On 3/29/12 3:17 PM, Magnus Danielson wrote:
just so we can re-transmit it, is probably not sensible. However,
studies of these remarkable pulsars is ongoing.
Hmm, wouldn't the space-located antenna have a good chance of better S/N
as the antenna sees cold space and could be kept cold itself?
I was also thinking antenna size would be a limitation. Then I was
thinking about what WMAP has achieved in measuring the background
temperature and look back at the very early years of the universe.
Yes and no.. depending on the frequency.
A reasonably high gain dish antenna that is under illuminated (i.e. the
feed doesn't "see" the earth behind the antenna) is basically looking at
cold space anyway. One reason that Cassegrain and similar designs are
possible.. the primary feed is looking at space with the secondary
reflector in the way, so "spillover" is going towards the sky.
(moisture in the air is the big factor here.. it's easy to tell when
there are clouds overhead with a microwave radiometer)
Cryogenically cooled feeds are pretty standard items and not
particularly expensive (compared to the cost of even the smallest launch
vehicle). Cryocoolers, not dewars of LHe, by the way.
As the frequency goes up, though, the atmospheric loss rises, and
eventually, it's worth it to get above the atmosphere (Mauna Kea and
Atacama are pretty close, but there's still moisture above them).
In the mm wave and far IR is where it's really worth while, so you have
things like SIRTF and JWST. The latter is a good example of high cost,
though..
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