[ Overview | In the field | Detailed plans | Flavioscope | Don Fessenden's Miatascope | Fiatascope ]

The MiataScope

The MiataScope is a 10" Truss-tube Dobsonian telescope designed to fit into the trunk of a Mazda Miata.

The Miatascope is a proud father. Flavio Vianna of Brazil has built a somewhat improved model based on the Miatascope.

The Miatascope is now a proud uncle. Don Fessenden has built his own -- brace yourself -- fifteen-inch Miatascope. Not based on my plans, but at least inspired by them.

Jason from CloudyNights.com has built a telescope based on my design which fits in a Fiat. Aptly named the Fiatascope. (pic), (Cloudy Nights)

Overview

The Dobsonian telescope is named after its inventor, John Dobson. John Dobson was an ancient monk who lived in a monastery back in the 20th century in a far-away place called Sacramento. Working secretly, late at night, he learned how to make his own telescopes using smuggled glass, scrap lumber, and sand from the garden. For instructions on how to make your own telescope out of scrap, see the How to Build a Dobsonian Telescope

Ok, maybe Dobson wasn't ancient when he invented the telescope -- which he calls a "sidewalk telescope", by the way, not a Dobsonian -- and he lives in San Francisco now, but the rest is true. His biography is at this web site: John Dobson, Dobsonian Telescopes, and The Early Days of the Sidewalk Astronomers.

Dobson teaches telescope-making at the Randall Museum in San Francisco several times a year.

A truss-tube Dobsonian is a Dobsonian telescope where the main tube of the scope has been deleted and replaced with a system of poles, called truss tubes, which connect the lower mirror section with the upper "secondary" section. Truss-tube telescopes are not as cheap to make as traditional Dobsonians because aluminum poles don't come cheap, but they have numerous weight and portability advantages.

Here are the parts of the MiataScope, starting from the outside in.

Rocker Box

Doubles as the carrying case. Circular cut-outs on the side are the altitude bearings. The front is mostly cut away, partly to allow the scope to swing out. In addition, it was necessary to cut away the front to clear the Miata trunk latch. It was the trunk latch that actually determined the height of the rocker box front.

Note the fact that the front corners are cut out. This is to clear the lip of the trunk lid. I measured this by building a cardboard mock-up of the rocker box, putting it into the trunk of my Miata, and closing the lid. Any part of the cardboard box that got crushed got removed from the final design.

There is a ½" hole drilled in the center to match a dowel attached to the ground board. The underside of the rocker box bears a layer of Ebony Star plastic laminate.

Dimensions are (TODO)


Mirror Box

Sized to fit snugly inside the rocker box. Shown with mirror installed.

The upper corners of each side contain brass threaded inserts. The truss tubes attach to these from the inside with ordinary 1/4-20 bolts.

Note the three pairs of holes on each side. These are to mount the altitude bearings. The three pairs of holes allow the altitude bearings to be mounted in different positions to adjust the balance of the scope. One hole of each pair goes all the way through for the mounting bolt. The other only goes in a short distance and mates with a small peg on the bearing. That way, the bearing can be attached with a single bolt, but will not move.

If I were to make any changes here, it would be to find a way to mount the side bearings without using a bolt that protrudes into the mirror box. The head of the bolt prevents the upper tube assembly from being stowed while the side bearings are in place. The obvious answer is to use threaded inserts.

Also, using bolts to attach the truss tubes is cumbersome and there's too much chance of dropping one onto the mirror. I won't change this scope, but the next one will have a much better clamping mechanism.


Upper Tube Assembly

Goal here was light weight. Two plywood rings are held by four plywood spacers. A fifth removable spacer holds the focuser.

This UTA is sized to fit upside-down inside the mirror box. The mirror cell just clears the hole in the upper ring. My original UTA was three inches shorter and rested above the mirror cell. I rebuilt it because I had a bright idea for a sliding-board focuser that didn't pan out. I now wish I had kept the old UTA, as weight is critical.

