures covering most of the area of the four IRAC arrays.

This characterization of the IRAC PRF is extremely important because it allows the scientific analysis of data taken in presence of very bright objects:

  1. ‣ by comparing the intensity of the structures in the HDR PSF with the ones of saturated sources (sources brighter than the maximum allowed flux that can be registered by the detectors) it is still possible to measure their flux;

  2. ‣ by subtracting a careful aligned and rescaled HDR PSF from the image of a bright point source it is possible to remove a large fraction of its scattered flux, and then search for emission of fainter nearby objects.


For examples of these two techniques in action, see: characterization of AGB stars with IRAC and the search of planetary mass companions around ε Eridani.




The IRAC High Dynamic Range PRF (Marengo et al., 2006 [2])



For more information:

[1] The Infrared Array Camera (IRAC) for the Spitzer Space Telescope, Fazio, G.G. et al. 2004, ApJS 154, 10

[2] A Spitzer/IRAC Search for Substellar Companions of the Debris Disk Star ε Eridani, Marengo, M., et al. 2006, ApJ 647, 1437

[3] Post-BCD Pipeline Processing Software Suite for Spitzer/IRAC Data, Schuster, M., Marengo, M., Patten, B. 2006, SPIE Meeting, Orlando, #6720-65

Return to Research - Last update August 9, 2008 - mmarengo@cfa.harvard.edu