View Full Version here: : 200mm f6 dobson with metal tube is operational

17-07-2017, 08:50 AM
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.

17-07-2017, 09:03 AM
Forgot to add, i plan to install a compter fan blowing across the front of the mirror a la Alan Adler's sky and telescope article for better cooling and warm boundary layer elimination.

I also have a 200mm f3.75 spherical mirror from aliexpress which i bought thinking it was focal length of 750mm rather than what it actually is, radius of curvature. I might try to parabolize it for figuring learning and then build as a wide field scope maybe astroscan style to be durable for my son. Or maybe is that just too fast and difficult a job?

17-07-2017, 03:52 PM
waiting for photos :)

17-07-2017, 04:07 PM
Photos or it never happened!

17-07-2017, 04:56 PM
That is a hell of a mirror to use to learn how to figure! The re-aluminizing will cost you more than a new fully figured 8" f4 mirror from GSO will cost.
A new 8"f4 GSO is $219. I suggest you teach yourself mirror figuring on the Aldi mirror but don't re-aluminize it. Spend the re-coating budget on a new GSO mirror and use the change to buy some of the other bits and pieces then make him a telescope wrapped around a decent GSO optic.

Good luck whatever you do.

17-07-2017, 05:36 PM
Congratulations on completing the scope, nothing like viewing the universe through a telescope you made yourself.

18-07-2017, 08:21 AM
Yes, agree re coating cost. probably the only other possible use for it is to attempt microflexing but would be a lot of pull and likely end up astigmatic.

I definitely won't spend on another mirror as house spending committee chair would veto/be cross. :-)

18-07-2017, 08:39 AM
Hamish congratulations on finishing the f/6, should be a nice scope.

IMHO focal ratio around f/6-f/7 is ideal as the useful magnification from typical modest eyepieces can easily span a 10:1 range without resorting to Barlows, and the secondary is not so large as to degrade the image.

Best to buy a mirror - as Joe suggested buying one costs less than having one coated locally.

Only reason to make mirrors - or recoat them - is because what you have is both quite special and irreplaceable - such as the mirrors in my 9" Santel mak. This is a rare beast and the complete optics are 1/20 wave RMS.

FWIW the Edmund Astroscan was f4.2 and only suited to low-power - it was quite disappointing on planets or the moon which will be most novices first targets.

PS your idea of making a trackball is a good one, if you have a reasonable sphere of the right size to start with.

18-07-2017, 11:19 AM
Hey, excellent work using all the local resources available. It's all about function and not necessarily form. My first build looked like a plumbers nightmare with alum struts and large wooden boxes but it got me well and truly started.
I rebuilt it about a year later ( 10" F5) into a Serrurier Truss lightweight OTA and have since built another 4 (I think) scopes of various focal lengths and diameters. Some of them just for fun.
Hang on to the spherical mirror, you might get all ambitious one day and turn it into a Schmidt (SP?) camera, a weird beast which works with even weirder optics.
Keep us posted on your progress and projects, plenty of sage advice and skills to tap into from everyone here. Good on you to get the young fellow out there too, hope he enjoys and learns more.


21-07-2017, 07:46 PM
Ugly but works ok.

Although it looks like a lot of duct tape, the big crossed strips are just because I have not yet screwed the tube to the box and it slowly creeps down. I haven't done that because I may trim the excess 80mm or so from the back end of the tube, so the balance will change.

Then there is the tape around the fan. There is nice soft foam weather stripping under it to prevent vibrations getting to the tube from the fan. I guess I could make a better surround for the fan. But it isn't "mission critical" and it works. though it will probably fall off in summer.

The shiny brass plug in the focusser is a brass fitting with peephole I was using for collimation (a cheshire has been ordered). It's been raining since I did the latest collimation with peephole, so hopefully things will be a bit sharper (could only just see the biggest gap in saturn's rings, but my own astigmatism may be a factor there too).

tailgate and mirror-front fan (and too much duct tape, but much will go soon)

"from the business end" (wooden spider, tube needs more paint, main mirror already dusty):

profile and base (little plywood handle at the top to avoid cold hands on winter night observations)

Oh, forgot to leave the saucepan lid on for the photos.