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Stephen -- your design and machining skills as displayed in this thread are nothing short of "awesome". While I do not know what engineering principles have been invoked to assure rigid collimation of the optics; the attention to detail in the construction and aesthetics of this instrument are first class. It looks like something that NASA would build. Thanks so much for sharing. Dave O

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Thanks for the kind words, Dave.
I've certainly tried to make it work mechanically. This is my first home designed/built telescope so we'll see how it turns out. Optically, I am hoping the optics will be good. Sandwich mirrors clearly make them lighter but their mechanical retention is more of a challenge. I will have to wait several months before I find out.

I am getting ready to start machining parts for the equatorial mounting. This will be a challenge.

I've taken another picture with an object that will be better to get an idea its size.

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More importantly, however, to maintain critical collimation, the spider vanes should be mounted to or directly above the trusses and not in between them. This is how it's done professionally.

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More importantly, however, to maintain critical collimation, the spider vanes should be mounted to or directly above the trusses and not in between them. This is how it's done professionally.

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No Mladen, the best position is at the imaginary crossing point of the trusses. If you mount them higher than that, further away from your primary, your create an extra force in your truss.

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Yes, Benach, that is the best position and that's how the professionals do it (which is obvious from the picture I referenced).

I was suggesting what should be done with the existing setup. If you look at the finished truss and spider couplings, it would be difficult to remedy this problem (i.e. mounting the spider at the truss crossing point) without redesigning and refabricating the couplings - and pretty much the whole UTA.

As for the increased peripheral moment created by moving the spider above the ring, it would be offset by the shorter truss length, which would be necessary to maintain the correct primary-secondary distance.

Regards,

Mladen

Edited by MKV (09/03/12 04:36 AM)

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Then also RCOS is doing it the wrong way. Nothing to be ashamed of yourself, we're all too human, but it is still undesirable.

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I was referring to this: http://www.rcopticalsystems.com/telescopes/16truss.html the spider vanes start from the median between 2 truss pairs.

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But I'm curious and I'd really like to understand what is really fundamentally wrong with having the spider vanes at the median of 2 truss hubs.

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If you look on page 2 of this thread, strength members are built into the ring via a pair of channeled groves between each truss and secondary vane mount point.

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Yes, I have seen them. Nice touch, but I see more value in making the wafer lighter then stiffer. That still doesn't explain why not simply mount the spider on the trusses which is logistically just as easy as mounting them midway between them?

Without any data it's not possible to figure out how much flexure will be present, and if it might be detrimental to maintaining critical collimation. I am assuming you did the calculations and found the deflection acceptably small, but someone copying your configuration may not be as savvy in mechanical engineering skills, or use the correct type of aluminum, etc., and end up with less than optimum results.

My point was that it's just as easy to simply put the force on the pillars rather than midway between them (the "suspended bridge" configuration), the latter being inherently more flexible (which is a good thing for a suspended beridge but not necessarily for secondary mirror supports! ).

Regards,

Mladen

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You also don't know the thickness of the spider vanes and the type of aluminum used. All that and more is part of the equation.

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Spider vanes are 316 S-S and 2 mm thick (painted flat black)

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It is not a matter of different views John. We're no philosophers here in the sense that there're no final answers. In the field of engineering and science there are usually final answers and usually they are right of wrong and in this case it is wrong. Too bad for OP, but this is the result of the method.
Are you also one of the opponents of Einstein who'd say in 1905 that he's wrong because "it is picking apart Newton's magnus opus Principia"?

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RCOS and myself are not the only doing it this way: you can Alluna-Optics's RC's, Sky-Vision (their OT structure), and I am sure

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Thanks Benach for your post: I think you'd agree there isn't such thing as right or wrong here unless the ring ends up not being stiff enough. If a deformation of X is acceptable, doing 300 better than X is still going to be nothing else but acceptable.

I obviously can't speak for RCOS but as mentioned in earlier posts there were other factors that I took into considerations that lead me to this choice. But I have a hunch that RCOS had reasons as well.

RCOS and myself are not the only doing it this way: you can Alluna-Optics's RC's, Sky-Vision (their OT structure), and I am sure others.

I hope this will turn out OK for me, if not, a little bit of re-design won't be a problem.

thanks again Benach.

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What kind of hardware and electronics are you planning on using to operate the mount? Something off the shelf or scratch built?

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I would do some imaging before changing too, why not.

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I have looked at the alignment issue a bit. All Casses are sensitive to mis-alignment but the Dall Derivatives tend to be the least sensitive. With a spherical secondary, all the secondary mis-alignment terms more or less drop out as long as the mis-alignment is fairly small.

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Hope you don't mind, but I'm following this discussion with interest. I have a mirror set for a 300MM F20 DK and sifting through designs from which to build the Mechanics for a truss. I'll be using an AP1200 for the mount. Of particular interest are the details of the tube connections-

It would be nice to have access to my own machine shop, but the CFO would prefer I keep this simple.

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I totally don't mind , but I don't see what your point is. You want advice on how to do the mechanics better than Zythrahus? I think he's done some things correctly, but certain things leave much to be desired for.

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How steady is your air there in Southern California? How do you make the distinction between a pure optical aberration and an aberration due to misalignment?

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When all the speculation, simulations, and postulations by engineers and designers were done and said for the Hale 200”, the final star testing and optical figuring was done by Don O. Hendrix under the steady air of southern California’s Palomar mountain. Too bad a star test wasn’t performed for the Hubble before it was launched instead of relying on the assumptions, postulations, and simulations of the engineers and designers.

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It's unlocked, but only because of a couple of passioned pleas by participants. I ask that if you can't add to the discussion without your editorial comments, then don't bother posting. Your expertise is an asset to the others, but if it's accompanied by infantile remarks, it becomes a distraction.

With that said, consider this thread on probation. This isn't a school yard and you're not school kids.

David

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It's unlocked, but only because of a couple of passioned pleas by participants. I ask that if you can't add to the discussion without your editorial comments, then don't bother posting. Your expertise is an asset to the others, but if it's accompanied by infantile remarks, it becomes a distraction.

With that said, consider this thread on probation. This isn't a school yard and you're not school kids.

David

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Thanks David - I hope the thread stays on topic. Many of us have an interest in watching this project unfold.

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Me thinks that giving feedback on an important detail of a project is still on topic.

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Me thinks that giving feedback on an important detail of a project is still on topic.

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I agree, though think of it differently. Tilt of a spherical surface can be though of as decentering. The reason I down play this it that it's the only alignment issue theat's fairly easy to see and correct (with a laser colimator). This is also in contrast to an asphere, that is down right ugly... tilted or decentered :-)

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