Telescope Reviews: 17" CDK and Equatorial Mounting

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Benach
Pooh-Bah

Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new

new

[Re: zytrahus]
#5415741 - 09/11/12 05:12 PM

Just an offer from me to Zythrahus: Can you contact me offlist and I'll give you an e-mail adress. You can send a SW model of your scope to me and I am willing to have a critical look at certain other mech. engineering thingys. However, be prepared that I might conclude that some things have to be redesigned.

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zytrahus
professor emeritus

Reged: 06/16/09

Loc: Long Beach, CA

Re: 17" CDK and Equatorial Mounting new

new

[Re: Benach]
#5415844 - 09/11/12 06:11 PM

Thanks but at this point parts are made so I think I'll just wait till the optics come in so I can do some actual imaging tests to validate my design. It doesn't make sense to change anything now and until I've done some actual imaging.

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gregj888
sage

Reged: 03/26/06

Loc: Oregon

Re: 17" CDK and Equatorial Mounting new

new

[Re: zytrahus]
#5416046 - 09/11/12 08:07 PM

Benach, thank you. It's the dynamic analysis that I think would be interesting.

Stephen, sorry no log as such. Just the files from foucault testing. The real problem is time to would on it, with a clear head. BTW, I would do some imaging before changing too, why not.

I'll post an interferagram of my mirro when I get one here soon.

Greg

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Benach
Pooh-Bah

Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new

new

[Re: gregj888]
#5416494 - 09/12/12 12:56 AM

Greg: a telescope is not a dynamic instrument, at best quasi-static. Buckling is, after all, quasi-static. So I am not going to invest a lot of time in a full dynamic analysis here. Simply a matter of priorities.

Zythrahus: ehm, you're going to image your aberrations? Good luck with that one. 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? Having seen a couple of CDKs myself, I know that they are extremely sensitive to misalignment and especially when the mirror position is overdetermined, you cannot determine with certainty that the position and/or correction is at optimum.

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David Pavlich
Transmographied

Reged: 05/18/05

Loc: Mandeville, LA USA

Re: 17" CDK and Equatorial Mounting new

new

[Re: zytrahus]
#5416537 - 09/12/12 01:26 AM

Looking forward to the next few pictures of your progress. And may I be the first in line for an autographed copy of your first APOD!

David

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gregj888
sage

Reged: 03/26/06

Loc: Oregon

Re: 17" CDK and Equatorial Mounting new

new

[Re: Benach]
#5416550 - 09/12/12 01:40 AM

Benach,

IMHO Vibration dampening and resonant frequency are a big part of telescope design, especially as you get larger. It's not so much the mount moving as wind buffeting or something in resonance with say a motor drive frequency. The static flexure (which is quasi-static) is also important. I'm not suggestion one over the other, just that it would be nice to have both analyses during the design process.

A side note, with my 20" starting out as a CDK (is now planned as a corrected Classical Cas.) 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. "Reflective telescopes” has a section on mis-alignment sensitivities.

I needed to be able to select the final f/# of the scope using different secondaries, hence the Classical.

Thx,

Greg

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Benach
Pooh-Bah

Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new

new

[Re: gregj888]
#5416588 - 09/12/12 02:25 AM

Greg: There is no active damper in your telescope, is there? So at best it is Rayleigh dampening. I've never heard of an amateur calculating their resonance frequencies and comparing them to actual measured vibrations. So yes, theoretically speaking you're right, but then there is so much more engineering you have to take into account, that this is just the final part.
I am willing to help you to think along the way from a professional (opto)mechanical PoV but that is not the same as doing all the analysis for you

I agree that a secondary of a DK is the least sensitive to misalignment of all Cassegrain derivatives but the optical alignement of the primary is extremely sensitive and the final focal plane quality is also sensitive to this. also the conical constant and radius of curvature requires quite some accuracy. I remember that in a friend's 14" f/4.7 CDK the primary's Conical constant had to be within 0.02 and the RoC within 1mm. That is quite wild for a f/3.6 primary and beyond the abilities of most amateurs.

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Tom and Beth
Carpal Tunnel

Reged: 01/08/07

Loc: Tucson, AZ

Re: 17" CDK and Equatorial Mounting new

new

[Re: Benach]
#5416632 - 09/12/12 03:35 AM

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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MKV
Carpal Tunnel

Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new

new

[Re: gregj888]
#5416686 - 09/12/12 05:44 AM

Quote:

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

Imaging is relatively low resolution. If it's going to be a camera then the tolerances are much more relaxed.

