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Equipment Discussions >> ATM, Optics and DIY Forum

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johnC
member
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Reged: 08/24/06

Loc: Hucknall Nottingham UK
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5398156 - 09/01/12 01:31 PM

Would it be possible to stand next to the scope so we may get an idea of its size?
The completed ota pick is now my desktop wallpaper!
It really is beautiful.
Thank you again for sharing.
Will be looking forward to optics and mount construction.
Best regards
John
UK


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: johnC]
      #5398763 - 09/01/12 09:32 PM

Thank you John,
I will take a picture with a sizable reference object.


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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5399065 - 09/02/12 02:30 AM

next to the 10" f/4 Astro-Tech. they're about the same length



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Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5399077 - 09/02/12 03:03 AM

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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johnC
member
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Reged: 08/24/06

Loc: Hucknall Nottingham UK
Re: 17" CDK and Equatorial Mounting new [Re: Dave O]
      #5399132 - 09/02/12 05:04 AM

Thanks Stephen.
A fantastic work of art.


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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]
      #5400347 - 09/02/12 11:06 PM

Quote:

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



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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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5400555 - 09/03/12 03:42 AM

Stephen, in your configuration, it my be better to mount the spider above the UTA ring. This would shorten your OTA even more. The shorter the better.

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.

Now, having a spherical secondary does relieve some of the strict axial collimation requirements of other Cassegrains, but image degradation is still subject to tilt, if not micrometric centering. A 300 mm f/8 DKC, with a 120 mm secondary, for example, must maintain a tilt to within ± 6 arc minutes. This turns out to be as little as ± 0.1 mm or 0.0004 inches for that configuration. You want all the rigidity and support you can get, so proper spider support is quite critical.

Regards,

Mladen


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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: MKV]
      #5400569 - 09/03/12 04:08 AM

Quote:

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.





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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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5400585 - 09/03/12 04:35 AM

Quote:

Quote:

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.





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.



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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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: MKV]
      #5400911 - 09/03/12 11:31 AM

RCOS does have the spider vanes in between each truss pair. My original 3D model had the vanes right on the truss pair but I ended up doing some FEA to identify which of the two would work best. Unless I completely failed my analysis, the solution I opted for gave better results.

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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5400991 - 09/03/12 12:21 PM

Zytrahus: That is quite unlikely. Your position is de facto unstable and not statically determined wereas the position I suggested is statically determined.

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zytrahus
professor emeritus


Reged: 06/16/09

Loc: Long Beach, CA
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5401050 - 09/03/12 01:04 PM

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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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: zytrahus]
      #5401060 - 09/03/12 01:09 PM

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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johnC
member
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Reged: 08/24/06

Loc: Hucknall Nottingham UK
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5401222 - 09/03/12 02:54 PM

With the greatest of respect, this is a wonderful project and being graciously shared with us.
With the engineering skills being evident this is not the thread for picking apart the OP project.

Again i do not mean offence but find your above post a little unsavoury.


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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: johnC]
      #5401236 - 09/03/12 03:05 PM

John: I like amateurs that are willing to go beyond the usual Dobsonians. In that sense I have full respect for this project. However, that does not mean that this project is without a critical view. So either give arguments against mine and we can continue or stop blaming the messenger.

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Datapanic
Post Laureate
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Reged: 10/17/09

Loc: Tucson, Arizona
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5401248 - 09/03/12 03:11 PM

Nice project! Can't wait to see the mount design. The Maynard Clark scope was also designed and built by an expert Machinist several years ago and was made to last.

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johnC
member
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Reged: 08/24/06

Loc: Hucknall Nottingham UK
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5401250 - 09/03/12 03:12 PM

I do not have the technical knowledge to argue, this i freely admit.
I apologise for what may have been a Knee jerk reaction to your post. But as in all things i suppose there are differing views.
In hindsight if the OP is not offended then that is that.

Best regards
John.


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Benach
Pooh-Bah


Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new [Re: johnC]
      #5401322 - 09/03/12 03:45 PM

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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johnC
member
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Reged: 08/24/06

Loc: Hucknall Nottingham UK
Re: 17" CDK and Equatorial Mounting new [Re: Benach]
      #5401342 - 09/03/12 03:52 PM

I bow to your superiority.
We just do things differently here.
I shall continue watching this project evolve.
I apologize to the OP for diverting the thread.
End of.


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Mirzam
Post Laureate
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: 17" CDK and Equatorial Mounting [Re: johnC]
      #5401358 - 09/03/12 04:01 PM

We really have heard no critical analysis. A real analysis would examine the allowable flexure and whether the design meets the necessary criteria.

Simply to claim that it is "better" to do something differently is meaningless.

JimC


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