On Mar 11, 2:20*pm, L D'Bonnie wrote:
L D'Bonnie wrote:

snip all

L D'Bonnie wrote in news:47d59fdf$0$26249$6c5eefc5
@news.maximumusenet.com:

Perhaps my question should have been more along the lines of
Has nature imposed a limit to the maximum size a telescope could
be, where it would no longer be of any use to build bigger?
LdB

Looking at the question from a hypothetical prespective - free of the
engineering practicalities that limit materials - the answer is a
qualified no. For radio astronomy, there probably are no limits on
the
size of a useful telescopes.

For optical astronomy, the presence of interstellar and intergalactic
dust probably makes a theoretical limit on making a clear image - for
example - of a star or a planet in another galaxy. But, as we have
seen
in the past with microwave radio telescopes like COBE - even blurry
images of distant objects (the cosmic microwave background radiation)
-
can result in groundbreaking discoveries.

For radio astronomy, because of materials limits and for economic
efficiencies, some extremely large _synthetic_ telescopes are
currently
in use. These are much larger than you might intuitively expect.

Radio astronomers use very long baseline interferometry to create
synthetic telescopes many times larger than the size of the Earth.

http://en.wikipedia.org/wiki/Very_Lo...Interferometry

In the United States, the Very Long Baseline Array, a series of
interconnected radio telescopes -

http://en.wikipedia.org/wiki/Very_Long_Baseline_Array

- work together to create a synthetic radio telescope half the size
of
the Earth. The longest baseline (the diameter of the synthetic
aperture
of the radio telescope) is about 8600 kilometers.

The Europeans have a similar array - the Joint Institute for Very
Long
Baseline Interferometry in Europe (JIVE).

http://www.jive.nl/

When the two networks of radio telescopes work together - they form
the
GVLBI - the Global Very Long Baseline Array. This is a synthetic
radio
telescope with an aperture effectively the diameter of the Earth.

A larger synthetic radio telescope has been created in the past.
Through 2003, the Japanese space agency operated an orbiting radio
telescope called HALCA -

http://www.isas.ac.jp/e/enterp/missi...ca/index.shtml

- which had a maximum orbital altitude of about 24,000 kilometers.

When combined with Earth based arrays, HACLA created the largest
experimental aperture radio telescope ever known - one much larger
than
the Earth itself. As noted on the JAXA site, the Space VLBI telescope
-
even though it operated on radio wavelengths and not optically -
achieved an angular resolution "in these observations was [of] 0.3
milli-arcseconds in the 5 GHz frequency band, which is equivalent to
300
times the resolution of the Hubble Space Telescope."

http://www.isas.ac.jp/e/enterp/missi...a/achiev.shtml

But you do not need a multi-billion dollar space radio telescope to
create a truely massive synthetic telescope.

An intermediate-advanced amateur observing project is the detection
of
parallax shift of stars - and can be done will a 10 inch reflector
and
equatorial mount. In this type of synthetic telescope, a picture of
star is taken twice six months apart. The two images can be combined
to
see how the position of the star against the background stars has
changed.

http://en.wikipedia.org/wiki/Parallax#Stellar_parallax

This effectively creates a synthetic telescope with an aperture of
twice
the average radius of the Earth's orbit. The aveage radius of the
Earth's orbit is called an "astronomical unit" or "a.u." and is equal
to
about 150,000,000 kilometers. 2 a.u. is 300,000,000 kilometers.

For the future in optical astronomy, NASA has planned an orbiting
optical synthetic telescope - the Terresterial Planet Finder -

http://en.wikipedia.org/wiki/Terrestrial_Planet_Finder

- which, if realized, will be able to resolve planets orbiting nearby
stars. In 2007, the Bush administration and Congress defunded the
project for budgetary reasons - not technical reasons. The TPF's
future
remains unclear. The analogous European Space Agency proposal is
called
"Darwin", which is in the "assessment" stage of projects for
consideration after 2015.

http://sci.esa.int/science-e/www/are...cfm?fareaid=28

Finally, your pickup on the gravitational lensing was one that I
enjoyed -
using natural events - the chance positioning of large galaxies as a
"telescope".
I guess using a 200,000 light year diameter galaxy qualifies as the
largest "lens" ever used!

Hope that helps.

Peace - Canopus56