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Old 17-07-2017, 09:50 AM
hamishbarker
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hamishbarker is offline
 
Join Date: Mar 2017
Location: Nelson, new zealand
Posts: 63
200mm f6 dobson with metal tube is operational

Maybe nothing too exciting, but having been previously only binocular equipped, it's a big jump for me.

I built a 200mm f6 dobson telescope with 50mm secondary, pretty much according to a scan of John dobson's instructions.

The primary is a gso mirrro from agena astro. Secondary was i think from ebay.

I couldn't source cardboard form tube feom concreters here in nelson ( don't you love that reception from suppliers when you ask for something they've never heard of?) But the fan shop at the bottom of my street had some galvanised metal duct at $28 for 1.2m of 250mm diameter so that looked like it would do.

The tube box is of 20mm thick pine, while the rocker is mostly 2 layers of 12mm pine plywood. ( i had it in the woodpile so used it rather than buy more) i figured one layer would be too flexy. The bottom of the rocker and the base board are both 2 layers of 12mm ply sandwiched over 3 layers of 5mm hardboard for extra mass (for low centre of gravity stability and less vibration) and more stiffness.

I bought a rack and pinion focusser on ebay. Seems ok although it's a bit too long. I re-sited the tailgate board up the tube 100mm in order to keep to focus tube out of the optical path.

A coat of urethane sealant and filler in all the nail and screw holes ( i glued and screwed when laminating the double thicknesses of plywood), done!

The 3 leg spider is 40mm wooden dowel, siliconed to the secondary, with tapered wooden blades, friction fit inside the black painted tube.

A red dot finder has been variously duct taped, double side taped and finally screwed to a wood block itself screwed to the metal tube). It's a crap ebay job, flimsy and difficult to adjust. A telrad has now been ordered to sort that out!

Found a perfectly fitting saucepan lid for a dust cover at the local op shop.

Views so far have been nice, although just from our deck with a bit of light pollution both from the port here in nelson and from inside the house as the wife doesn't see the need to turn off the living room lights as so important. highlights have included the lovely jewel box, banding on jupiter, seeing a gap in Saturn's rings, and destroying my night vision looking at the moon.

Also seen eta carina nebula but with longest 20mm eyepiece i can't fit it all in. And even managed to see comet johnson c/2015 but that was really at the limit of visibility.

The slewing action is still a bit jerky as the alt bearings are Teflon on plywood while azimuth is Teflon on an old hard gramophone record. I'll source some rough formica sometime to improve that.

My 3 1/2 year old son has seen saturn and the moon and is excited but viewing is limited to couple of minutes as it's midwinter and chilly. In the plus side at least it's properly dark beforw his bed time.

Overall a satisfying 1st build. Next iteration i think may be a trackball mount with drive so I'm looking for suitable spheres locally. Nelson has a big fishing port so I'm hoping a big plastic net float should be easy to come by.

Will post photos soon.
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