Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:wise.wise_camera::1.1
NAME WISE CAMERA
TYPE IMAGER
DESCRIPTION The Space Dynamics Laboratory in Logan, Utah, designed, fabricated and tested theinstrument. They also manufactured the electronics used to control the instrument andperform onboard processing of the detector images.The instrument includes a 40-centimeter-diameter (16-inch) telescope and fourinfrared detectors containing one million pixels each, all kept cold inside an outercylindrical, vacuum-tight tank filled with frozen hydrogen, called a cryostat. Somesay the whole assembly looks like a giant Thermos bottle, while others see aresemblance to the Star Wars robot R2-D2. After launch, the hydrogen vents on thecryostat are opened and the instrument cover is ejected. Once these events haveoccurred, a scan mirror in the telescope will be the only moving instrument part.At the bottom of the instrument is a three-axis stabilized, eight-sided spacecraftbus that houses the computers, electronics, battery and reaction wheels needed tokeep the observatory operating and oriented correctly in space. Two star trackers forprecision pointing are mounted on the sides of the spacecraft bus. A fixed solarpanel that provides all the spacecraft's power is mounted on one side of the bus, anda fixed high gain antenna for transmitting science images to the ground is mounted onthe opposite side. The bus structure is composed of an aluminum skin backed byaluminum honeycomb panels. It has no deployable parts -- the only moving parts arefour reaction wheels used to maneuver the satellite.TelescopeThe WISE telescope has a 40-centimeter-diameter (16-inch) aperture and is designed tocontinuously image broad swaths of sky at four infrared wavelengths as the satellitewheels around Earth. The four wavelength bands are 3.4, 4.6, 12 and 22 microns. Thefield of view is 47-arcminutes wide, or about one-and-a-half times the diameter ofthe moon.The telescope was built by L-3 SSG-Tinsley in Wilmington, Mass. Its design uses atotal of 10 curved and two flat mirrors, all made of aluminum and coated in gold toimprove their ability to reflect infrared light. Four of the mirrors form an imagefrom the 40-centimeter primary mirror onto the flat scan mirror. The scan mirrormoves at a rate that exactly cancels the changing direction of the spacecraft on thesky, allowing freeze frame images to be taken every 11 seconds. The scan mirror thensnaps back to catch up with the craft as it continues to survey the sky.The remaining mirrors form a focused image of the sky onto the detector arrays.Before reaching the arrays, the light passes through a series of flat "dichroic"filters that reflect some wavelengths and transmit others, allowing WISE tosimultaneously take images of the same part of the sky at four different infraredwavelengths.The image quality, or resolution, of WISE is about six arcseconds in its 3.4, 4.6 and12 micron bands, meaning that it can distinguish features one six-hundredth of adegree apart. At 22 microns, the resolution is 12 arcseconds, or one three-hundredthof a degree. This means WISE can distinguish features about five times smaller thanthe Infrared Astronomical Satellite could at 12 and 25 microns, and many hundredtimes smaller than NASA's Cosmic Background Explorer could at 3.5 and 4.9 microns.DetectorsLight gathered by WISE's telescope is focused onto what is called a focal plane,which consists of four detector arrays, one for each infrared wavelength observed byWISE. Each of the detector arrays contain about one million pixels (1,032,256 to beexact). This is a giant technology leap over past infrared survey missions. TheInfrared Astronomical Satellite's detectors contained only 62 pixels in total.The 3.4- and 4.6-micron detectors convert light to electrons using an alloy made ofmercury, cadmium and tellurium. The electrons from each of the million-plus pixelsare measured on the spot every 1.1 seconds, and the result sent to the instrumentelectronics. These detector arrays, a type known as the HAWAII 1RG, were manufacturedby Teledyne Imaging Systems, Camarillo, Calif. They need to be warmer than the restof the instrument to improve their performance. The 12- and 22-micron detectors senselight using silicon mixed with a tiny amount of arsenic. They have readoutelectronics specially developed for the low-temperatures of WISE and weremanufactured by DRS Sensors & Targeting Systems, Cypress, Calif.CryostatBecause WISE is designed to detect infrared radiation from cool objects, thetelescope and detectors must be kept at even colder temperatures to avoid picking uptheir own signal. The WISE telescope is chilled to 12 Kelvin (minus 261 degreesCelsius or minus 438 degrees Fahrenheit) and the detectors for the 12- and 22-microndetectors operate at less than 8 Kelvin (minus 265 degrees Celsius or minus 447degrees Fahrenheit). The shorter wavelength 3.4- and 4.6-micron detectors operate ata comparatively balmy 32 Kelvin (minus 241 degrees Celsius or minus 402 degreesFahrenheit). To maintain these temperatures, the telescope and detectors are housedin a cryostat, essentially a giant Thermos bottle.The WISE cryostat, manufactured by Lockheed Martin Advanced Technology Center, PaloAlto, Calif., has two tanks filled with frozen hydrogen. The colder, or primarycryogen tank, the smaller of the two tanks, cools the 12- and 22-micron detectorarrays. To achieve this low operating temperature, a larger 12-Kelvin secondary tankprotects the primary tank from nearly all the heat from the outer structure of thecryostat, which is comparatively warm at about 190 Kelvin (minus 83 degrees Celsiusor minus 117 degrees Fahrenheit). This secondary tank also cools the telescope andthe 3.4- and 4.6-micron detectors. Small heaters are used to warm the 3.4- and4.6-micron detectors from 12 to 32 Kelvin.It is important to maintain a vacuum inside the cryostat when it is cold and on theground; otherwise air would freeze inside it. It would become a giant popsicle. Adeployable aperture cover seals the top of the cryostat while on the ground toprevent air from getting in. After WISE is safely in orbit, a signal is sent to ejectthe aperture cover. Three pyrotechnic separation nuts will fire, and the cover willbe pushed away from the spacecraft.An aperture shade is mounted at the top of the telescope to shield the open cryostatsystem from the sun and Earth's heat.
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REFERENCES Wright, E.L., Eisenhardt, P.R.M., Mainzer, A.K., Ressler, M.E., Cutri, R.M., Jarrett,T., Kirkpatrick, J.D., Padgett, F., McMillan, R.S., Skrutskie, M. 2010. TheWide-Field Infrared Survey Explorer (WISE): Mission Description and Initital On-OrbitPerformance. Astron. J. 140, 1868-1881. doi: 10.1088/0004-6256/140/6/1868