12" Dobsonian Project

[stephenb]

I have decided to begin an new project and after a couple of months without the ability to get out into the workshop, I have decided to build a 12" truss Dobsonian - basically a scaled-down version of the "big ones". Some of you ATMers may remember my initial 18" Dobsonian project here (http://www.iceinspace.com.au/forum/showthread.php?t=23739), which I unfortunately had to end prematurely for family and health reasons. I hope this thread will last longer than two pages! This scope will be a low maintenance, basic Dobsonian - no bells and whistles - so family and others can use it. More importantly, this scope will not be "over engineered" which can be an asset to a person's ability or a hinderence, depending on who you are. If this is successful, I may venture onto a larger Dob in the future.

On with the show...

Flotation Mirror Cell Design.

The first step was to design the mirror cell and despite my previous comment "This scope will be a low maintenance, basic Dobsonian" I did decide on an 18-point flotation cell. I spent a few hours last night calculating the dimensions and drawing up the results on AutoCAD.

In most part, I rescaled the dimensions from Kriege's book and confirmed them with some software and internet resources. The tolerances of the balance and pivot points, and the size of the triangles and bars are within a millimetre or two. For me, quite acceptable. Oh, and all my dimensions are in inches, don't ask me why, I just like imperial measurements! Here are the results:

[Satchmo]

How thick is your mirror ?

[Starkler]

Is a 18 point cell necessary? My 15" 1.6" thick mirror rides on a 9 point cell no problems.

[PhilW]

..and my 1.6" thick 14" mirror rides on a mere 6-pointer, with the flotation points spaced at around 58% of radius.

The thing to do is download David Lewis's program PLOP and have a go at modelling the various options. I think you'll find that anything more than 6 points for a 12" mirror is overkill:

http://www.davidlewistoronto.com/plop/

[stephenb]

Thanks all for your advice. I actually used PLOP to do some of the calculations and I understand that an 18-point may be an overkill. By recalculating to a 9-point cell, the balance points for the triangles are only about 2mm further out from the centre of the mirror. No big deal as far as I'm concerned. I can easily make a 9-point cell with the same frame dimensions. I am making the triangles this week.


Flotation Cell Frame Welding Jig.

I knocked up a welding jig this evening from some old scrap. Normally, I wouldn't have done this, but welding can be a bugger of a job, especially when you want your work to be completely square. As anyone who welds knows that the heat can cause bowing and twisting so I wanted to minimise that as much as possible.


Flotation Cell Frame Parts.

Easy! 20mm square tube for the three horizontal rungs and 20 x 8mm flat bar for the two sideframes. All these materials were lying around in the shed. All steel cut and waiting to be welded. Money outlaid so far: nil.

[stringscope]

G'day Stephen,

Good to see another ATM scope on the way :thumbsup:

A 6 point cell should be OK for a 12" mirror, unless it is unusually thin. A bit easier to make and lower profile than a 9 pointer.

Cheers,

[Satchmo]

Analysis with Plop shows that when you allow for refocus , a 6 point cell actually gives you a smaller wavefront deformatioon that a 9 point , too.

[PhilW]

So in summary of the ideas below & adding another, a 6-pointer:

- is lower profile
- gives better theoretical results when modelled in PLOP
- is easier to fabricate, because you have no triangles to cut out, just bars.

[Starkler]

And.... allows airflow to a greater area of the glass (not covered by triangles), aiding faster cooling ;)

[stephenb]

Flotation Mirror Cell Design (continued).

Thank you Geoff, Ian and Phil for your advice and input. I have been discussing a 6-point cell with Satchmo (Mark) at length today, and the pros and cons of various designs and construction methods. All your advice sounds quite valid and makes sense, so I will sit on it and weigh it all up over the next week or so. I have designed a 6-point and 9-point cell based on my calculations.


Flotation Mirror Cell Frame.

I welded up the frame today, using 20mm square tube for the horizontal rungs and 20x5mm flat bar for the side rails. I had to cut a slot for the welding clamp as I had nowhere to attach it! I am pretty happy with the end result. I might even get it powder coated as I know someone in the industry who may be able to help.

[rmcpb]

Stephen,

The design of the 6 point would worry me a bit as it appears that the bars are not straight with the balance points inside the collimation screws. This would put a twisting force on the plate and make collimation a bit sticker than if the bars were simply straight. Worth considering??

