The SIRTF telescope
Posted: April 14, 2003

Illustration of SIRTF. Credit: NASA
The heart of the Space Infrared Telescope Facility is an 85-centimeter-diameter (about 33.5-inch) telescope and three cryogenically cooled science instruments that will perform imaging and spectroscopy at wavelengths from about 3 to 180 microns. The instruments are the infrared array camera, infrared spectrograph and multiband imaging photometer.

Like the Hubble Space Telescope, this mission's telescope is of a Cassegrain design, named for the French sculptor Guillaume Cassegrain, who invented it in 1672. In this design, light from distant objects in space enters the telescope and is reflected by a primary mirror at the telescope's rear. The light is then gathered onto a smaller secondary mirror suspended in the middle of the telescope near the front end. The light in turn reflects back toward the rear of the telescope, where it passes through a hole in the middle of the primary mirror. All reflective surfaces in the telescope's optics are made of aluminum.

At the rear, behind the primary mirror, is the sensor that records the image. Three centuries ago, this "sensor" would have been the eye of the astronomer peering into the telescope. Later, the living eye was replaced by photographic film. However, infrared radiation does not pack enough energy to leave a suitable image on a photographic plate. Instead, contemporary astronomers rely on electronic devices to detect infrared light.

The mission's telescope is based on a refinement of Cassegrain layout called a Ritchey-Chretien design. This design, developed in the 1920s, uses primary and secondary mirrors in the shape of hyperbolas in order to prevent an optical problem called a coma.

All its parts, except the mirror supports, are made of lightweight beryllium. Beryllium is a very strong material that works well in infrared space telescopes because it has a low heat capacity at very low temperatures. As a result, the telescope weighs less than 50 kilograms (about 110 pounds) and is designed to operate at an extremely low temperature.

Although many different wavelengths of light enter the telescope, the various science instruments are sensitive only to certain infrared wavelengths.

The telescope is attached to the top of the vapor-cooled cryostat shell. The telescope and cryostat shell are warm at launch, and are then actively chilled to an appropriate operating temperature once in orbit. Only portions of the three main science instruments that need to be chilled are contained in the multiple instrument chamber, which is mounted directly to the helium tank in the cryostat shell.

The sensors for each of the three science instruments are mounted within a structure called the cryostat, the cold-storage portion of the spacecraft that is maintained just a degree or so above absolute zero. The science instruments' electronics that don't need to be chilled are mounted elsewhere on the spacecraft structure.

The chamber that contains the science instruments is designed so that no light can get through it except for the beam reflected by the telescope's mirrors. The chamber is 84 centimeters (33 inches) in diameter by 20 centimeters (7.87 inches) high. It has an aluminum base plate and cover, and is mounted directly to the tank that contains the telescope's helium coolant.

The detectors that the observatory uses to record its observation take advantage of infrared-sensitive materials made from alloys of exotic metallic substances, such as silicon, mercury, cadmium, germanium and tellurium. When infrared radiation strikes an infrared detector, the electrical resistance of the detector changes. This change in resistance is directly proportional to the amount of infrared light. Since infrared detectors are extremely sensitive to heat, they must be kept chilled by a coolant system, such as liquid helium in the case of this mission.

Riding an awesome wave of research advances in light-detection technology made by industry and academia over the last decade, mission scientists now refer to the detectors as the "heart and soul" of the Space Infrared Telescope Facility.

Each of the observatory's three main science instruments is equipped with one or more "detector arrays," lattice-like arrangements of individual detectors. The arrays convert energy from infrared radiation into electrical signals, which are then converted into "bits" of digital data. To give an idea of the leaps and bounds of such technology, the Infrared Astronomical Satellite of the mid-1980s boasted an array of 62 detectors. The Space Infrared Telescope Facility will carry arrays of up to 65,536 detectors. Such light-gathering prowess coupled with state-of-the-art cryogen chilling capabilities gives this new observatory unprecedented infrared sensitivity.

Infrared Array Camera
The infrared array camera is a general-purpose camera that can take images at nearand mid-infrared wavelengths. Observers will use the camera for a wide variety of astronomical research programs.

The camera has four channels that provide simultaneous images at wavelengths of 3.6, 4.5, 5.8 and 8 microns. Each of these images is a square 5.12 by 5.12 arc-minutes. (An arc-minute, 1/60th of a degree, is the width of a quarter held at a distance of 100 yards. For comparison purposes, the Moon as seen from Earth has a diameter of 30 arc-minutes, or one-half degree. )

The camera uses two sets of detector arrays -- two for short wavelengths, and two for longer wavelengths. The two short-wavelength channels have composite detectors made from indium and antimony. The long-wavelength channels use silicon detectors specially treated with arsenic. Each detector array captures an image 256 by 256 pixels in size.

The principal investigator is Dr. Giovanni Fazio, Harvard Smithsonian Center for Astrophysics, Cambridge, Mass.

Infrared Spectrograph
The infrared spectrograph provides both high- and low-resolution spectroscopy at midinfrared wavelengths. Spectrometers, or spectrographs, are instruments that break light into its constituent wavelengths, creating spectra that can be studied to identify the chemicals that make up the object giving off the light.

The instrument has no moving parts. It has four different modules that make observations in different wavelength ranges: a low-resolution, short-wavelength mode covering wavelengths from 5.3 to 14 microns; a high-resolution, short-wavelength mode covering 10 to 19.5 microns; a low-resolution, long-wavelength mode covering 14 to 40 microns; and a high-resolution, long-wavelength mode covering 19 to 37 microns. Each module has its own entrance slit to let in infrared light. Each detector captures an image 128 by 128 pixels. The shorter-wavelength silicon detectors are treated with arsenic; the longer-wavelength silicon detectors are treated with antimony.

The principal investigator is Dr. Jim Houck, Cornell University, Ithaca, N.Y.

Multiband Imaging Photometer
The multiband imaging photometer will provide imaging and limited spectroscopic data at far-infrared wavelengths.

The photometer has three detector arrays. One array creates 128- by 128-pixel images at a wavelength of 24 microns and is composed of silicon, specially treated with arsenic. Another array creates images 32 by 32 pixels at wavelengths from 50 to 100 microns, while another is 2 by 20 pixels and operates at 160 microns. Both of these latter arrays use germanium treated with gallium.

The instrument's field of view varies from about 5.3 by 5.3 arc-minutes at the shortest wavelength to about 0.8 by 5.3 arc-minutes at the longest wavelength. The only moving part in the photometer is a scan mirror, used to efficiently map large areas of the sky.

The principal investigator is Dr. George Rieke, University of Arizona, Tuscon.

The observatory's cryostat will keep the science instruments at temperatures as low as 1.4 C (about 2.5 F) above absolute zero. The cryostat functions by venting helium vapor from a liquid helium tank. This tank holds about 360 liters (about 95 gallons) of liquid helium at launch.

In order to protect the science instruments, the cryostat shell that encloses them must be sealed during ground operations and launch. After the spacecraft cools down following launch, a door on the top of the shell is opened to allow light from the telescope's optics that pass into the chamber that contains the science instruments.

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