10 inch Twin-Truss Dobsonian Project

I wanted a telescope with a good sized apeture for visual deep sky observing, but the portability and storage of a large tube was impractical, so a truss tube dobsonian was the practical option, but after looking at the usual 8 pole designs I wanted something which was quick and easy to erect and dismantle with the minimum of fuss, and as few parts as possible. I tend to loose things in the dark too easily, and having lots of poles and fixing didn't look like a good option.

 

Dob

 

The Optics

The primary mirror is a 10 inch f6.3 coupled with a 50mm (minor axis) eliptical flat set made by Beacon Hill Telescopes in the UK, obviously the optics are the most important part of any telescope, as they are the components which make the instrument what it is. These were bought rather than made, my skills lie in the carpenty and mechanics, rather than optical craftmanship of mirror makers, a good mirror set was important to me, as a telescope is only as good as its optics, so I wanted to get started on a good footing.

The Build

I started with the two aluminium poles, I got these from a friend who runs an engineering fabrication company and were the seed from which my design grew. The poles are 32mm diameter with a 3.5mm wall thickness which makes for a lightweight but rigid structure on which to base the rest of the telescope. I wanted the telescope to be simple to erect so I decided that having parts which slid on and off the poles without any fixings would be the way to go.

With this in mind I went about building the secondary/focusser holder from two discs of 9mm plywood. The central hole was cut just larger than the mirror diameter, and the outer edge was made with ‘tabs’ on either side to accomodate the tubes. All this was done with a router being guided from a pivot point in the centre of the plywood, and as both discs needed to be identical they were sandwiched together so that they could both be cut at once.

ring

The holes for the tubes were cut precisely to 32mm to ensure that a good slide fit was achieved, this was done with the plunging action of the router with a bit which I had ground down to cut this diameter hole exactly. This was important as I knew that any slack introduced here would result in difficulty keeping the scope rigid and in colimation.

 

rings2

Two 3mm circular grooves were then cut into the discs to accomodate the hardboard wall. The hardboard was carefully rolled into position in the grooves in both discs and left clamped together for several days and dampened to help it adopt its new shape, this was then glued into the slots with the poles inserted in the holes and carefully aligned so that they remained parallel right down to the base of the scope. I then applied a series of resin based glue to the whole assembly to make sure it was strong and rigid.

cage

 

The Secondary Mirror Holder & Spider Vane

The spider and secondary mirror holder posed a few design possibilities, I built the secondary holder itself from a solid 40mm diameter bar of alluminium which was cut at 45 degrees and mounted on another offcut via a single spring tensioned bolt, three small adjustment screws were fitted to put pressure against the spring and also to allow precise tilt of the mirror to allow for collimating the telescope. Finally a flat was filed on one side and two 6mm holes tapped to bolt it to the spider vane.

sawing

secondary holder

The spider vane itself was made from a 12 inch stainless steel engineers ruler, this was found to have good spring-steel like properties and when held under tension proved to be very rigid and therefore became the curved vane spider assembly. The two ends of the ruler were bent to an angle in a vice to suit the inside of the tube and fixing holes drilled. Two additional holes were also drilled centrally to allow the mirror holder to be fixed to it.

spider

The foccuser unit was bought from ebay and is a 2inch Skywatcher rack and pinion assembly, this had a few problems with slop but I improved it by adding thin offcuts of teflon sheet to the inside of the drawtube, and adjusted the rack and pinion gears.

A hole was cut into the side wall with a jigsaw and the focusser carefully positioned for the fixings, I used a set square to make sure that the drawtube was running at 90 degrees to the truss poles and fixed it into position with 4mm fixings.