The secondary holder is completely primitive -- taken straight from Dobson's sidewalk telescope plans. The holder is a short section of 1½" closet rod with four slits cut down it's length with a table saw. Four wooden vanes are glued into the slits and the whole rig is painted black. The spider is not attached to the upper tube assembly by anything but friction. So far, it hasn't budged a millimeter, but I will probably add a safety wire to it, just in case.

Here is a closeup of the top of a pair of truss tubes, showing the mounting hardware.

The ends of the tubes are simply hammered flat, and ¼" holes drilled through. A 2" x 1½" x ½" wooden block has a slit cut into it wide enough to accommodate the flattened truss tubes. ¼" holes are drilled, and pieces of ¼" dowel are forced through and cut off flush.

There is a small dowel peg in the block that mates with a hole on the underside of the UTA to align them. An ordinary draw hasp from the hardware store holds it all together.

In addition, the part of the draw hasp attached to the UTA acts as a hanger when the UTA is stowed in the mirror box.


Focuser

Here are two views (outside and inside) of the focuser. Extremely primitive. The white part that didn't photograph too well is a section of 1¼" diameter PVC sink drain extension with male threads. There is a hole drilled through the plywood just large enough for the threads of the sink drain extension to slip through.

The grey part on the inside is a piece of 1¼" PVC electrical conduit with female threads. It just so happens that the threads of these two parts are a perfect low-friction fit. I got the idea from Russ Orr's web page World's Cheapest Low-Profile Focuser. On his page, you can see that he painted all the parts black. Also, while the electrical conduit piece on mine is simply epoxied onto the backside of the focuser board, his protrudes all the way through to the front of his.

The focuser mounts to the UTA with two small dowel pegs on the bottom, and a single screw at the top.

The only change I'm likely to make at this time will be to install a Crayford focuser instead.


Accessories Box & Ground Board

Shown here is the small accessories box and the ground board. The ground board is sized to fit exactly on top of the mirror box, acting as a lid. The accessories box is sized to take up the space from the top of the ground board to the top of the rocker box.

On the sides of the accessories box are two bits of wood that match the bearing cut-outs on the rocker box. These hold the latches which hold the accessories box in place when everything is stowed.

The underside of the accessories box has enough space to accommodate the feet of the ground board and the two rocker bearings.

The biggest mistake I made here was making the accessories box too shallow to hold eyepieces. In retrospect, I should have made the mirror box about 1" shorter so that I could have made the accessories box 1" taller.


Stowing

First picture: Side bearings are removed from mirror box. Mirror box goes into rocker box. Upper tube assembly goes into the mirror box.

Second picture: Ground board is flipped over and covers the mirror box. Side bearings sit on top of the ground board.

Third picture: Accessories box goes on top and is latched into place.


General

The entire telescope is made from ½" birch plywood. Baltic Birch plywood would have been a better choice, as ordinary plywood tends to have occasional voids and defects in it which can be annoying.

All plywood raw edges are finished with birch veneer which is made by slicing thin strips off of a solid birch board. This is pretty easy if you have a table saw, and well worth it in the final result.

All screw holes are plugged with birch plugs. This greatly improves the final result, but makes it so you can never take it apart again.

Wood is stained with Watco "Golden Oak" stain, followed by a coat of polyurethane varnish. Golden Oak looks great on oak, but I don't think I'll use it on birch again.

star test The mirror is 10" Pyrex. I ground and polished it in John Dobson's telescope making class. I then took it to the telescope workshop at Chabot Observatory where they (a) told me how nice it looked, and (b) told me how much more work I still had to do. It was several more months before it was finished.

On the left is a photo of the scope being star-tested in Dobson's class (that's Dobson in the background.)


Acknowledgments

Much inspiration and help for this design came from:

Kittens sold separately.


[ Overview | In the field | Detailed plans | Flavioscope | Don Fessenden's Miatascope | Fiatascope ]


This site is a member of the
Amateur Telescope Making Web Ring
TM logo Telescope Making
Web Ring
Next | Previous | Random | List Sites
Next 5 | Previous 5 | Join
This page maintained by Ed Falk