Re: Building first dob - questions

From: Len Philpot (len_at_philpot.org)
Date: 10/23/04 

Date: Sat, 23 Oct 2004 17:44:20 -0500

In article <8664d343.0410231359.79a02ec5@posting.google.com>,
n_perkins2003@yahoo.com says...

> A couple of questions for you more experienced ATM builders:

...can't guarantee I fully qualify there, but a few comments.

 
> - What's the optimum eyepiece height to design the dob at? I observe
> with my little Mak-Newt using a drummer's stool, but the dob is going
> to be much higher. So do you try to design it to do most of the
> observing standing or sitting? I'm relatively short at about 5'7"
> tall.

The eyepiece height will be mostly dictated by the focal length, with
some variance available in the height of the rocker box, ground
board, etc. I suggest making them overall as short as possible for
stability.

 
> - What's the optimum eyepiece angle? Do you normally set the eyepiece
> axis to be parallel to the ground, or do you orient the secondary cage
> so that it tilts up a few tens of degrees?

This is apparently a personal preference. On the 10" I built back in
'91, I initially had it 90 degrees (flat), but later changed it to
45. On that scope, it worked great. On my current scope (14.5"), it's
about 20 degrees above horizontal. In general, you're trading off
ease at low altitudes against difficulty at higher ones. Given your
size scope, I personally would err towards making it easier at low
altitudes (i.e., I'd probably make it about 30 degrees above
horizontal). Just keep in mind that the "higher" it is, the more it
ends up "behind" the tube at high altitudes. With a 12.x" scope, the
mirror/rocker box combo probably won't be so large as to get in the
way of observing at that position, even if the eyepiece is pointing
relatively backward.

 
> - Why the widespread use of Baltic birch and apple ply? These woods
> make sense to me around the ground board and azimuth bearings, for
> stability and flatness. But it seems like the secondary cage could be

True. However, stability and flatness are desireable attributes
pretty much /everywhere/ on a Dob.

> of other materials, and in particular you could make the primary
> mirror box and the base for the alt bearings out of some more
> attractive woods (say walnut, maple, or cherry in 1/2" thickness with
> a poly varnish finish).

I understand your desire for a nice looking wood - Me too! If a 15-
ply 3/4" oak plywood was available, I'd use it (mayb it is...)!
However, BB and AP /are/ nice looking woods, too, IMO. Also, I'd
strongly recommend keeping the rocker box walls double thick to
prevent flexure. That's also the reason to keep them as short as
possible. In my old Sky Designs Dob, you could pull on the end cage
and see the single-thickness rocker box walls flex and bend before
any of the bearings gave way and moved (and they weren't that stiff,
either). Bottom line is that they were too tall and too thin to be
very stable. That translated to a spongy feel with jumpy images at
the eyepiece.

If you haven't already, I strongly suggest reading "The Dobsonian
Telescope" by Dave Kriege and Richard Berry. LOTS of good info in
there. It's published by Willmann-Bell and is available from their
website (www.willbell.com). The cost of the book will be repaid many
times over the course of designing and building a kit. Plus, it's
just fun to read if you like to build telescopes.

There are also various companies out there that sell telescope
components that could be useful (e.g., AstroSystems, etc.).

Have fun though, at any rate! 

-- 
 -- Len Philpot -> len@philpot.org     <--
 ------ ><> -----> http://philpot.org/ <--

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