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

[Greg Troutman <mor@crl.com>]

Brent Hutto wrote:

Hi!  I've been lurking for a month or so and will unveil my incredible
ignorance with some comments on this message!

> 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.

Me too.  I finally got off the pot and started buying some cheap doo
dads that I plan to use in an 8" "purchased optics" dobsonian.  I
already own some incredibly cheap binoculars and majorly cruddy 2.5"
refractor, which have been fun for a beginner, but I'm ready to get a
little serious here.

> Recently, I came across a couple of interesting Web sites. University
> Optics seems to sell everything you need to build a Newtonian OTA
> (yes, I know there are numerous other vendors, they just have a
> convenient Web site). 

Besides a pretty thorough line of products, the quality _appears_ very
high to me.  I was hoping someone who has used their components could
comment on that and contrast to the components in commercial (both
cheapo and higher end) dobsonians. I'm thinking of getting primary,
secondary, mirror cell, and 4-vane from them.  Maybe I'll build my own
mirror cell and vane though, unless I hear that the offerings from
University are really fantastic quality, in which case I'll probably
just spend the bucks.  The prices seem good.  Cheaper than equivalent
parts from Orion, despite seeming to be higher end...

> 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. FWIW, I'm looking at the mount from the
> sidewalk plan but would prefer more of a "high-tech" focuser and
> secondary mirror mount than their do-it-yourself versions - not that
> there's anything wrong with that ;-)
> 
> 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. 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. Surely something made
> out of 1/2" or 5/8" ply, maybe with a 3/4" ground board would be
> sturdy enough. What would be a target weight for a sturdy rocker box
> for a 6" f/8 'scope, 25-30 pounds?

The SFSA plans I've seen are for 8" or 10" designs.  I would think high
grade 1/2" or 5/8" plywood would be just fine for a 6".  The cheap
commercial mounts for 6" scopes weigh in the 20-24 lbs. range from the
specs I've seen, so if you're building something from heavier material,
you probably should expect 25-30 lbs. I'd guess.  But you can probably
figure out ways to lighten it (hole saw?) without sacrificing much
sturdiness, and maybe get it down to 20 lbs...?
 
> What are the "hidden" difficult steps in building a basic, ugly
> Dobsonian mount like in the sidewalk 'scope plans? 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?
> 
> 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?

I got a 10" x 5.5' raw Sonotube for $12 and 4 x 1.5" round teflon discs
with screw holes and screws for $7 from a local building supply place. 
So I'm $19 plus tax in the hole right now.  I'm sure I can build the
rocker and tube box, and the mirror cell looks pretty easy as well... 
Though there aren't plans, the written description of the cell for the
Scope Like Alice at one of the ATM sites sounds nice to me.... I've got
some access to assorted scrap plywood and formica that I can probably
get free, but if I can't scrounge up good enough pieces, it won't kill
me to buy it.  Maybe another 70 or 80 bucks in miscellaneous there?  Now
the purchased optics can go anywhere from $170 at Coulter, to $300 or so
at University, to...?  Then there's a good focuser (also from
University?  I guess my main reason for responding here was to try and
evoke some commentary on just how good their stuff is...)  At least just
to get started I already have some cheap kellner eyepieces and can
possibly rig a finder from the binocs or 60mm refractor I've already
got--if I feel the need.  I'm hoping to build a "better than Celestron"
dob for about the same cost (or less), but it's hard to tell, huh?

Obviously, like Brent, I'm hoping to hear from the pros.  Especially if
you've got any great optics and/or hardware you can sell for less than
the web dealers ;)


--
mor@crl.com
http://www.crl.com/~mor/