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'[EE]:: [AR] cooling in space -- Actual Data'
2007\11\27@070354 by Apptech

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Just for interest.
>From ARocket.
Cooling a telescope in space.




Subject: Re: [AR] cooling in space -- Actual Data

One can get a better feel for cooling in space from Spitzer
Space Telescope
data.  Spitzer is an infrared imager that operates at about
5.5 Kelvin.  It
is in a heliocentric orbit at 1 AU.  It trails the Earth by
about 0.083 AU,
because the IR heat load from the Earth would degrade its
life.

Its operating temperature is partly achieved by having the
solar panel
between the sun and the telescope at all times, partly
through evaporation
of liquid helium (II) from a cryostat on board.  The
instrument was launched
"warm," and reached 34.5 Kelvin within a few days just
through radiation.
After that, the cryostat was brought on line.  The telescope
reached 5.5
Kelvin about 41 days after launch.

Spitzer carried about 49 kg of helium II at launch, and
after the chill down
had about 43.4 kg left.  With the shadowing from the solar
panel, the total
heat load on the telescope is only 5.8 mW, so the remaining
helium II is
good for a total of 5.3 years mission life.

So "just" shadowing Spitzer from the sun and earth allowed
it to reach 34.5
K.  As Henry notes (below), a simple sunshade isn't quite
enough.  Spitzer's
design is quite ingenious, and it must be operated very
carefully.  However,
the operational experience shows what CAN be done.

MSK


WHICH WAS A RESPONSE TO:


{Original Message removed}

2007\11\27@080003 by Alan B. Pearce

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>(See, for example, the JWST design, at just under 50K --
>they'd run it colder if they could.)

Umm, not necessarily according to a presentation we had here from one of our
local staff involved in the JWST design. The aimed for temperature is
dependant on what the instrument is searching for, and the technology you
are using. It is quite possible that the noise in the detectors being used
is going to overwhelm any detector, so making it heaps colder than a certain
amount below the detectors noise level doesn't gain you anything, only adds
to the cost.

We currently have the Spire instrument that is going on either the Hershel
or Planck satellite (I can never remember which of the two it goes on) under
test, and the guy responsible for organising the pumping and general
mechanical aspects of the test rig would take delight in putting on the
status board "pumping at 0.25K (probably the coldest place in the Universe)"
...

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