Xref: utzoo sci.space:4187 sci.crypt:740
Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!karn
From: karn@faline.bellcore.com (Phil R. Karn)
Newsgroups: sci.space,sci.crypt
Subject: Re: satellites
Summary: aperture synthesis
Keywords: resolution
Message-ID: <1708@faline.bellcore.com>
Date: 13 Jan 88 23:15:30 GMT
References: <873@uop.edu> <2166@umd5.umd.edu> <4910@well.UUCP> <1952@netsys.UUCP> <990@devvax.JPL.NASA.GOV>
Organization: Bell Communications Research, Inc
Lines: 23

> ...You don't need a huge round mirror to get the
> aperature you want--just build a frame that will stay rigid in microgravity
> and hang several smaller mirrors on it with a common focus.  What's the
> resolution for a mirror with an effective aperature of, say 20 meters?  How
> many 1 meter mirrors would it take to get the interferometry to come out right?

Yes, but those mirrors will have to be positioned to accuracies of a
fraction of a wavelength. That's easily done at radio frequencies, but
optical frequencies are a different story. I'm not saying it can never
be done, only that it's well beyond our capabilities at present to do it
on spacecraft. Not to say that there probably isn't a few billion buried
somewhere in NRO's black budget for R&D into this sort of thing. (Can
you say 'Welfare for Engineers?' Good! I knew you could!") :-)

Don't be *that* sure you don't need light-gathering capability, even
when imaging the sunlit earth. Remember you are imaging a narrow field
with a very long "lens", so the f-rating will be very large. You're also
moving along at a good clip (~7 km/sec) so there's something to be said
for being able to use short exposure times.  Any photographer can tell you
that ASA 400 film isn't all that fast when you're using handheld telephoto
lenses, even on a sunny day.

Phil