My 7 inch f7 Dobsonian Telescope


Introduction

The telescope shown is mostly made of aluminium with the only exception being the upper and lower formica shrouds and the plywood base box. The telescope itself has a black satin anodised finish and the rocker box and altitude bearing yoke assembly have been powdercoated texture black. All components except the secondary mirror, quick-point and eyepieces were made by myself. The scope performs well, with bright sharp images and very smooth movements.

Construction Details

Construction started with the purchase of "How to make a Telescope" by Jean Texereau and a 7" (174mm) Duran primary blank from Schott Garsco. Grinding and Polishing went smoothly with only a few misadventures. I ground the mirror in the garage but soon found out that I had to find a cleaner place to polish as I badly scratched the mirror and had to return to 800 grit to remove the scratch. I moved into the bathroom, washed it from floor to ceiling and painted my grinding table and had clean drop sheets everywhere and proceeded to polish. I had a good polish after 4 hours and the first test on the knife showed a slight hill in the centre, which was quickly removed with a few appropriate strokes.


Two tools, the grinding tool on the left and the polishing tool on the right.


Yours truely, at the polishing table and the polished out mirror with reflection of a light globe.

The mirror was tested on a Focault Tester as described by Texereau.

The focault tester and mirror holder were mounted on the wall in the lounge room and positioned at a height that would put the knife edge at a comfortable standing height. An additional benefit of this setup was that I could leave the mirror setup overnight so I could test it in the cool steady air of the morning before anyone else got up. After overdoing the correction and having to return to a sphere twice, I was happy to achieve a figure of 1/22nd wave. Artificial star tests showed the mirror had no astigmatism.


The mirror cell was of a simple 3 point mount with air vents under the mirror and in the mirror baffle ring.





When the telescope is not in use the vents are closed and a dust cover fitted to seal the lower assembly to keep the dust out and protect the mirror. Brass columnation screws and locknuts were machined and Teflon pads fitted where the screws contact the mirror.




The Top Ring was machined from a solid piece of 230 x 230 x 20mm aluminium. The rough sawn piece only just fitted into my lathe with only 0.5mm between the blank and the bed of the lathe. (a 9"Hercus) The bore was screw cut and lightning holes were drilled to lighten the ring and tapped to prevent reflections reaching the mirror.


The spider hub was cut from solid aluminium and 4 slots 0.25mm (0.010") wide were cut on the lathe by a broaching process.


The focuser was based around the helical section from a damaged 135mm telephoto lens that was purchased for $5.00 at a jumble sale. It was quite fun to hacksaw a lens in half.


An adapter was machined to accept the helical section and bored to match the curve of the focuser mount plate. Another piece was machined to hold the eyepieces 15mm within the focuser, this was to position the eyepieces closer to the secondary mirror to reduce the required size of the secondary thereby increasing contrast. The Spider Vanes were cut from 0.25mm stainless steel.


The altitude bearing was made from a reject motorcycle sprocket that I found at a machine shop. At the time I didn't know what I was going to use it for, but I just couldn't leave it sitting there when it obviously wanted a good home. I cut it in half and wrapped some 3mm aluminium strip around each half, secured with M3 countersunk screws and designed a yoke to mate the bearings and telescope tubes together. Teflon blocks were cut from a 20mm thick off cut. They were cut to size on my table saw, holes drilled and tapped M5 and mounted on the faceplate of my Lathe and bored to suit the radius of the bearings. The powdercoated altitude bearings ride on the Teflon blocks with an action that is smooth and even.

Return to Kuriwa Observatory last updated: 2nd November 2013 email Dave: (d a v e 4 g e e @ y a h o o . c o m . a u)