Well, the time has come to quit talk'n and start cutt'n to rework my Odyssey II.
After much consultation on the matter of design and material selection, I've finalised the design to a modified 'Albert Highe' tri-pole truss scope.
My change to his original design is the use of a more conventional solid timber mirror box. I'm just not satisfied that the original light-weight mirror box is stiff enough for a 17.5" mirror. It should also see the centre of gravity dropped down much further.
I've got the 50mm drawn aluminium tubes, 15mm hardwood ply, made braces for the spider & have had them powder coated. I've also made a couple of router jigs for the various arcs and circles needed to be cut.
I spent the last couple of hours scribing the various panels onto the plywood. Because of its unique design, my intention to make 750mm diameter bearings may not be feasable. Their ultimate diameter I will need to determine once the OTA has been completed. My intention here is to also make them removeable.
Cutting will finally start tomorrow. It got me all giddie to see the shape of the single sheet secondary cage scribed onto the plywood.
What do you think of renameing the scope 'Odysseus'?
I remember when I helped you load that big dob up in your 4WD back in October. That was a lot of work!
But that looks great, and i guess you wont need that elastic rope hook as a counter weight any more. Looks like it can be folded up like a accordion now too!
Thank you for the encouragement, gentlemen. The first cutting session went slowly, as to be expected. Though no disasters.
I should clarify, the completed scope at bottom is one of Albert's three-pole scopes, not mine. I've used it to give an idea of the direction I'm going.
All the OTA panels have now been cut, as too where the clamps cut out today. Hickny helped me out here with the clamps, giving me use of some very nice power tools. Ta mate, .
We also put together the mirror box. It has been reinfored with triangular brackets, should be able to jump on it now. The bottom panel holding the mirror cell is being held in place with cross-dowel connector bolts and a couple of side screws to allow access to the mirror (pic. 1). This should only be a rare occarance. I used epoxy glue to fix the cross-dowels in place to prevent them from rotating.
The second pic shows the secondary cage being cut out with the router jig I made. The jig shows the various centres measured out for the various radii. Tricky part here is to keep in mind the out-side and in-side edge of the router bit. The third pic show the finished 'cage'. The teat on the top is the continuation of the stop of the top strut. All three poles stop on the underside of the cage.
The fourth pic shows the completed clamps. The were made by joining three layers of the 15mm ply. Because of the size of the scope, for me the clamps make or break this design. Having such a long clamping surface makes for less flex. I'm looking at using very fine emery paper on their pole contact area to provide an exceptionally strong grip when engaged.
I'll paint the cage and clamps before fixing the clamps to the cage to avoid having the paint 'glueing' the moving part of the clamps to the cage. The clamps in the centre of the image are larger as they sit on the outside half of the cage and the top edge of the mirror box.
Glad to see you are underway with your project. Not sure what you mean about the emery paper - never seen anyone bother to do it before - though sometimes wooden pole blocks can expand and contract a little with changing weather, making the poles a little harder to slide in or requiring a bit firmer clamping.
In my experience, the drilled holes in the blocks would have more than enough friction to hold the poles tightly in place. Adding emery paper on the inside of the pole blocks would seem to me to be complicated, problematic and unnecessary.
Of the top of my head- you'd probably have to drill or sand the holes slightly oversize to make the paper fit. The layer of paper and glue might make the fit more "spongey". The paper would likely get torn off in the process of putting the poles in and out. If the paper did not go all the way around the pole, go the whole length of block or had any bumps or wrinkles in it, you would have a less snug fit around the pole than a precisely drilled piece of wood.
Tnott, I've been mulling over the emery paper. What you've mentioned has all popped into my head. I'll see how I go without it first.
One set back. I'm having to recut the clamps. The first batch was too loose. I used a 2" forstner bit after having measure one end section of one pole to gauge the pole diameter. No other section of pole length is within 1mm of the cut hole. I'll be using a 50mm forstner now. Amazing what a 1mm disprepency can do.
