[ATM] The Public Terrestrial Telescope Project: An Update

["David Sleeter" <d.sleeter@roadrunner.com>]

First of all, my thanks for everyone who's replied to my previous posts on
this subject. Your suggestions have been very helpful, and here's a report
on the current state of the project.

Regarding the optics, the objective will be one of the inexpensive 80mm.
clear aperture lenses available from the Surplus Shed or Hands-on Optics. As
of today, I've ordered an F5 from the Surplus Shed ($29.00) and an F7 from
Hands-On ($59.00). I need to experiment with them to decide which one I
want.

A big limiting factor in the scope's design is the availability of a
suitable erecting prism, and through a process of elimination, I've chosen a
1-1/4" GTO 45 degree "Premium" erecting prism ($69.00), which I ordered a
few days ago from Hands-On. It has a clear aperture of 25mm., it's
identical to the 1-1/4" Williams Optics prism, except it costs less, and
it's the only prism with a 25mm. clear aperture that's small enough to fit
in the tube assembly I'm building. Most of the other prisms have smaller
apertures, and the ones that don't are too big.

On the eyepiece, I've eliminated the zoom eyepieces because I want an
inspiringly large field of view, and the zoom eyepieces all have
comparatively narrow fields of view. Also, I'm going to seal the eyepiece in
a stainless steel weather proof housing with an optical window over its eye
lens, and this housing would eliminate access to the zoom control. A turret
is impractical because it would greatly complicate the scope's design.
Remember, I can't use a commercial turret (they're too expensive anyway).
I'd have to design and machine one myself.

Unless I'm willing to spend hundreds of dollars on a high-end 1-1/4" wide
field eyepiece (which I'm not), I'm limited to the less-expensive 2"
versions, and this causes an additional problem. Unless someone on this list
knows of an alternative that I've missed, there is only ONE inexpensive 2"
wide field eyepiece (of the right focal length) that will work with the
1-1/4" erecting prism. All the others would cause vignetting problems. This
single exception is a brand new eyepiece that's just hit the market. It's
being distributed by Baader Planetarium. It's their 24mm. "Hyperion", and it
works with both 2" and 1-1/4" systems. It has a 68 degree field of view, and
a HUGE 1-3/8" dia. eye lens with a generous 20mm. of eye relief. I've
ordered one from OPT (Oceanside Photo & Telescope) at their introductory
price of $99, but they're on backorder with a projected delivery time of
December or maybe January. For a $99 eyepiece, the specs I quoted above are
so exciting that their initial shipment sold out on the first day.

With this 24mm. eyepiece, the F5 lens will give me a magnification of about
17X, and with the F7 lens, I'll get about 23X. I want to experiment a little
before I make my final decision.

The exact design of the mount (a simple alt-azimuth) will vary a little
according to the length of the optical tube assembly (the tube assembly's
balance point will have an effect), and to test the lenses and decide which
magnification, F-ratio, and tube-length I want, I need that eyepiece which
is on backorder. So for now the project is on "hold", and I'll resume work
when the eyepiece arrives.

David Sleeter/Moreno Valley, CA


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