----- Original Message -----
From: "Andreas Derwahl" <andreas@duda-derwahl.de>
> Hi James,
Hi Andreas
> I had a look at your "Dadscope" and must confess that I've never seen
> anything quite like it before. It certainly is a unique telescope (and with
> unique colors, too :-) ).
Thanks, and I've fallen in love with powder coating.
> What matarial combination do you use for the altitude bearings?
They started as 2.25" aluminum bar stock, and have a Delrin plastic 'Friction
Disk'
>It looks a bit like steel to plastic, right?
Aluminum to plastic, but yes. Here's a picture I've added showing detail on one
of the Alt bearings:
http://lerch.no-ip.com/atm/Projects/Dad_Scope/Alt_Bearing.jpg
http://lerch.no-ip.com/atm/Projects/Dad_Scope/Alt_Inside.jpg
> I understand the recommended ratio of
> mirror to bearing diameter is something like 1 : 1 / 1.3 (from
> Kriege/Berry).
I'm pretty good at ignoring rules and regulations :)
> I guess you don't need alt bearings that big, since you can balance your
> design very well, but how are you satisfied with the handling?
Two things I should have done
#1 I should have extended the internal OTA mounting plate such that it tied in
with the vertical aluminum framing member. As is the alt bearings are attached
only to the 1/4" thick birch plywood. This causes the scope to fail the 'bump'
test at high powers, as it oscillates on the plywood.
#2 Where the alt bearing rests in the saddle of the rocker 'box', I should have
added a method of preventing the Alt bearing from rotating in the saddle.
Something simple like a slot in the alt bearing, and a pin in the saddle would
work.
>Can you adjust the drag on the alt bearings?
Yes, it works very much like the drag adjustment on a fishing pole. By
tightening the external nut on the alt bearing, it applies more pressure to the
delrin friction disk. Works very well until the Alt bearing starts to rotate in
the rocker box saddle (see things I should have done #2 above)
>
> I am asking because I am currently building a 10" f4.9 travelscope (rough
> design of the OTA can be seen here:
> http://www.duda-derwahl/ATM/aluscope2.jpg) and want it to have a collapsible
> rockerbox made from a plywood ring and aluminium tubes
I can't seem to find the web-page / image. Is it actually this link?
http://www.duda-derwahl.de/ATM/aluscope2.jpg
> For static reasons one of the designs I was thinking off, is very similar to
> yours, with maybe a slightly larger bearing which is covered with ebony star
> formica on the side and a piece of springloaded teflon pushing against it
> for adjustable drag. That way I could have the favourable near triangle
> arrangement of your rockerbox, and still finetune the altitude drag to match
> the azimuth drag for optimum performance.
>
> I know, it is a difficult question to answer, but from your experience, do
> you think this could work in principle? Has anyone ever done a design like
> this?
I don't really have a lot of experience with things like this, I pretty much
make it up as I go and hope and pray it works. If it turns out to have
problems, I make up ways to solve to the resulting problems, and repeat :)
My aluminum on delrin alt bearings seemed to work out pretty good. They are
adjustable from very little stiction, to so much that its nearly impossible to
move in Alt. In both cases, once the static friction was overcome, the kinetic
friction was very smooth. However, I may have just gotten lucky...
> Certainly your rockerbox looks like it could easily take a much heavier
> scope, very solid.
Indeed, I way over built it, but I worked with the materials I had left over
from building the 16" Bino-scope.
Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"
" Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent. "
Calvin Coolidge
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