12" Dobsonian Project

[rmcpb (Rob)]

Looking good!!

[stephenb (Stephen)]

Mirror cell completion.

Today I successfully completed the mirror cell by fitting +-45 side pins and the top pin with the PVC dowel and stainless steel fittings and mirror clips, and I added the two split pins for a sling for peace of mind. I am undecided if I will actually use it. All these pin assemblies were kindly custom-made for me by Peter Read of SDM Telescopes. I also cut some thin Kydex strips to connect the pivot bars, and adhered them with double-sided foam tape. The flotation mirror cell is now officially complete.

Secondary cage rings and struts.

Also today I cut the secondary cage rings out of A-A grade 20mm 9-ply. I then screwed them together and ran the plunge router around the inside radius to ensure that both internal radii match.

I also cut the secondary cage struts form 25mm aluminium extruded tube and fitted the captive threaded nuts inside each end.

Next week, the focuser board and varnishing the timber...

[GrahamL]

Its coming along nicely ..I'm more than happy with my 12" trusser ,,And It took me two attempts over several years to finish a scope ,, things do get in the way sometimes.. Great to see you at it again.

[stephenb (Stephen)]

Thanks Rob and Graham. A long road, and as it was commented to me the other day "Why on Earth are you making a 12"? You could buy a 12" dob cheaper than you could make one". This is true as this dob will end up about 15-20% more expensive than a 12" GSO dob, but I couldn't explain to the person concerned why I was building my own unique scope. I think the reasons go without saying.

[Starkler (Geoff)]

Originally Posted by redesford

. I think the reasons go without saying.

Quite right if you want a scope built to a standard and not just to a price
Looking good

[fguihen (Fintan)]

great job! For the bushings holding the mirror in around the edge, where did you get the rubber from? I cant find it anywhere and was contemplating just drilling through a wooden dowel and using that for the task. id still much rather use rubber for the job.

[Merlin66 (Ken)]

On the 18" I sold to Gove, the main mirror was sitting on an 18 point suspension. This was easily achieved without any major machining work by cutting six triangles from 6mm aluminium plate, fitting three felt pads at each corner on the top surface and partly drilling an 8mm hole in the bottom surface in the centre.
Arms, from 6x 12x18 brass bar were cut to act as dividers between the triangles, two 6mm drilled recesses on the top of the 6mm surface and one in the middle of the bottom surface. These "balance arms" were then mated with centres of the triangles using 6mm ball bearings embedded in silicon; this gives a 6 point support onto a single point contact underneath.
The base plate was then drilled and tapped in three places and 12mm screws with the ends ground to 6mm points placed from the back; the "balance arms" then sat on these screws; the mirror on the front.
Three restraining clips, just to the side and above the mirror gave lateral and radial protection.
Hopefully it's still working well!!!

[stephenb (Stephen)]

Hi Fintan,

I do not know what the material is call for the side, but it is not rubber, it is a hard pvc plastic in dowel-form. The do not actually touch the mirror but sit approximately 1/4 - 1/2 inch from the mirror. Peter Read from SDM Telescopes here in Australia supplied them for me. I am sure he could supply them if you provided the dimensions, but honestly, I almost made them out of timber dowel myself. Timber dowel will work quite satisfactory.

If you read this thread and other threads, there is a debate as to whether the side pins are actually needed on a mirror of my size at all (I don't know the size of your mirror), but I decided to go all the way with mine.