Re: [ATM] holding a tube in place in an inexpensive dobnewt

[Dave Smith <smithersscope@yahoo.com>]

--- Guy Brandenburg <gfbrandenburg@yahoo.com> wrote:

>  They work OK except that the tubes have an
> excessive tendency to slide around in the cradle. 

> I am now considering two possible remedies:
> 
> (1) a split ring made out of either plywood or metal
> strapping material or a combination of the two, 
> 
> (2) An entirely different sort of clamp that fits
> into one of the corners of the cradle and uses some
> sort of a large-handled screw to wedge it onto the
> tube. 

Hello Guy,
I was "forced" to do the same with my son's 4.5" dob
that we're currently re-re-rebuilding. What worked
well for us was option number 2. I should post some
pics on my website (note to self). I used a longish
section of 1/4-20 all thread and a self tapped union
to reach from the outside corner of the square cradle
to a piece of plywood between the cradle and the OTA
tube. On the outside the union connects to a hand nut.
On the inside the all-thread mates into a pre-drilled
pocket/hole on a piece of ply cut at 45 degree angles
to fit between the cradle inside corner and the tube.
Gentle thumb pressure on the hand nut drives the 'free
floating' piece of ply into the tube and holds it in
place. I wound up lining the cradle and the contact
surface of my ply pressure plate with felt. This forms
a snug fit that holds the OTA (without crushing the
tube) and still allows the user to rotate the tube AND
slide the tube up and down to adjust for eye height
AND adjust for different weights of EP's. It was
straightforward to tap the hole in the cradle and
reinforce the threads with thin CA glue.
This solution was easy (2x45degree cuts and a couple
of holes drilled and running a tap) and it worked
well--at least for a small 4.5" scope.

On a side note regarding this re-build, a while back I
built a wooden crayford to replace the crappy rack and
pinion original version. Although it was fun to build
and does work, with time the teflon surfaces get dirty
and it starts misbehaving. It's annoying to have to
keep clean. I just finished a new aluminum & ball
bearing crayford and will be placing the optics in a
concrete form tube in the very near future--the tube
is just TOO short for a short focuser. Looking forward
to MUCH improved mechanical performance!
Thanks and good luck!
Dave

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