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polaraligned
professor emeritus


Reged: 12/26/08

Re: 17" CDK and Equatorial Mounting new [Re: Mike I. Jones]
      #5403469 - 09/04/12 05:18 PM

I agree with Mike above!!!
Beautiful project and thanks for sharing. There is more than one way to skin a cat. If it works, great. If not, you correct it and learn. You could stiffen the top ring easy enough if need be. Don't pay the howling dogs any mind.


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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: Mirzam]
      #5412031 - 09/09/12 05:53 PM Attachment (92 downloads)

Jim&Zythrahus: this has nothing to do with weight. Weight is not stiffness and vice versa. RCOS is simply doing it the wrong way. period. Look at how it is done at the VLT, the Subaru, the Keck, the Gemini etc etc.

What is wrong with putting the spider at Zythrahus' position, is that the upper ring has the least amount of stiffness there. It will bend by itself the most there and you even put a large weight on that position. If you put it at the position between the trusses' crossing, the deflection is far less, almost zero.
Mladen's comparision is correct, it is like a suspension bridge. In fact, it is the same equation (Euler-Bernouilli beam theory) that determines the deflection. You can also see exactly the same effect on every suspension bridge. Look eg. at the collapse of Tacoma Narrows Bridge, aka Galloping Gertie, http://www.youtube.com/watch?v=j-zczJXSxnw
The biggest deflection always happens between the biggest distance between the constraints. To give any evidence for my statements, I did the following comparision: I designed a ring in SolidWorks. Ring is a model of the upper cage. I put the material properties of Aluminium 6061 on the ring. The ring has a outer diameter of 370mm and an inner diameter of 350mm. Thickness is 5mm. I applied a force (purple arrows) of 100N on four points (four-vane spider) and I constrained with 4 pairs of constraints, green arrows, first between the forces (Zythrahus' method) and then the method that I am advocating.
In the attachment you can see the results of my analysis. It is pretty convincing. The deflection of Zytrahus' method is about 300x bigger than in my method. No, this has nothing to do with the design of the upper cage. It scales inversely linearly with the stiffness of the upper cage. Neither does it have anything to do with the dimensions of the cage. I used the same design for the comparision and even if you'd use a design with different dimensions, the deflection would again scale linearly.
I am wiling to answer questions concerning this. I am also willing to help here. But I don't want a flame war because I am advocating something which is non-mainstream without arguments (Yes John Jarosz&Polaraligned, I am also referring to you now.)

Mike: You disappoint me here.


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5412423 - 09/09/12 10:02 PM

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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David PavlichAdministrator
Transmographied
*****

Reged: 05/18/05

Loc: Mandeville, LA USA
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5412458 - 09/09/12 10:23 PM

Ok, kids. You've beaten this horse well past its death. Time to put this one to bed. This thread has two engineering philosophies that aren't going to change.

David


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David PavlichAdministrator
Transmographied
*****

Reged: 05/18/05

Loc: Mandeville, LA USA
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5412536 - 09/09/12 11:27 PM

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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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5412775 - 09/10/12 04:41 AM

Stephen, would it be too much to ask what is your estimate of maximum deflection? Just curious. Thanks.

Mladen


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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5412808 - 09/10/12 06:46 AM

Quote:

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




Stephen, SkyVision is actually mounting their spider on trusses and not midway between them (see top portion of their official site):

http://www.skyvision.fr/wordpress/?page_id=81&lang=en

The lower portion, however, does show a spider mounted midway between the trusses, but they compensate for it with a much thicker and heaver (solid square tube) ring structure, the way RCOS and Alluna do with their tall thin rings.

Ragards,

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]
      #5413154 - 09/10/12 11:46 AM

I'm glad the thread has been unlocked and I'd like to keep it that way. This topic has been discussed enough and unless I have to make modifications to it down the road, I won't reopen the topic.

Feel free to open a new thread to discuss it though, I am sure David P. will love the entertainment.

Thanks.


I've started the machining of the parts for the base of the equatorial mounting. I will post some pictures in a few days.


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johnC
member
***

Reged: 08/24/06

Loc: Hucknall Nottingham UK
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5413194 - 09/10/12 12:07 PM

Looking forward to further updates Stephen.
Best wishes with the rest of the project.
Really interested in seeing the mount.

