Re: [ATM] My 20"

["bill kocken" <wmkocken@hotmail.com>]

We probably need more details.  What is your tube made of? How long is it? 
Your advice may be good, but it's hard to tell with what you've said so far.

With a 20" mirror, most ATMers build a truss tube design.  There are many 
examples and web pages on the net where people proudly display their 
project.  There is also an excellent book by Kreige and Berry, called The 
Dobsonain Telescope, a  Practical Manual for Building Large Aperture 
Telescopes.  Of course, people can and do build 20" size scopes with 
concrete form tubes ( Sonotube) or other materials but they get pretty hard 
to handle at that size and weight.

Bill Kocken

----- Original Message ----- 
From: "David Williams" <dwilliams1128@earthlink.net>
To: <atm@atmlist.net>
Sent: Sunday, October 03, 2004 12:19 PM
Subject: [ATM] My 20"


Hello

Everyone my 20" telescope is a very nice project, but I still have some 
problems.  Recently I tried to use it to look at the moon with.  But the 
tube was too flexible.  Because the front end was pointed to the moon as 
there was a sag in the middle.  So I talked to a physicyst named Jerry 
Barkley who works at the Glen Oaks Community College at Centreville MI and 
he advised me to strengthen the tube by using longitudenal (length wize) 
pieces of wood not only the circles or rings (you can tell and see that I 
have used a ring or plywood to act as a baffle and as a stiffener).  And he 
says that I should notch the lengths and rings together and when it's in 
good shape use liquid nails.

Is this good advise?

So basically I haven't seen anything with my telescope except what I showed 
you guys with, the trees.  And I need more info and knowledge about figuring 
and parabolizing.  I know that a spherical mirror doesn't reflect all the 
light rays to the similar plane as much as a parabolic mirror does.  But my 
friend Jerry (another Jerry) who made some telescopes says that that's the 
reason that the trees aren't coming into sharp focus.  And the contrast 
isn't good.

Is that right?

Also I wondered about the floating point mirror cell.  I see that you guys 
say that the mirror, my mirror isn't mounted right.  But I don't understand 
how a floating point cell works and why it works.  And what's the advantage 
of it.  So far I've seen people using a sling to support one side of the 
mirror, but it looks like the mirror could fall forward.  And if you use 
silicon to hold the mirror that isn't very sturdy.  The mirror could come 
free of that.  Also I don't know what is PLOP.


Thanks to you all.
Bye
David
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