........Cryostat
Because WISE is designed to detect infrared radiation from cool objects, the telescope and detectors must be kept at even colder temperatures to avoid picking up their own signal. The WISE telescope is chilled to 12 Kelvin (minus 261 degrees Celsius or minus 438 degrees Fahrenheit) and the detectors for the 12- and 22-micron detectors operate at less than 8 Kelvin (minus 265 degrees Celsius or minus 447 degrees Fahrenheit). The shorter wavelength 3.4- and 4.6-micron detectors operate at a comparatively balmy 32 Kelvin (minus 241 degrees Celsius or minus 402 degrees Fahrenheit). To maintain these temperatures, the telescope and detectors are housed in a cryostat, essentially a giant Thermos bottle.
The WISE cryostat, manufactured by Lockheed Martin Advanced Technology Center, Palo Alto, Calif., has two tanks filled with frozen hydrogen. The colder, or primary cryogen tank, the smaller of the two tanks, cools the 12- and 22-micron detector arrays. To achieve this low operating temperature, a larger 12-Kelvin secondary tank protects the primary tank from nearly all the heat from the outer structure of the cryostat, which is comparatively warm at about 190 Kelvin (minus 83 degrees Celsius or minus 117 degrees Fahrenheit). This secondary tank also cools the telescope and the 3.4- and 4.6-micron detectors. Small heaters are used to warm the 3.4- and 4.6-micron detectors from 12 to 32 Kelvin.
It is important to maintain a vacuum inside the cryostat when it is cold and on the ground; otherwise air would freeze inside it. It would become a giant popsicle. A deployable aperture cover seals the top of the cryostat while on the ground to prevent air from getting in. After WISE is safely in orbit, a signal is sent to eject the aperture cover. Three pyrotechnic separation nuts will fire, and the cover will be pushed away from the spacecraft.
An aperture shade is mounted at the top of the telescope to shield the open cryostat system from the sun and Earth's heat.
The expected lifetime of WISE’s frozen hydrogen supply is 10 months. Since it takes WISE six months to survey the sky, this is enough cryogen to complete one-and-a-half surveys of the entire sky after a one-month checkout period in orbit.
Gefunden hier:
http://www.nasa.gov/mission_pages/WISE/spacecraft/index.html
Besser zu verstehen....WARUM.... hier (ab Seite 8):
http://wise.ssl.berkeley.edu/documents/WISESPIE_2008.pdf?product_id=790087
Tom