[ATM] The Public Terrestrial Telescope: Further Reflections and an Update on the Project

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

Hi again Folks:

This past week several of you have commented on my public terrestrial
telescope project, and in the process, provided some good suggestions and
other information that's been helpful in refining my design. Here are some
new thoughts of my own, and a report on the current status of the project.

THE TUBE ASSEMBLY

When building a refractor with an eyepiece and an erecting prism, there are
3 ways to focus the instrument. The first is to keep the distance between
the eyepiece and the prism fixed, then move them back and forth together
with a rack and pinion or a slide-focuser; this is how a standard refractor
works. But in a scope that's going to be mounted permanently outdoors where
it's permanently exposed to the weather, neither configuration is suitable
for the following reasons.

1. The rack gear in a rack & pinion has to be greased, and because the
grease is be exposed to the elements, it picks up dust and dirt that
in-short-order render it less like grease, and more like valve-grinding
compound.

2. With either an R&P or a slide-focuser, to keep dust and dirt from being
transferred into the telescope tube, there has to be an o'ring seal. In a
simple slide-focuser the o'ring can run against the circumference of the
focuser tube, but for the focuser to move smoothly, the o'ring has to be
lubricated. As the focuser is used, a thin film of lubricant is transferred
to the exposed part of the focuser tube where it attracts dust and dirt that
abrade-away the o'ring as the focuser is used.

In a rack-and-pinion, you CAN'T use an o'ring seal because the rack gear
gets in the way. My conclusion is that for an essentially untended,
remotely-located and permanently-mounted mountain-top telescope, both the
rack-and-pinon and slide-focusing mechanisms come with serious design and
maintenance problems.

A second alternative is a helical focuser (aka a "spiral focuser"). You can
make it so the eyepiece-prism assembly moves in and out of the telescope
tube, or so that the eyepiece moves toward and away from the prism. But
AGAIN, whatever the configuration, you have to seal it with an o'ring, and
dust and dirt on the exposed part of the focuser will abrade-away the o'ring
as the focuser is used.

The third alternative is to entirely seal the telescope tube, then focus the
intrument by moving the objective lens back and forth INSIDE the tube. This
is how a Celestron C8 works, and in the past few days I've come to the
conclusion that THIS is the best solution to the problem. Here's how the
tube assembly will work as I picture it now.

The objective-end of the tube (the front of the tube) will be sealed with an
inexpensive (i.e. 1/4" plate glass) optical window and a rubber o'ring. The
eyepiece and prism will be mounted inside of a sealed housing in a fixed
position on the eyepiece-end of the tube (the rear of the tube), and the
eyepiece will be covered with a smaller optical window that's also sealed
with an o'ring.

The objective will be mounted in a cylindrical cell INSIDE the tube directly
behind the larger optical window, and focusing will be accomplished by
sliding this objective cell back and forth inside the tube. Like the tube
assembly on a Celestron C8, there'll be a small knurled knob on the rear,
connected through a small bearing and o'ring seal to a threaded rod inside
the tube. The threads will run in a female nut mounted on the rear edge of
the objective cell. When the user turns the knob, the objective will move
back and forth.

The tube will be machined from 4" dia. schedule 40 steel water pipe. The
prism housing will be a modified 3" 45-degree schedule 40 steel pipe elbow.
The cell for the front optical window will be machined from steel, the
housing (with its small optical window) for the eyepiece will be machined
from stainless steel, and with the exception of this stainless steel
eyepiece housing, all exposed steel parts will be painted with Rustoleum
primer and one of the Rustoleum "Hard Hat" spray paints designed for outdoor
use on things like construction equipment.

Though the parts for the sliding objective-lens cell COULD be machined from
aluminum, then black-anodized, I'm going to make them from black ABS
(plastic) schedule 40 water pipe fittings. Four-inch dia. aluminum barstock
isn't cheap, and local anodizing shops typically have $50 minimums. By
making the parts from ABS pipe fittings, I can keep the total material cost
down to about $9, and I won't have to paint the cell because the ABS is
already colored black. This past week I found everything I need at Lowe's
and Home Depot.

According to several references on the Internet, ABS has a long working
lifespan (up to 50 years), and its other properties are ideal. Its only
drawback is that it has a poor resistance to U.V light (ultraviolet), but
this doesn't matter because the cell will be sealed INSIDE the tube assembly
where it will be exposed to the Sun ONLY when the telescope is actually
being used. The rest of the time it will be protected by the objective-lens
cap.

To accomodate the sliding cell, I'll bore the inside of the tube a little
over-size. Then I COULD line it with a thin sheet of Teflon, but Teflon is
"pricey", and it might be difficult to find for someone trying to duplicate
my design, so instead, I'm going to use one or more of the standard plastic
"report covers" you can buy at Staples or Office Depot. They're made of a
stiff grade of vinyl, I think, about .020" thick, and they come in different
colors including black. I can cut them to the exact dimensions I need with a
pair of scissors, then curl up the cut pieces up inside the i.d. of the
tube. A bearing surface made of this material will work smoothly against the
ABS cell, and it will be cheap and easy to replace.

