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Equipment Discussions >> ATM, Optics and DIY Forum

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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: Mirzam]
      #5401465 - 09/03/12 04:54 PM

Mirzam: the arguments were already given above, statically determined vs. non-determined.

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Mirzam
Post Laureate
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5401515 - 09/03/12 05:28 PM

If there is a requirement to absolutely minimize the weight and thickness of the top ring, then your arguments are relevant. Otherwise, if the top ring of this rather small telescope provides adequate stiffness to support the spider irrespective of the truss connection points then arguing for a change in design becomes a matter of philosophy rather than engineering.

JimC


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5401632 - 09/03/12 06:39 PM

Quote:

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.




Since it (obviously) doesn't bother RCOS then it pretty much any concern I could have. 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. Maybe if you talk real slow I will be able to follow. =)


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5401813 - 09/03/12 08:39 PM

Quote:

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



The upper ring of the RCOS design appears to be shaped like an inverted "L", or "T" (see the magnified image below).

This ring is tall and thin with all the weight bearing burden placed on the tall aspect of the ring.

The UTA ring on this thread has it the other way around: the greatest stress is placed on the thin aspect of the ring.

Placing the stress on a structure suspended between two points (as in the configuration described on this thread), so that the maximum force is midway between those points, results in maximizes flexure. One doesn't have to be a rocket scientist to understand that.

So, to compensate for the "suspended bridge" design, one can do what RCOS did - create a tall thin ring - or, if minimal structural changes are desired, simply mount the spider on the trusses, where the maximum stress should be, and not midway between them.

http://www.rcopticalsystems.com/telescopes/images/12inchTrussSmall.jpg

Reagrds,

Mladen

Edited by ausastronomer (09/10/12 06:42 PM)


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5401854 - 09/03/12 09:13 PM Attachment (67 downloads)

Quote:

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.



How about a picture? Does that answer your question? Which one of these three examples will results in least deflection where the force is applied? Example A is the configuration described on this thread. Example B is the configuration seen on RCOS, and Example C is the spider mounted over the trusses.

Both, B and C will work well, but A is in effect a suspended bridge with maximum force applied midway between the pillars. Or think of it as a shelf suspended at both ends, and all the books placed half way between them. You think it's going to bow? You bet! And when the bowing cannot exceed 0.0004 inches you might want to consider a slightly stiffer option.

Again, if your are going to limit your work to wide field, low power deep sky imaging, then it's a moot point. But if you are talking high power visual observation or imaging, then a slight sag in your secondary could seriously degrade your image quality, especially in an RC.

Regards,

Mladen

Edited by MKV (09/03/12 09:20 PM)


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Datapanic
Carpal Tunnel
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Reged: 10/17/09

Loc: Tucson, Arizona
Re: 17" CDK and Equatorial Mounting new [Re: MKV]
      #5401933 - 09/03/12 09:48 PM

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. I don't know what I did with my old Strength Of Materials book, but that design will greatly affect the stiffness, no matter what the material.

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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: Datapanic]
      #5401984 - 09/03/12 10:15 PM

Quote:

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.



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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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: MKV]
      #5402048 - 09/03/12 10:53 PM

If that matters at all (maybe it doesn't) I chose to most of the calculations with the scope pointing at an altitude 45 degrees. For some reason I thought pointing straight up was a rather special case and it may not represent reality as well. Maybe one of the reasons the calculations brought me in this direction.

One other thing was that I tried to minimize the distance between 2 truss on the secondary. On my first model I had a much larger secondary ring with enough room to fit 2 truss and the bracket for the vane. With a ring of its current size I would have needed to have the bracket for the vane in between the 2 truss ends. But the strength of the truss is also function of the angles which boiled to having the truss ends as close as possible.

One thing to take into consideration was: although it looks decent, in regards to professional instruments, I don't expect it to be remotely comparable. Because I had the resources to design/machine doesn't mean I have the funds for a 5 digits mirror, unfortunately.

edit: It will be used for imaging only, at an imaging scale of .63 arcsec/px.
I am sure I already made some mistakes but nothing that can't be solved by redoing one or several parts, not a big problem. I'll find out quick enough if my optics will be good enough to evidence some of the mechanical problems I created by using shortcuts or just making mistakes.

edit2: in response to your last paragraph: Maybe I should write off a disclaimer ahah More seriously, although I am a mechanical engineer I spent most of my time in grad school doing thermal engineering and this is still what I do everyday. There is quite a bunch of critical aspects of the design of an astrograph or telescope in general that goes above and beyond simple mechanical engineering that clearly call for experience more than anything else. Before starting this project I had been looking for resources for a year or two. Any good book you know that talks in details about the one mechanical aspect we just discussed? I also posted threads here and on the yahoo ATM group looking for plans or resources to make an astrograph. I am not going to lie, I didn't get any response. If you are concerned about readers making mistakes copying a wrong design maybe it would be a good thing to design the perfect one and offer it to the public? There are much more people looking for such plans than actual plans... especially a perfect one

Edited by zytrahus (09/03/12 11:44 PM)


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psi_chemie
sage
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Reged: 05/01/11

Loc: Leawood, KS, USA
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5402086 - 09/03/12 11:11 PM

Sorry I didn't see if you mentioned what software you used for CAD? SolidWorks?

Thx,
-mike


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: psi_chemie]
      #5402099 - 09/03/12 11:17 PM

Yes, Mike.

