From: arnold@clipper.ingr.com (Roger Arnold)
Newsgroups: sci.space
Subject: Re: Putting telescopes on the moon
Date: 23 Nov 92 20:52:56 GMT
Distribution: sci
Organization: Intergraph Advanced Processor Division - Palo Alto, CA

roberts@cmr.ncsl.nist.gov (John Roberts) writes:
>
>[..]
>Would speckle interferometry, or integration of the incoming signals over
>very long periods of time (hours to weeks) help with reception? (Other than
>the fact that planets move over such time intervals.)
>
>I think you've made your point that the resolution formula can't be extended
>out to infinity. I'd be interested in how far it *can* be extended with, say,
>100 high-precision 10-meter optical telescopes.
>
>John Roberts
>roberts@cmr.ncsl.nist.gov

To be pedantic about it, speckle interferometry isn't applicable to 
space-based telescopes; it's a specific technique for partially 
overcoming distortions and blurring caused by the unstable atmosphere 
that ground based telescopes have to look through.  

Signal integration over long periods of time does help.  The trouble
is that the improvement in signal to noise ratio is only proportional
to the square root of the observing time.  To get 10x in single-to-
noise over some reference observation, you need 100x the observing
time.

But if each element in the LBI array can individually resolve the
signal you're interested in, then you can derive spatial structure
to a level commensurate with the size of the virtual aperture that the
array synthesizes.  Given 100 high-precision 10-meter optical scopes
on the moon as the array elements, then mapping the continents on 
planets of nearby stars should be quite do-able.

In my earlier reply, I may have come across as more negative on the 
idea than I really am.  Actually, I'm a staunch advocate of space 
telescopes, precisely because they offer the potential for direct
observation of extra-solar planets.  Consider the impact of even one
fuzzy point of bluish light, whose spectrum revealed an atmosphere 
with significant water vapor and free oxygen; it would revolutionize 
our thinking every bit as much as the Apollo images of earth from the
moon.

The point I was making is that before you can start playing LBI games
with extra-solar planetary images, you need individual telescopes that 
are extremely impressive in their own right.  They have to be capable
of resolving the traces of light from the planet from the overwhelming
glare of light from its primary.  Calculations that I once did showed 
that, for the dozen nearest stars, a 10-meter aperture would do the 
job--if the optics were nearly perfect.  For a reflector, the mirror 
would need to be manufactured to the same level of tolerance as the 
Hubble mirror.  It would have to be of one piece, not segmented, and 
it would have to focus off-axis.  Light diffracted from mirror segment 
boundaries, or from an on-axis secondary mirror and support struts, 
would overwhelm the feeble light from the planet.

A telescope mirror of that size and quality, 4 times the diameter and
16 times the area of the Hubble mirror, could theoretically be built.
It would, however, be a daunting task.  Telescopes incorporating such 
mirrors would be far too large to be built on earth and shipped to a
lunar base for setup and operation by a handful of astronauts.  So--to 
get back to the original thread from which this subject split--I don't 
think one gets to count extra-solar planetary observatories in the 
justification for a lunar base.

--
Roger Arnold
arnold@clipper.ingr.com
Newsgroups: sci.space