[ATM] Public Terrestrial Telescope: Re: Bob May's Reply

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

Hi again:

On Saturday the 10th, Bob May said:

"Remember when you are doing the focuser that you don't need to
have inches of travel as the E{ isn't going to be changing."

Dave Sleeter asks, what is "E("? I haven't heard that before.

The Bob said, "Also, yo can limit the near field by not allowing for that
much
focus.  Generally, there is no need to focus to something a
hundred feet away so yo can take that into consideration."

Dave Sleeter says Bob's correct. The nearest things of interest (like maybe
a hawk perched on a rock) will be several hundred yards away. Almost
everything else (the city below and the surrounding mountains) will
essentially be at infinity. The objective is an 80mm. F/7 with a 560mm.
focal length, and I DO have to accomodate for people's vision. I am
nearsighted, and if I remember correctly, I think I have 20/40 vision in one
of my eyes. Using my little Celestron Nexstar 80 (it's an F/5) as a test
scope with a 26mm. Plossl, there's a difference of maybe 1/8" in the focuser
position between looking with my glasses on and looking with my glasses off.

Then Bob said, "The Rak and pinion focuser will work fine in that regard as
yoe
will only need to put an o-ring at the EP tube and the two (or
better yet the one) end f the knob shaft to gain waterproofing.
Another way is to do a twist type focusing with a cam insidewhich
will rapidly change the position of the EP.  You don't need to do
a real fine focusing like the helical focuser that we use as the
user doesn't need to have that fine a focus but rather the speed
rate that the variable fofocal length EPs have.  That ends up
with only one o-ring needed for waterproofing.
FWIW, Washing machine grease is a nice waterproofing grease for
the seal."

Dave Sleeter replies that... I'm trying to create a scope that not only
works well, but that's easy for people to duplicate, so I'm trying to
simplify the necessary machine work to the greatest extent possible. As of
now, I've decided against a rack & pinion, a spiral focuser, and anything
like a cam for the following reasons.

The rack & pinion requires the cutting of a longitudinal slot for the rack
gear to run in, and there's no way to seal the rectangular hole where the
rack runs in and out of the tube assembly. Then there's the issue of
machining the parts that hold the pinion and pinion shaft in place and keep
proper tension on the gears. This is all time-consuming, and it
unnecessarily complicates the design.

I've decide against a spiral focuser because it requires the cutting of
threads, and though many hobbyists own lathes, a good portion of those
lathes do NOT have thread-cutting capabilities. And I've decided against
other things like cams, again, for the reason that it complicates the
machine work needed to duplicate the scope.

At this point I've decided to go with the type of focusing mechanism that
Celestron uses on their Schmidt-Cassegrains because the parts are easy to
make, and there's no lathe-thread cutting involved. The objective lens will
be mounted in a cell BEHIND a clear glass optical window to protect it from
weather and vandalism, and it's VERY easy to simply arrange for the cell to
slide back and forth inside the tube. All I need to do is bore the inside of
the tube smooth, lathe-turn the o.d. of the cell smooth, and provide a
cusion of smooth bearing material in between.

I can machine the parts for the lens cell from the 3" ABS (plastic) pipe
fittings sold by Lowe's and Home Depot, that is, a 3" ABS coupling, a 3" ABS
cleanout adapter, and a 3" ABS cap. And for the bearing surface between the
i.d. of the tube and the o.d. of the cell, I can curl up sheets of stiff and
springy vinyl cut from the plastic "report covers" sold at places like
Staples and Office Depot; they're about .020" thick, and in addition to
other colors, they come in black. To move the cell back and forth, a
threaded rod will run in a female nut mounted at the rear edge of the lens
cell, and there'll be a single sleeve-bearing and o'ring where the shaft
exits the rear of the tube assembly. Like a Celestron C8, focusing will be
accomplished by turning a knurled knob on the end of the shaft.

As I make the prototype, I'll take pictures as I go, and when I'm done, I'll
publish an eBook, complete with CAD drawings of all the parts, on how to
make the scope. In the late 1990s I designed a homemade laboratory scale
that's accurate to 10 milligrams. It's made with simple hobby tools and
things you can buy at a hardware store. I'd originally intended it to be a
separate chapter in my book, "Amateur Rocket Motor Construction" (if your
curious, you can Google the title in quotes, or look it up on Amazon.com).
But by the time the rocket motor book went to press, it was so large that
there simply wasn't room, so last year (2006) I published it as a 41-page
eBook. The eBook I'll publish on how to make the telescope will be similar.

As a matter of fact, if any of you are curious, and would like a copy of
this eBook on how to make the lab scale, send me a separate email, and I'll
send it back to you as a pdf attachment. It requires some of the same basic
skills you use when "ATMing", and it's a interesting project that combines
science with simple woodcraft and metalcraft. Email requrests for this eBook
to d.aleeter@roadrunner.com.

David Sleeter/Moreno Valley, CA


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