Todd Pattist wrote:

Robert Ehrlich wrote:

Astronomers use huge telescopes because they are primarily concerned
by resolution and resolution is better with big mirrors.

I used to work on mirrors larger than 50" looking at the
satellites of a country whose satellites were looking at
mine. Trust me (I hate it when someone writes that :-) -
almost all the large mirrors astronomers use were built to
gather light, not improve resolution. Over a period of
about a second, atmospheric motion produces image wander
that limits resolution to about the same resolution as a
6-12" diameter telescope. Any exposure longer than a few
100 milliseconds is blurred out to the resolution limit of
the isoplanatic patch. All the rest of the diameter used to
be to there to just gather light. The big scopes were
called "light buckets" as they just served to gather more
photons into the blurry patches set by the atmospheric
resolution limit. The "light bucket" ability to gather lots
of photons from dim objects is still a major reason for
large diameter.

In the last 20 years, active image correction has been
developed that can eliminate much of the image wander and
recover the lost resolution, even on the older scopes.
Space telescopes don't suffer from this resolution loss and
they get both the higher resolution and the higher light
gathering power of a big scope mirror.

Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)

Yes, I agree that the very huge mirrors used are primarily
designed for collecting more energy. What I was meaning is that
the resolution is the motivation for using diameters larger
that what you find in e.g. terrestrial binoculars.

Anyway resolution had also its motivation for bulding big
instruments in the previous century. IIRC I read in a book
from the famous astronomer Eddington, written in the mid thirties,
that Michelson built an interferometer (which is essentially a very big
mirror reduced to two pieces of its border) with which he was able to
directly determine the diameter of the star Betelgeuse. In the
same book it is mentionned that the companion of the star Sirius
was discovered by its gravitational perturbation on Sirius before
it could be optically observed, which became later possible with
instruments with better resolution.