PROCEEDINGS ARTICLE | July 9, 2008
Proc. SPIE. 7014, Ground-based and Airborne Instrumentation for Astronomy II
KEYWORDS: Cooling systems, Telescopes, Imaging systems, Cameras, Nitrogen, Head, Space telescopes, Charge-coupled devices, Cryocoolers, Liquids
DECam, camera for the Dark Energy Survey (DES), is undergoing general design and component testing.
For an overview see DePoy, et al in these proceedings. For a description of the imager, see Cease, et al in
these proceedings. The CCD instrument will be mounted at the prime focus of the CTIO Blanco 4m
telescope. The instrument temperature will be 173K with a heat load of 113W. In similar applications,
cooling CCD instruments at the prime focus has been accomplished by three general methods. Liquid
nitrogen reservoirs have been constructed to operate in any orientation, pulse tube cryocoolers have been used
when tilt angles are limited and Joule-Thompson or Stirling cryocoolers have been used with smaller heat
loads. Gifford-MacMahon cooling has been used at the Cassegrain but not at the prime focus. For DES, the
combined requirements of high heat load, temperature stability, low vibration, operation in any orientation,
liquid nitrogen cost and limited space available led to the design of a pumped, closed loop, circulating
nitrogen system. At zenith the instrument will be twelve meters above the pump/cryocooler station. This
cooling system expected to have a 10,000 hour maintenance interval. This paper will describe the
engineering basis including the thermal model, unbalanced forces, cooldown time, the single and two-phase
flow model.