Cheers

[Satchmo]

For the sake of clarification about the 6 point cell, the bars must be straight.

I've run the numbers in Plop for an optomised cell for Stevens case where he has allready drilled collimation bolt holes at the 56% zone. Including a little optimisation of pad angle, the flotation points on the bars will be 96mm apart ( which effectively works out at about 63% zone ) with the collimation points/fulcrums at the 56% zone. This yields a surface RMS error of 1/366 wave for the 35mm thick mirror, quite good enough by anyones standards I think.

Don't get too obsessed about the precision of the cell , there is quite a bit of leeway before you would ever notice anything at the eyepiece.

[stephenb]

Flotation Mirror Cell Design (continued).

Okay, I think we are finally getting to the home stretch. Thanks to Mark again for those figures (I still cannot get Plop to play ball). Here is the revised drawing, and I am glad you agree when I say that 1mm out here or there is not going to matter in the scheme of things. I have made the bars straight and 1" (25.4mm) wide.

[PhilW]

Looks great Stephen.

At the risk of opening a new can of worms: how are you planning to do the edge supports?

[tnott]

Interesting hot topic at the moment. Many builders of scopes with big,very thin mirrors are making sure to support the mirror on its centre of gravity. Usually with +/- 45 degree points or a cable sling. http://www.cruxis.com/scope/mirroredgecalculator.htm

But with this thickness of mirror any solution would probably be OK. That is, simple +/-45 points, seatbelt sling, or RTV-ing the cell directly to the mirror(if it doesn't wobble).

[stephenb]

tnott, you answered Phil's questions regarding the mirror supports. +/- 45 points, sling and silicon the base of the cell onto some 1/2 inch rubber o-rings to create an uniform height on each of the six pivot points. And yes, I will be using a 6-point system base on Mark's calculations and advice.

I managed to get hold of a GSO 12" with perfect optics but a damaged tube for less than the cost of a new GSO mirror.

Last weekend I made good progress with the flotation mirror cell. Instead of threading the square tube for the collimation bolts, I fitted three 1 1/2 inch all-thread joiners (3/8 thread) into the square tube and silver braized them in place rather than welding. They protrude out approximately 8mm either side of the tube.

I then rummaged through the shed for some material to use to support the cooling fan. I couldn't find any perspex or plastic, as is commonly used, and I was determined not to spend any money, but I did find some 1mm mild steel plate. This may sound heavy for some to use, but after the 70mm hole was cut for the fan, there is not much weight in the scheme of things. It is rigid enough. Tomorrow, I'll make a small bracket for the cooling fan connector, buy the connector from the local muppets at Dick Smith, then off to the powder coaters. I discovered a powder coating firm about 3kms from me and they will put it in another batch and only charge me about $50-70.

[Satchmo]

Steven, Looking good. Id only recommend either +/-45 degree edge support pads or Silicon down to float points for this mirror , a sling is not necessary.

[stephenb]

Today I made a small panel to mount the DC connector, power switch and LED using silver braizing. It consists of a long, low-profile panel with two gussets at either end, then I mounted it in between the middle and lower rungs of the cell, and braised it in place. It looks all discolored because of the silver, but once it is painted it will look okay.

Mark, I feel more comfortable using the silicone method and the +/-45 supports as a back up. I will not be bothering about a sling, although I have drilled the holes in the frame.

[Satchmo]

By all means have some 'peace of mind' safety clips safety clips positioned wherever around the mirror , but you cant both glue the mirror to the cell and have anything else touching the edge, its not good opto-mechanical engineering. A mirror glued to its cell behaves as a sling support with its C of G at the rear edge of the mirror. There is no way of co-ordinating the forces between a glued down mirror and another form of edge support ( from the rear) and you will get some kind of mirror distortion that you can't control. I'm not a fan of silicon , I think Velcro is much better , as the mirror has more freedom to relax on its mount. All that being said you can get away with a fair bit mounting a 12" mirror so I wouldn't lose any sleep over it.

[stephenb]

I change my decision and had the mirror cell frame powdercoated today, and it turned up a treat! I mounted the switch, LED, fan and connector, wired them and tested - all working okay!

[rmcpb]

Looking good!!

[stephenb]

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 (http://sdmtelescopes.com.au/). 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.:thumbsup:

[stephenb]

:thumbsup: 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]

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

[fguihen]

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]

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]

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 (http://sdmtelescopes.com.au) 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.