From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 




Ken Hunter wrote:

> I wonder if the reduced light transmission through the relay lens and
possible

> degradation to the image from a less-than-perfect lens will make up for the

> slight gain caused by having a smaller diagonal size...

One of my pipe dreams is building an observatory dedicated to public
observing that would be really well equipped.  One of the centerpieces
would be a really big (by public observatory standards) scope designed for
deep sky objects. Perhaps 1 meter aperture.  Now for deep sky work, we want
this to be capable of a fairly low power, say to make an exit pupil of 6
mm.  This is roughly 170x. You need a scope around f/6 or a bit less to get
this with a reasonable eyepiece.  Another objective would be to make the
eyepiece position as accessible as possible.  I think this would best be
accomplished by bringing the
eyepiece position through the center of the mirror.  Then, with a very
light tube and upper end, and perhaps an offset single fork mount, the eye
position would move through a quite small arc.  Except near the horizon, it
should be possible, assuming the observing floor can be raised and lowered,
for most wheelchair bound persons to get to the eyepiece.  (I am thinking
equatorial for the mount.  If alt-az, the focus could be brought out to a
Nasmyth position and not change height at all.)  If the scope is managed,
to keep the single fork arm
on top while observing, then only the lower half of the mirror cell would
tend to obstruct access.  Circumpolar gets a bit awkward, but the scope
doesn't go crazy near the zenith.

Anyhow (to get back to the relay lens topic), I have thought that perhaps a
folded Newt with relay lenses would be the way to go.  Or maybe an R-C with
a low magnification factor, the R-C focus inside the scope and relay lenses
to bring it out.  Since this monster would entail a lot of work and money,
it would
be worth it to get a good optical designer to do the design.  Perhaps then
coma correction could be included in the design.

In this pipe dream, the relay lenses are mainly a tool to get the rear eye
position with an f/number near 6, and a reasonable central obstruction.

BTW, I realize that there are several very nice deep sky objects for which
1 meter is too big for visual observing, so this pipe dream includes 0.5
meter and
0.25 meter scopes as well, and perhaps a few 8 - 10 inch Dobs, or even
smaller, that people could use manually to look at whatever part of the sky
caught their fancy, or sweep through the Milky Way.

Now the pipe dream also includes one reasonably large, perhaps 0.5 meter,
scope for the planets and a couple of smaller ones dedicated to the moon. 
One moon scope would be set up to always show the whole disc.  Another,
with higher power, would be controlled through a computer with a point and
click moon map interface.  Point and click on the part of the moon you want
to see, and the scope would pan to it.

During the day, a white light projection scope to put a 0.5 meter diameter solar
image on a white table top where just about anyone could see it, a solar
spectrum projector with enough light gathering power and dispersion to
project a
spectrum a couple of meters long on a wall, an H-alpha full disc scope,
perhaps a Ca-K full disc scope and an H-alpha prominence scope.

Although many of these images are available over the internet, I think
there is something both magical and concrete about looking through an
eyepiece that no computer screen can duplicate.  When someone looks through
an eyepiece and sees something new, the experience is more vivid and
complete than any screen can create.

OK, is that pipe dream complete enough?  What other goodies should this
dream observatory have?  Remember, the objective of this pipe dream is to
give Jane and Joe Q. Public a really first class visual experience of what
astronomy has to offer.

Mark Holm
mdholm{at}telerama.com

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