Re: ATM 4" scope plans...

["Philip J. Tramdack" <tramdack@TCNJ.EDU>]

Ron- I have experimented with a number of the elements of your design.
See below for comments-- PJT 

Ron Forrester wrote:
> 
> Some of you may have read about my 'friends and family' scope making
> plan, where I plan to make a handful of simple 4" scopes as gifts for
> friends and family (to encourage them to look up once in a while).
> 
> I have the glass for the first 3 of these, and I wanted to run my
> basic plans by you all to get a reality check. As a point of
> information, my only experience to date is the 6" f/6 (spherical)
> dob scope I just completed
> 
> - first goal -- keep it simple and inexpensive.
> 
> - 4" f/5's. Is this a reasonable focal length? Will it be difficult to
> parabolize (unlike the 6", I want to go all the way with these as
> experience for the 16" will do for myself next).

My understanding is that the f/5 will be a considerably greater
challenge to parabolize than an f/6 or f/7. Others will comment 
authoritativly on that point I am sure.

> 
> - PVC pipe for the OTA -- I know it sweats, and isn't too stable, but
> I don't see many non-wood alternatives for this, and this is small
> enough I don't think it will hurt (we're talking like 18" total ota
> length here).

I think the thicker wall PVC will be ok for f5/or f/6. My own experience
is that at f/10, say, the sagging will be a pain.
> 
> - A simple fork type mount -- I am envisioning these as being table
> top scopes, not used for anything but casual moon and planet gazing,
> and I really want to make them easy to setup, move, and not be in
> multiple pieces. Is a fork mount (wood?) reasonable?
> 
Having made several "mini dob" type mounts for light telescopes such as
this my experience is that if you go with a simple fork type mount some
kind of altitude slo mo on is very desirable. Balanciing the short tube
is hard enough. I make a slo mo with threaded rod running through 1/2"
pvc with an end cap drilled and threaded for the rod. The 1/2" slides
through a block of some dense material like nylon which has a knurled
bolt to fix it where you want. Another knob on the end of the threaded
rod allows for fine adjustment in altitude. It is almost simpler than it
takes to describe and really makes a difference.


> - A stiff single wire diagonal holder.
> 
> - Some kind of homemade friction fit focuser -- I have heard of these
> being made from pipe fittings, etc, is that right?

I think a homemade helical focuser is better than a slip fit, if you
know ahead of time which eyepieces you are going to provide (or if you
intend only to provide one) and the eyepieces are more or less parfocal,
or can be parfocalized with O rings. I have made several focusers with
stuff picked up after thoughtful searching in a large plumbing
department. The main thing is making it so you can accurately square it
on to the tube. If your heart is set on a slip fit, you can get chromed
(or not) brass tube about the right ID from the plumbing dept. that
slides nicely in a focuser with leather patches bearing on the brass
tube.
> 
> - A simple push/pull bolt cell made from some thin hardwood.
> 
> - Should I make the diagonals too? Will I save much money?

I don't think so.

> So that is the plan in a nutshell. I am trying to keep construction of
> each under $100. The glass was $20, so I have $80 left for the rest.

I think this is do-able.

Another possibility for the mount is an alt az mount made from 1 1/2"
pipe fittings with the fitting joining the tube on the side. I made one
of these once and it was pretty steady. 
> 
> Any comments, suggestions, etc would be really appreciated.
> 
> Thanks.
> rjf&

-- 
Philip J. Tramdack
Associate Dean for the Library
Roscoe L. West Library
The College of New Jersey
Box 7718
Ewing  NJ 08628-0718
vox 609-771-2343
fax 609-637-5177