John


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Chucke
member


Reged: 03/12/10

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5413699 - 09/10/12 04:47 PM

Hi Zytrahus,

It will be interesting when your get your optics to find out how good they are. If they are ok I may look into a similar setup.

Chuck


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: Chucke]
      #5413812 - 09/10/12 05:45 PM

Hi Check,
I am a little anxious about this I hope they turn out to be decent enough!


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5413820 - 09/10/12 05:48 PM

Hi John,
I made a few renderings of the complete project. You'll get an idea of what I've done.

The pier is only a temporary thing as I didn't have a practical bench to assemble the mounting on and it will be a practical thing to have when I start testing things out from home.





I made a little blog entry with some quick explanations of what I've tried to do. More details about that will follow as I progress through the machining / assembly.

if you want to take a look: http://cooledpix.com/2012/09/10/eq-z-introduction/


Cheers,


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gregj888
professor emeritus


Reged: 03/26/06

Loc: Oregon
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5414087 - 09/10/12 08:55 PM

Can I ask a naÔve question? In the analysis above it looks like the only forces being considered are along the optical axis and the only constraints are those of the tubes. Isnít this a 3 space problem and doesnít the spider constrain the ring in 2 dimensions?

My question-- how would one do a more complete model to look at this in 3 space and at different orientations?

Stephen, beautiful work!!

thx,


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: gregj888]
      #5414252 - 09/10/12 10:12 PM

Thanks Greg.
It looks like the analysis you referring to was done in the specific conditions of the OTA aiming at Zenith. In any other orientation there would be forces in the other 2 directions.


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gregj888
professor emeritus


Reged: 03/26/06

Loc: Oregon
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5414371 - 09/10/12 11:57 PM

Stephen,

That's what it looks like. Again, I'm not trying to start anything, I'm just curious. It seems like a lot of things are missing and Iím not smart enough to know if it will matter.

With the 1.5m for the Alt Az initiative the Cal Poly students ran a lot of FEA and dynamic simulations looking at resonant frequencies, Q (in my world) and dampening. Much of this relates back to mass and stiffness, but in 3 dimensions.

Again very nice scope, I can only hope my 20" turns out halfway close.


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: gregj888]
      #5414383 - 09/11/12 12:06 AM

Greg: do you have a link to your work-log (if you've got one)? I'd love to check it out.
Re: simulation: I won't comment as it's not mine but I think it was mainly there as an illustration to give a general idea of the deformation, that's all.


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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: gregj888]
      #5415225 - 09/11/12 01:12 PM

Greg: the only difference is then that the force vector will be decomposed in a vector downward and a horizontal vector. The effects of downward force on the truss are fairly small because this will result in buckling. However, a truss is not sensitive to buckling although I must admit that in Zythrahus the chances of buckling are much bigger because the force vector will be excentric to his trusses. The horizontal force is nearly not important because the upper ring should be stiff already. Hope this is clear. If not, please rephrase your question.

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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5415258 - 09/11/12 01:26 PM

Quote:

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.




Zythrahus: I cannot speak for other's considerations. But this stiffness difference should make you reconsider your thoughts. If you think your design is still acceptable, which I doubt, I'd like to see a tolerance analysis of your telescope supporting a FEM analysis proving that your secondary mirror will remain within tolerances for all positions. I have already seen a very sketchy tolerance analysis by MKV and if that one is correct, if it is applicable to your telescope, it is highly unlikely that it will be good enough.
And to make myself clearer: the deflection of a beam supported by two constraints (This particular case) goes with the third power (of the smallest distance) between the force vector and the constraints.

But suppose it fails the analysis mentioned above, what are you going to do to correct this? Readjust your system for every altitude?
Even worse: what is so difficult in changing the radial position of your upper cage and increase your stiffness about 300x? I don't see the problem.


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TopherTheME
sage
*****

Reged: 02/11/11

Loc: Rochester, MI
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5415317 - 09/11/12 01:50 PM

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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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting [Re: TopherTheME]
      #5415348 - 09/11/12 02:01 PM

Quote:

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




Using 2x byers gears/worms a pair of Maxon motors/gearbox/encoders controlled by my Gemini-II.

Cheers,


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