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From: karn@thumper.bellcore.com (Phil R. Karn)
Newsgroups: sci.space.shuttle,sci.space
Subject: Re: Moon or Mars? Editorial in _Air & Space Smithsonian_
Message-ID: <1037@thumper.bellcore.com>
Date: 9 Apr 88 03:13:02 GMT
References: <47032@sun.uucp> <1290@hubcap.UUCP> <4076@whuts.UUCP> <1481@bigtex.uucp>
Organization: Bell Communications Research, Inc
Lines: 43
Summary: radio, not optical telescopes

> I am not an astronomer, but it seems to me that if it is useful to place
> an observatory on the far side of the moon so that Earth isn't radiating at
> the telescope, it would also be useful to place the observatory to avoid the
> Sun shining on it.  Is this not the case, or am I missing the benefit of
> placing an observatory on the moon?

The question dealt specifically with RADIO telescopes, not optical ones.
The idea is to use the moon to shield radio telescopes from the intense
din of artificially generated signals coming from the earth and
near-earth orbit. If you are doing very low frequency observations I
suppose the moon would also help shield against most of the naturally
generated signals coming from the earth's atmosphere (e.g., lightning)
and the region of space near the earth. (VLF through HF astronomy would
in any case have to be done from space to get past the earth's
reflecting ionosphere, as would microwave astronomy much above 20 Ghz or
so in order to avoid atmospheric absorption).

Although the sun does emit radio frequency energy, it emits
proportionately far more at infrared and visible wavelengths than does
the earth. Thanks to things like FM, VHF and UHF TV broadcast
transmitters, the earth already rivals the sun at meter wavelengths, and
when the radars crank up at Arecibo, Goldstone or Haystack, the earth
becomes one of the brightest objects in the entire galaxy at the right
frequencies and directions. Since the earth's atmosphere doesn't scatter
RF like it does visible (especially blue) light there's no problem in
operating surface radio telescopes during the day. Of course, in space
or on the airless moon there is no optical scattering either, so you can
also operate optical telescopes during the day. You just put a baffle
around the front, like the one on the Space Telescope.

In fact, with small antennas operating at VHF frequencies it's actually
easier to "see" the hotter parts of the galaxy than it is to "see" the
sun.  The sun may have an effective noise temperature at VHF of a
hundred thousand kelvin vs a few thousand for the galactic center, but
the hot parts of the galaxy are much larger and fill much more of the
antenna's beamwidth.  I can easily see the difference when I aim my
amateur satellite antenna (14 element yagi on 145 Mhz with GaAsFET
preamp) first in and at right angles to the galactic plane.  However, to
really "see" the sun takes somewhat larger antennas (so the sun fills
more of the smaller beamwidth) and this is in fact a very common method
for evaluating the performance of large antennas.

Phil