With the focusser attached I could now work on the position of the spider vane fixings to suit the mirror holder. I made a cardboard substitute of my secondary mirror and mounted it in the position that the mirror would finaly go. On the false mirror I had marked up the optical axis of the mirror and used a cheshire peep-hole colimator to align and mark the accurate position of the spider. This was then fixed in place with the tension in the curved ruler providing the rigidity to hold the mirror stable in the centre of the tube.

focuser

structure

alignment

Mirror Box

With the scope built I could now concentrate on the mirror box. Again using plywood two identical forms were fabricated with the router, but this time square in shape to create the box, holes for the tubes were again made so that they ran parallel to suit the alluminium tubes. Once final positions and dimentions had been decided upon the sides of the box were made and fixed using glue and screws. I left one side hinged to allow access for removal of the mirror if required, this was held closed using a simple spring catch.

The poles needed to be able to be attached securely to the base of the scope with minimal use of tools or fasteners, to do this I used two 16mm bolts which were fitted into the base of the assembly protruding into the inside of the mirror box. Two corresponding 16mm nuts were then press-fitted into the bottom end of the tubes and firmly secured in place with a 6mm grub screws creating a captive thread inside the alluminium tubes.

A 32mm diameter recessed hole was also cut in the base around the protruding bolts to allow for precision registration of the poles when hand tightened into the holes, and a nylon washer fitted between the pole and the wooden base. Two pieces of plywood were also fitted inside the box next to the mirror to prevent the poles from accidentally damaging the mirror when inserting or removing the poles.

mirror box

thread

 

The Rocker Box

The rocker box is a simple plywood box with one low wall to provide room for the mirror box to swing, with cutouts for the bearings to sit in with 2 small teflon pads screws in for the bearings to rest on, the bearings were made from 12mm formica sheet which was cut using a router to give a smooth edge for the bearings to run on the teflon pads. The baseboard was made by cutting a disc from 40mm plastic coated plywood, and 3 more teflon pads were fixed to the underside of the rocker box to provide the azimuth bearings, finaly a 10mm pivot bolt was fitted to hold it all together

A 12v Battery sits on a 'L' bracket which provides ballance for the scope as when built it is just slightly top heavy, this provides, via the switches and potentiometer, power to the cooling fan on the base of the mirror, and also regulated power to the resistors situated on the secondary mirror for dew control

The power is fed to the top of the telescope by using the two support trusses as positive and negative, with the wiring commected to the bolts at the base of the scope, whwn the poles are attached they make the electrical contact, and two small 'bullet' pins fit into holes in the tubes at the top to continue the electrical circuit along the spider vane

top

The dewcontrol consists of a series of resistors soldered together and heatshrinked to prevent moisture ingress, this glued to the back of the secondary using epoxy resin, the temperature is controlled by a small potentiometer on the rocker box

 

The Mirror Cell

I opted for a 9 point supporting cell, made from 3 triangles cut from 3mm alluminium sheet, and used 3 rubber electrical gromets in each triangle which suited the job perfectly. These were inserted through holes drilled in the aliminium triangles and then a central ‘pivot’ point hole was drilled which was then bolted with minimal tension through the plywood cell base, this created a similar pivotal effect as a wiffle tree support used in large dobsonians.

triangles

mirror cell

I also fitted 3 sprung loaded colimation bolts to the plywood cell base which fit through the base of the telescope to allow adjustment of the mirror.

mirror

 

The Shroud

The shroud was made from black 'Rip-Stop' nylon fabric, and is attached to the base of the telescope wuth press-studs, an eleasticated drawstring enables the top of the shroud to fit tightly around the top of the secondary cage assembly, it's not perfect, as I had quite a bit of trouble trying to make a square tube into a round tube, and my skills on the sewing machine dont match that of the workshop, but it serves its purpose to keep out stray light and also helps prevent dewing of the primary mirror during use

Summary

to observe the skies with a dobsonian is probably one of the most rewarding areas of astronomy, on a good clear dark night, nothing beats being behind the eyepiece of a telescope that has the ability to show you the faintest, most elusive parts of the sky that you often only read about in the books and magazines, of course for me, I started with a mirror, and a few basic plans in my head, the mirror arrived before I built a single thing or made a single sketch, so for me the telescope, and the fact that it did finally materialise, makes the observing much more of a pleasure, that coupled with my own personal want to have a usable piece of kit that was ready to go in an instant, I think I found a balanced solution which solve one of lifes little problems