Jeff Albro: One Trip Dobsonian

One Trip Dobsonian

See how I get this scope into carry on luggage!

Background:

It all started with long hair. When I was 16 I had long, shaggy hair, and my mother *hated* it. She said she wanted me to cut it. I said I wanted to build a telescope. Several hundred dollars and a haircut later we were both happy.

That was my first telescope, an 8" f6 Dobsonian. Later on, after college, I got a car that was too small for the 48" tube. I had to replace the mirror, cut the tube down, to end up with an 8" f5. Living in an apartment, it was still too big to take out for short sessions. I wasn't outside looking because of the time and trouble it would take to make two or three trips through two doors and down a set of steps. I started using my binoculars more than my scope.

At the 2003 Stellafane, I decided it was time to make my portable dream scope. I promised myself I would have it done in time to compete in the 2004 Stellafane contest.

The Design Process:

Aperture fever is a terrible thing. It makes you waste time thinking about cramming a 10" collapsing tube assembly into carry-on luggage. That is where the sensible girlfriend comes in. Every time I'd show her a sketch of a complicated contraption, Amy would remind me that I was trying to make it easier to observe, not harder.

Our discussions made clear what I actually meant by the word "portable": a one trip system. I needed to get outside and set up in one trip out the door. I also decided on some secondary goals: design for airline carry-on capability, and use the best materials and components I could get to build something nice enough for display in my living room. This was a scope for a lifetime.

After much layout work in AutoCAD, I came up with a design that fulfilled all my requirements. I had plans for a 6" f 4.5 truss tube Dobsonian telescope. It would break down into a space 7" x 12" x 20", giving me 1" of padding to fit into the airline carry-on standard of 9" x 14" x 22".

Construction:

I built a 8" x 8" x 7" tall maple plywood box with splined miter joints for the mirror box. Without the splines the miter joint is weaker and MUCH harder to glue up. The secondary cage is a 8" diameter, 6" tall maple plywood tube. Instead of pulling my hair out trying to bend maple, I bought a maple drum shell to get a super strong and great looking tube. Connecting the two parts are aluminum C channel struts. The struts connect with small bolts and t-nuts in the plywood. I didn't need quick release clamps, because I *only* disassemble it for carry-on. I wouldn't assemble it in the field, only a well-lit hotel room.

The maple plywood rocker box is a bit unusual as it is open both front and back and narrower than the mirror box itself. This allows for the rocker box to be the same height as the mirror box for packing into a smaller space.

Many hours and hundreds of dollars later, I entered the 2004 Stellafane competition with my new scope.

Special Features:

Retractable secondary shield - A semi-circular shield is connected at the center of the circle to the inside of the secondary cage opposite the eyepiece with a single screw. When properly sized and located, this allows the thin shield to be rotated up inside the tube for security.

Well tuned balance - Because the scope is so light and the balance point is close to the bottom, I had to take special care to get the balance right. I built the whole tube assembly and found the balance point before placing the altitude bearings. Even then I had some trouble. I had to move the bearings deliberately off center towards the top of the telescope because at the zenith the weight of the eyepiece wanted to pull the scope over backwards. I also had to plan for a set of eyepieces that are all the same weight as my primary eyepiece, the Tele Vue 24mm Panoptic. (29x, 2.3 degree field)

Carrying handle - Because getting out of the house in one trip was so important I added a dowel for a handle right at the balance point. At under 12 lbs with eyepiece and finder, the tube assembly is easy to lift with one hand.

Tripod mount - I decided on a tripod mount instead of a tall Dobsonian rocker box for three major reasons: A tripod was much lighter than a chair and a rocker box tall enough to use with a chair, a tripod gave me much more height adjustment, and the rocker box could be small enough for airline-carry on. The base of the ground board has both a tripod socket, and three small feet for use on a table if I arrive at my destination without a tripod.

Primary cell - The full thickness 6" primary is silicone cemented to a 1/4" thick aluminum plate in three locations. The plate is attached to the mirror box with three spring loaded collimation screws. This thin plate allows me to get the back of the mirror within 1/2" of the back of the telescope for a more compact design. Also, the screws are at right angles to each other, making collimation easier. I only ever turn two of them to keep the plate from going in or out too far.

Extra thin Ebony Star Formica - I had a heck of a time getting the Ebony Star Formica to stick to the small, 7.5" diameter altitude bearings. I decided to thin them on a thickness sander. I liked the results so well, that I have started selling Ebony Star Flexi Strips for other ATMs to use.

Dust covers - I cut a square piece of Kydex to cover the top of the mirror box. There is a knob in the middle of it, and when I flex it, it can slip under the protruding altitude bearing attachment bolts to lock into place. The secondary is covered with a plastic tube that friction fits over the secondary housing.

The Results:

It turns out the judges liked it, and I won 2nd prize for craftsmanship and an innovative component award for my retractable secondary shield. I'm very happy with it, and it has provided wonderful views of star clusters, nebulae and even the occasional galaxy. I took it into the mountains in Vermont and could not believe the view of the Veil Nebula. Of course, now I'm planning a larger (12"-16") telescope for when I'm taking a car trip to a dark sky site. However, I will always have time to take this little guy out under the stars.

Materials and Component Sources:

Ebony Star Flexi Strips: http://www.InspirationToolworks.com

Aluminum, Teflon, and assorted hardware: http://mcmaster.com

Helical crayford focuser: http://kineoptics.com/HC-1.html

ApplePly maple faced plywood: http://boulterplywood.com/

Drum shell for secondary cage: http://www.aitwood.com/

Spider (with custom upside-down vanes) and diagonal: http://www.fpi-protostar.com/

Ebony star Formica: Home Depot

Rigel Quickfinder: Various astronomy stores.

AutoCAD Drawings