Blasted rain: it means that I bring out old and incomplete projects and start fiddling with them.
The currect one is designing a truss OTA to house a Dall-Kirkham combo I have had sitting here for a while.
I'm aiming for an open truss system similar (if not identical) to the ones that Royce now makes using open channel struts. I'm wondering about the placement of the central box-section strut and it's positioning in the obverall design. Is there any magic ratio of weohts and distances that should be adhered to? I'm presently planning on a rear section of 400mm and a front of 700. The point of balance, with the optics etc in place would actually be at about 275 from the rear though, as far as my rough maths goes. It's a full-depth mirror of 12 1/2 inches and weighs a bomb.
And does anyone else have any thoughts on the open channel struts?
Peter
As both Rolf and myself have found it's a matter of mocking it up and hanging it literally from the rafters. Build the major components, sling them all from a suspended beam and start measuring.
In reality close enough is good enough. You will add and remove components all the time and it will never be perfect. Besides which a wee bit of weight bias in one direction keeps the tracking system happier. My system also has a long dovetail which means if I make any major weight additions, temporary or permanent ( cameras etc ) I can slide the dovetail back to compensate.
Current all up weight on my 10" Serrurier OTA is about 11 kg and then add about 3 kg for current cameras, finders etc. Rolfs, if I remember correctly is about 17 kg (?).
Check Rolf Olsens website for the whole story. My 'copy' of his is on here.
I would tend to go with tubing of a suitable wall/diameter rather than open channel struts. My experiments with angle and channel said they flexed too much and therefore had to be thicker and bigger therefore heavier, than tube struts.
If you are building this yourself you most important thing will be accuracy in parts measurements. There are a lot of duplicate parts, Struts, angle brackets, etc so jigging and careful measuring during fabrication are extremely important.
Keep us posted on progress and if I can offer any advice let me know.
Thanks Brent. Just clinging to the open channel idea for a moment longer - because this is a Dall-Kirkham, the front end is relatively uncomplicated - no focuser in particular, so all the weight is in the front ring, spider and secondary, plus the struts themselves of course. I was hoping also to simplify the connectors this way by bolting the overlapping pairs straight through to the ring; so a bit more weight gain there too. Do you still thing flexure will be a problem? With the side and bottom struts (pairs of struts actually), all resisting compression, I was hoping this would be rigid enough.
Peter
My Strut ratios (If I remember correctly ) are about 3:4. Upper struts are nearly 600 long and bottom struts about 400 plus about 140 for the cage structure in the middle. Tube diameter is I think 15 mm, maybe 18mm. Upper struts are thinner walled than lower ( just what I had available at the time ....free ). 8 struts at each end, 4 triangulated pairs and it is the most rigid open structure I have ever built. I reckon I could stand it on end and stand on the top and it would hold my weight without moving at all.
The short length of each set of struts makes for good bracing. The longer they are the more possibility of flex as is noted in bigger truss dobs.
If you want more accurate information happy to provide some measurements but I think you will find that with a relatively light front assembly and short lengths due to the folded nature on the light path you can build something very rigid with care.
Take a look at all the big scopes, they are all truss\strut builds.
Pic of my build just after assembly and before all the bits were added. Sitting in its work\dob base but it mounts to the EQ6 for serious stuff.
Another lightweight solution I tried many years ago on my 12" was to use 4 triangles cut from marine plywood, with 1 layer of fibreglass mat on each side instead of 8 poles.
You will need to make a simple jig to drill holes accurately so they wind up being precise isosceles triangles and interchangeable. With a jigsaw, cut out most of the interior to minimise weight.
In practice it was light and quick and easy to assemble with rather fewer nuts & bolts than traditional trusses. 600 long is peanuts, and will be easily stiff enough.
FWIW il s'appelle 'Serrurier' pas 'Surrier'. Mon dieu, ces anglais !
Just a further correction....
The Serrurier tube design is not just a "truss" design.
The detailed write up on the design from the 1930's shows the true nature (and elegance!) of the design.
The top and bottom trusses as well as the joints should allow the top and bottom "rings" to deflect BY THE SAME AMOUNT and also maintain the top ring parallel to the bottom ring. This means that any deflection only results in a change of focus position without rotation.
These criteria are obtained by varying the cross section of the trusses and their relative lengths...
( I was at Grubb Parsons works during the late 1960's when they were actually testing a "Serrurier Truss" telescope tube.)