At last the clamps have been sorted. And it all works really well. The poles fit nicely and the whole rig is very rigid. No need for Emery paper. The clamps now donot need to be closed all the way to grip like Tarzan.
In the pics you can see that the base of the mirror box contains ply wood rings which provide an assistance to the clamps to maintain alignment. This way the whole mirror box serves to provide rigidity to the rig, not just one surface.
Next step is to begin painting the clamps, mirror box and cage ring before glueing and screwing the clamps in place. This is followed by placement of the spider and determining the size of the focusing board. Then, the primary's placement and finalizing the length of the OTA poles.
One question though - in that example photo at the start, there's a big black stray light shield. Are you planning to put that outside the light path, or hard up behind the secondary and well inside the light path, as on your 10" ?
The reason I ask is that on your 10", while the shield works, the diffraction it caused on bright objects (Jupiter) was pretty bad and personally I think the scope will work much better if the shield is outside the light path. But the best would be a lightweight closed cylinder for the top end, if you can manage it...
This is one for a light baffel outside the light path as you suggest. Yes in my 10" the diffraction spike is big, but at the same time I'm rarely looking at bright objects. Tnott mentioned he uses a signwriting corrugated plastic board as a light shield on his scope. I'm contemplating using the same for the baffle. Thoughts?
By the way, I've changed the focuser on the 10 to a 2" focuser, also low profile helical. The change in image quality was immediate. M42 looked like billowing smoke.
Thank you all for the encouragement. Blue Skies told me "it will take longer than you think". I initally thought "no way! I'll get it done by the end of January". Tick-tock, tick-tock!
After a detour, and work, I was able to get a bit more done. Painted OTA, installed the mirror, spider and focuser. Looks so cool just like this- a real scope!
Little more tweeking of the focuser needed to better align. Gee laser collimators are unforgiving!
I'm assembling the OTA first on these short struts to reduce any excessive pressure from the screwing of components. Once completely satisfied I'll do the final pole length determination.
Also cut out the mirror box cover. None too soon either! First thing I did before installing anything. Already paid for itself!
Man, I thought the mirror was dirty! Gawww, I've seen a couple of CLEAN mirrors lately, and this one is in real need of a clean. Won't do it until finished. Kept it covered all this time though! Don't need more dirt to then remove, .
Alex,
It looks like the hard work and effort are finally paying off. Cannot wait until first light. We may have to drink something other than home brew.
Keep the pictures coming. It is inspiring me for my mod later this year.
I cut mine on my "little one" on my CNC Machine ( I love my CNC too!). It made my project very straight forward for me. I commend the job you are doing now, but if I can help in the future please don't hesitate to contact me. I hope this does not sound like to much of an advert - I really do enjoy building stuff.
I look forward to seeing your completed project; it certain looks impressive.
Looking mighty fine and going well! When is first light?
I just suggested to a 16" Lightbridge owner this weekend that they consider this design to make it more portable, rather than the traditional truss dob setup. I reckon the Highe/Plettstone/Dobstuff design is my favorite one for scopes 10-18 inches. Only disadvantage is that you cannot use a shroud if you want one (most experienced ATMers don't).
Just to clarify, for the baffling on my scopes opposite and just below the focuser, I have used thin black polypropylene from a plastics place, covered in Protostar adhesive flocking. Works well and holds it shape there.
For the big removable tube around the primary mirror on the 22", this stuff was too floppy to hold shape. Instead, I used black plastic hollow core signboard bent along its length around in a circle. If you try and bend it the other way it kinks, unless you slice the outside along the hollow tubes underneath with a stanley knife. It's very stiff and holds its own weight well.
It is probably overkill and might be too heavy or thick for the baffle behind the secondary on the top end though. But you could just try whatever you can get your hands on the easiest.
Looks like it can be folded up like a accordion now too!
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