THE MOUNT

The mount will be the simple alt-azimuth type with sleeve-type bearings (NOT
ball bearings). Garlock makes a sleeve-bearing that they call their "Garmax"
line that's ideal, and they come in all the standard sizes. Garmax bearings
are specifically designed for heavy-duty applications where they're exposed
to the weather. In a typical application, they're used as the hydraulic
cylinder-end bushings on heavy earth-moving equipment. Incredibly, they're
designed to work at dynamic loads of 20,000 psi, and static loads of 30,000
psi, and they retain their initial lubrication longer than any other porous
bearing material. They're not expensive, you can order them from most
bearing dealers, and their use in the mount for this scope will minimize and
possibly eliminate concerns about bearing maintenance.

The mount will be made of heavy steel plate. I'll probably use 3/4" for the
altitude bearing supports and 1/2" for the base plate. But for the base,
I've also got some 7" steel channel iron that I might use. Vertical motion
of the scope will be limited to 10 or 15 degrees above the horizon, and
maybe 30 degrees below; enough to bring all the surrounding scenery into
view.

The base plate will be rectangular, and on its sides and its ends I'll mount
vertical steel walls (probably 1/8" thick), and this will make the scope
look like its mounted inside of a steel shoe box. The ends of the box will
be hinged so they can be unlatched and swung down out of the way when the
scope is in use, and I'm doing this for the following reasons.

1. Twice in the past 20 years I've seen brush fires sweep over the top of
the mountain. In each case the peak (right where the scope will sit) was
licked with flame for several minutes as the fire passed, and in the event
of a brush fire, the steel shoebox will protect the scope from direct
exposure to the flames.

2. The top of the mountain is occasionally subjected to extremely high
winds. For a week or two each year, Southern California experiences what the
locals call "Santa Ana Winds". During a Santa Ana Wind, the top of the
mountain acts like an airfoil, accelerating the wind to speeds of up to 100
mph.

I personally experienced this effect in the late 1990s when, during a Santa
Ana Wind, I thought it would be fun to hike to the top. Having reached the
top, I was walking along the top ridge, and leaning at a 45-degree angle
into a steady approx. 70 mph flow of air (it was like trying to walk inside
of a wind tunnel), when a sudden 100 mph gust lifted me off the ground,
carried me back a few yards, and set me gently back on my feet.

For about 5 seconds I floated in the air in a horizontal position with my
arms at my sides (my back acted like an airfoil), and though the experience
was exhilarating (I briefly felt like Superman!), it also convinced me that
being on the mountain during a Santa Ana Wind was dangerous. Without further
reflection, I turned around and went back down. The steel shoe box will
protect the scope from the sandblasting effects of these high winds.

Finally, to give the scope some protection from the rain, I'll cover it with
a peaked heavy sheet metal roof, supported by 4 vertical posts mounted at
the corners of the base, and this roof will be mounted (high enough) so that
it doesn't get in the way of the viewer's head. Together, the rectangular
walls, the vertical columns, and the peaked roof will give the scope the
general appearance of an architect's model of a Greek Temple, and I'll
probably paint the whole thing "battleship gray".

This "temple" on its azimuth bearing will be mounted on a vertical post made
of 4" schedule 40 steel pipe sunk in concrete, and to discourage theft, I'll
assemble the entire thing with stainless steel button-head "security
screws". These special screws look like Allen screws, except that there's a
small pin in the center of their wrench sockets, and you need a special
wrench to tighten or loosen them. This will make it impossible to
disassemble the scope with regular Allen wrenches, and the button-style
screw heads will make it impossible to grip them with pliers or vise grips.

My only remaining concern is the effect of heat on the scope's plastic
components. Summertime temperatures here can reach 115 to 120 degrees F. I'm
GUESSING that the steel walls and roof of the "temple" will shade the scope
enough to prevent any problems. But if problems DO arise and the plastic
parts soften and distort, I'll replace the objective cell with one made of
anodized aluminum, I'll replace its sleeve with one made of Teflon, and I'll
replace the plastic, commercially-made prism housing with something of my
own design. When all is said and done, barring destruction by vandals, I
think the scope should last for many years.

In an email this past Friday, Richard Schwartz asked me how I'll get the
hundreds of pounds of water and concrete mix to the top of the mountain, and
the answer is that I'll carry it, 20-30 pounds at a time (in a 5-gallon
plastic bucket) to the top, where I'll hide it in the rocks. Then, when I've
cached everthing I need, a neighbor (a building contractor who's done some
other trail work on the mountain) has volunteered to mix, pour, and finish
the concrete (he does VERY nice work). And if we decide to do not just a
small footing, but an actual concrete slab that requires more materials,
another neighbor, a local Christian minister, has said that there are at
least a dozen people in his church who would LOVE to lift and carry and help
out in any way they can, and he's told me to call him if and when the need
for their help arises.

David Sleeter/Moreno Valley, CA

















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