Re: ATM 6" Dob: How Easy? How Cheap? How Heavy?

["Guy F. Brandenburg" <guyfbran@erols.com>]

Brent Hutto wrote:

> OK, so I'd like a telescope. I think a 6" Newt would be very nice and
> a slick Dobsonian mount should work well enough. One problem is,
> I don't want to vote with my dollars for what I perceive as shoddy
> workmanship on the part of the "Big 3" manufacturers. Other problem
> is, I have a very discouraging history w.r.t. anything involving
> "building" or "do it yourself" hobbies. Oh, and did I mention I can't
> afford to spend, say, $750 on something like the old Starmaster 7"
> Dob. So here I sit, waffling.
>

That is about how I felt when I built my first one. But I didn't have any
feeling about shoddy workmanship from the manufacturers. I did know that
I was only so-so handy with tools. But when I saw Richard Berry's first
book on making scopes, it was clear that even a klutz like me could make
a telescope.

> Then I saw the Sidewalk Astronomers site at
> http://members.aol.com/sfsidewalk/dobplans.htm which makes it look
> incredibly easy to build a plywood Dobsonian mount. So I'm almost
> persuaded that it's possible.

It is possible.

> For a 6" mirror in an 8" x 48" tube, is all that 3/4" plywood really
> necessary? I checked Home Depot last week and a 4' by 8' sheet of
> 3/4" birch ply cost 50 bucks and weighs more than my sofa.

Look at other plans. Heavy means stable, but there is no reason you can't
use AB softwood plywood and then just paint it. Berry & Kriege's second
book on telescope making, from Willmann-Bell, goes into great detail
about types of wood. I think HVHC (Hardwood veneer, hardwood core)
plywood is overkill, personally. I like the idea of using scraps of
plywood left over from other projects or from a dumpster at a
construction site, personally.

> By my
> calculations, the sidewalk plans would use over 40 pounds of
> plywood and that sort of argues against the convenience of a small
> Dob for spur of the moment backyard observing.

It is in two parts; the tube and the base; you can carry one in each
hand. It's not that bad. And unlike fancy GEM or MAK or fork-mount
Schmidt-Cassegrain telescopes, you are virtually ready to observe as soon
as you plop one on the other. (except for maybe installing a Telrad and
letting the mirror cool down).

> Surely something made
> out of 1/2" or 5/8" ply, maybe with a 3/4" ground board would be
> sturdy enough.

You really want the rocker box & ground board to be rock-steady. I do not
think 1/2" would be thick enough; but there is nothing to prevent you
from cutting holes in the sides at non-critical, carefully-chosen spots,
and losing weight that way.

> What would be a target weight for a sturdy rocker box
> for a 6" f/8 'scope, 25-30 pounds?
>
> What are the "hidden" difficult steps in building a basic, ugly
> Dobsonian mount like in the sidewalk 'scope plans?

Getting the mirror, focuser, and secondary aligned.

> I can probably
> handle the screwing and glueing and I accept as given that I'd have
> to get someone to cut out some rounds for elevation bearings but all
> that seems to be left is cutting a bunch of rectangular hunks of
> plywood carefully measured to match the diameter of the tube.
> Is it something I can do over at my father's house using his Skil saw
> and router over a couple of evenings?
>

Yes, really, you can. But check out the other 2 books at least, first,
before you start buying or cutting, and also go to some star parties
while it is still light out, so you can look at other folks' home-made
scopes.

> Finally, here's how I'm evaluating the financial feasibility of the
> whole thing. I'm just going to add up the cost of primary and
> secondary mirrors, primary and secondary mirror holders, a painted
> cardboard tube and a rack and pinion focuser based on the prices on
> the U.O. Web site. Assuming a sheet of plywood costs 50 bucks and
> some teflon and Ebony Star is 15 dollars from Crazy Ed, what else am
> I leaving out that would add up to more than $10 here or there?

  Don't forget EYEPIECES and FINDERS! They are not cheap! And--screws and
paint or shellac or polyurethane and maybe primer and brushes: they add
up to a surprising amount. Also, sandpaper and wood putty and maybe steel
wool or paint thinner.