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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5402257 - 09/04/12 01:49 AM

Again, it is obvious which configuration is most stable. Why people choose otherwise doesn't change that fact.

And resources have nothing to with it either. Come on, Stephen, in order to attach a spider on the trusses instead of midway between them doesn't require professional resources or a five digit mirror.

All the best in your project.

Mladen


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: MKV]
      #5402302 - 09/04/12 03:16 AM

And again, the most simple fact that some professionals choose otherwise makes the choice clearly not so obvious: At least for the majority of ATMers building an Astrograph, RCOS would appear as a pretty thrust worthy professional.

I'm sorry to disagree but I think the available web resources or literature have a lot to do with it. I understand the explanation you provided and can only hope the ring will be stiff enough, if not I'll redesign it, no big deal... no need for a bashing contest here, please and thanks.

Putting things into perspective, I am not sure if the quality of the optics I will be using is going to be good enough to see any degradation from this configuration.


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Datapanic
Carpal Tunnel
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Reged: 10/17/09

Loc: Tucson, Arizona
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5402303 - 09/04/12 03:26 AM

I think the ring will be plenty rigid enough for the secondary support.

First - There are technically 2 curved "T" beams around the ring with the embedded groves - if they were made for reducing weight or strength, it doesn't matter, end result is they add a lot of strength. The back mount brackets for the spider vanes also reinforce the whole ring. I think it would take a lot of force to bend that ring anywhichway, let along make it bend 0.0004 of an inch from gravity alone. Heck, I bet thermal expansion of high grade/low expansion aluminum is more than that!


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5402315 - 09/04/12 03:50 AM

Stephen, RCOS designed its UTA ring differently, as is obvious form their image. I hope your configuration works well, but I also wanted to point out why the spider is best mounted on the truss or why the UTA rings are made stiffer if it's not, the way RCOS did. There is no bashing contest here. Just facts.

Dan, I don't know how big that secondary is but if it's anything like a 12-inch f/8 DKC it should be somewhere between 6.5 and 7 inches. With all the hardware it's not a featherweight. 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.

No one should take this personally. It's all about facts. Thanks for your discussion.

Cheers.

Mladen


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Ryan W
sage
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Reged: 10/26/10

Loc: Port Matilda, PA
Re: 17" CDK and Equatorial Mounting new [Re: MKV]
      #5402604 - 09/04/12 09:50 AM

Stephen,

Beautiful project, very well thought out! I have only ever done civil engineering related CAD work and have a healthy respect for the well thought out CAD project followed by what looks to be a very well executed (so far) final result!

One question I have: Did you achieve considerable savings ($) doing all of this yourself as opposed to purchasing the final product from planewave or another? Or was it more about the journey and fun of doing it than the savings.

MKV,

I see the center spaced spider vanes as a non issue unless, for some reason, the material used for the upper ring is not rigid. Perhaps a placing the vanes over the truss connections would have been ideal, but if the critical analysis said otherwise then why argue with it? The flexure caused by that small of a load (secondary and vanes) on what I assume is an aluminum upper ring is almost certainly negligible. I am sure the upper cage was designed to resist the moment imparted on it by the center spaced spider and secondary.

Non issue IMHO.

again, great work, love the pictures!

-Ryan W


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Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: 17" CDK and Equatorial Mounting new [Re: MKV]
      #5402606 - 09/04/12 09:51 AM

Quote:

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.




Mladen ... had you read the thread, you would have seen:

Quote:

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




Cheers. Dave O


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: Dave O]
      #5402829 - 09/04/12 11:48 AM

Ryan: Yes it has a lot to do with the journey. In the end it will cost me less than the quarter of the price of the Planewave, but my optics won't be as good, so it makes it hard to compare.

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EddWen
Pooh-Bah


Reged: 04/26/08

Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5402843 - 09/04/12 11:53 AM

As a retired engineer and business owner, I find this a curious statement.

As the chief engineer of teams consisting of two to twenty engineers, there were always multiple solutions to just about any design facet.

My job was to cull out the best fit for the company, with regard to design requirements, cost, company capabilities, machining efficiency, etc.

I'm more of the opinion there is more than one satisfactory way to...........


Quote:

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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John Jarosz
Astro Gearhead
*****

Reged: 04/25/04

Loc: Fairfax, Iowa
Re: 17" CDK and Equatorial Mounting new [Re: EddWen]
      #5402863 - 09/04/12 12:05 PM

Stephen:

Beautiful work. Keep right on going and trust your instincts.

I can't really comment on the writings of the naysayers, as most of them are on my ignore list. I highly recommend using that feature as it makes reading CN much less stressful. Trolls need responses for their strategy to work. Constructive criticism will always filter thru and will be helpful in the end.

John


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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 17" CDK and Equatorial Mounting [Re: John Jarosz]
      #5403251 - 09/04/12 03:22 PM

Stephen,
WONDERFUL workmanship, the best I've seen since Preston Engebreton's 20" RC hit the forum.

Agree with JJ - you build it exactly like YOU want to. Thank you for going to the trouble to share it here on CN - a LOAD of people are diggin' it and learning from it, me included. Questions from everyone about how you thought out your design are constructive, with answers that educate. But if someone goes beyond just asking, and starts arguing with you about how you did something, and wants you to build yours their way, I say let 'em build their own! Constructive analysis degenerates to harassment if any critics are overly persistent in insisting their ideas be incorporated, with the goal of claiming partial credit for a finished product they had no part in funding or laboring over.

Mike


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