A girl and her dob.
My homebuilt 8" ultraportable dobsonian telescope!
The design goals were:
The top cage setup, with quick-release clamps, is described on a separate page.
The mirror is an 8" f/6 I ground as part of an SJAA telescope making class a few years ago under the expert guidance of Jane Houston, and finished at the Chabot Telescope Makers' Workshop (full of wonderfully helpful people who can tell you exactly what to do to figure your mirror right). Much to my surprise, the figure seems to be working quite well; so far it's given quite nice images of the moon and planets. The cell is homemade from plywood.
The baffle is made from cardboard-covered foam core, painted flat black. The holes on the outside are for ventilation (to let cool air flow out without getting into the optical path). I don't know whether either the baffle or the holes in it make any difference; it's held in by velcro, not glue, so that I'll be able to experiment. The mirror cover (white foam core, so I can find it in the dark) sits on top of the baffle.
The tube is a 4-tube "truss"; the tubes aren't trangulated as in an 8-tube truss design, but they're angled in because the top cage is smaller than the mirror box, and that angling probably adds some strength. The design seems to be very solid in practice, with a very short vibration damping time (much to my surprise -- I wasn't at all sure this was going to work. :-) Initially, I wanted to use four monopods, and tried several earlier designs using them, but the monopods just weren't strong enough.
The tubes are 1-1/8" aluminum from Alco (a local surplus place).
They don't need to be that big, that's just what Alcoa happened
to have that day.
The lower ends of the tubes have short dowel inserts epoxied in,
then holes drilled through tube and dowel.
1/4" bolts with plastic knob handles are captive in the tubes --
no tools or loose parts to fall on the mirror
(though there's a mirror cover made out of foam core to
guard against things falling on the mirror in any case).
The bolts screw into threaded inserts in the corners of the box.
This is one aspect of the scope with which I'm not satisfied:
it takes too long to tighten the screws. I'd much rather have
some sort of a clamp with quick-releases on them.
2/5/2001 update: I wasn't happy with this setup -- it took too long to fasten the tubes, and sometimes I had problems with cross threading when setting up in the dark, and I wanted this to be a very quick and easy setup. So I redesigned the lower end, and now use hinges to clamp the tubes into the corners of the box. Pictures coming soon.
Should the tubes be painted flat black, or covered in foam? Probably. I haven't done it because they happened to come anodized to a nice-looking grey color, and I haven't convinced myself that I'm actually getting reflections from them. One of these days I'll do some experiments and see. The lack of a foam cover to insulate the cold poles doesn't bother me because if it's cold enough to matter, I'm probably wearing gloves anyway.
I'm using Melamine edging around the altitude bearings, running on
teflon bearing pads. I have some ebony star for the bearings now,
but I'm not sure there's much difference between that and melamine
edging (though ebony star is a bit prettier and I may switch just
for that reason).
The shroud is described on a separate page.
As I expected, the scope came out top-heavy (a common problem with dobs under 10", since small mirrors aren't very heavy), though much less so than I expected. I was going to experiment with offset springs to keep from having to add a counterweight, but bungee cords turned out to work very well and are quiet, light, and easy to adjust. I've experimented a bit with anchor locations for the bungee cords on the side bearings, and I'm getting close, though it's not quite perfect yet.
Shown at left:
first light, at the SJAA Slide
and Equipment Night. I had made the final assembly of the 'scope
that day, and hadn't had time to stain, balance or bungee it yet.
I thought it wouldn't work at all, but even down at the horizon,
looking at the rising moon, all it needed was a copy of Sky & Telescope
sitting on the mirror box for balance.
With all the parts, the telescope weighs about 25 lbs, heavier
than I'd expected. I could bring that weight down a bit, but
it's mostly in the mirror and mirror box, and lightening the mirror
box would mean requiring more bungee strength or some other form
of counterweighting. It moves very nicely now, so I may leave
the weight where it is.
It's been to Hawaii once, packed in a drum case, with the primary mirror removed and packed in my carryon bag.
Here it is all packed up and ready to travel. The truss-tube bag is made from one leg of an old pair of Levis (Dave's -- my legs aren't quite long enough. :-) It's a button-fly, and the buttons are (now) gold stars. The bag on the left ... well, that's where I carry the koalimation tools!