An f/8's Ary disc is 11 microns. Even if quadrupled in size due to errors or scintillation, whatever, it's still a dot to the eye. And if you print it at 10x that scale it's still no bigger than the dot on your computer screen. It's still a dot to the human eye.

But if you want to use it visually, that's a different story. Visually you should be able to use 75x per inch magnification, or even higher, and see no imaging flaws. That's a whole new ball of wax.

M

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new

new

[Re: gregj888]
#5416720 - 09/12/12 06:46 AM Attachment (47 downloads)

Quote:

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.

Gregg, CDKs are relatively less sensitive to axial misalignment (since the spherical secondary really doesn't have an axis), but not to tilt. The same goes for the corrector elements, which are also spherical. Here is an example of a 300 mm f/8 CDK. In the upper left corner you see two columns of spot diagrams representing images on-axis, half way and at the edge of the FOV.

The column on the left has zero tilt. The column on the left is tilted by 1/10 of a degree (6 arcminutes), or 0.07 mm at the edge of the secondary. Clearly even the on-axis image shows blatant coma. The PSF on the lower right shows the same axial image if the secondary were titlted only 0.05 degrees (3 arcminutes), or 0.033 mm at the edge. Even here the image is decidedly assymetrical and the coma is still perceptible.

This is the effect of only one possible element being tilted and all others being perfect, which is of course never the case. If you add a mixture of tilt in the corrector lenses and the corrector as a whole, you'll get an even more complete idea of how critical this is, and why final collimation must be done by autocllimation.

Also, all along we are assuming that the very fast primary is a perfect ellipsoid. At ~ f/3.5 ot 3.6 its conic profile would have to be within 99.99% of the theoretical value to satify 1/4 wave limit at the best focus! Figure out what this would mean in terms of knife-edge readings!

But, like I said, if you're looking at a prime focus camera, rather than a visual telescope, then of course all this is moot. You can have a 1/2 wave camera and still make good astrophotos - especially with photo art software and stacking options. In fact 1/2 wav seems to be industrial standard for fine photographic commercial optics

Mladen

PS The vertical bar on the lower right is the normalization scale of the PSF function and is not related to the tilt in any way.

Edited by MKV (09/12/12 07:04 AM)

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Benach
Pooh-Bah

Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new

new

[Re: Tom and Beth]
#5416767 - 09/12/12 07:25 AM

Quote:

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.

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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don clement
Vendor (Clement Focuser)

Reged: 02/02/11

Loc: Running Springs, California

Re: 17" CDK and Equatorial Mounting new

new

[Re: Benach]
#5416904 - 09/12/12 09:24 AM

Quote:

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?

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.

Don

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johnC
member

Reged: 08/24/06

Loc: Hucknall Nottingham UK

Re: 17" CDK and Equatorial Mounting new

new

[Re: don clement]
#5416926 - 09/12/12 09:39 AM

Good post Don.
At the end of the day you have to credit the people that actually get out there and do it.
I am totally in awe of this project and look forward to seeing it progress to completion.
I am totally sure that if something required adapting it would not phase the builder in any way.

John

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Benach
Pooh-Bah

Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new

new

[Re: don clement]
#5416956 - 09/12/12 09:55 AM

Sure Don, but I think that you and I agree that with nowadays cameras the criteria of the optics have increased fairly much. Who was talking on eg. periodical error correction or subpixel imaging (and therefore imaging near the Rayleigh limit) when Mt. Palomar was build?

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: 17" CDK and Equatorial Mounting new

new

[Re: don clement]
#5416963 - 09/12/12 10:01 AM

Quote:

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.

Umm, sir, the Hubble fiasco was not due to engineering "speculation" and simulation (the engineered design was correct!), but on a faulty nulling device, better yet - on the individual(s) who made it incorrectly, and on their refusal to even entertain the possibility that it might be faulty (they either refused to make or use a backup), and on the arrogance of so many that just because something looks "perty" it must be beyond reproach, untouchable.

Mladen

Edited by MKV (09/12/12 10:52 AM)

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Datapanic
Carpal Tunnel

Reged: 10/17/09

Loc: Tucson, Arizona

Re: 17" CDK and Equatorial Mounting new

new

[Re: David Pavlich]
#5417001 - 09/12/12 10:32 AM

Quote:

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

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

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johnC
member

Reged: 08/24/06

Loc: Hucknall Nottingham UK

Re: 17" CDK and Equatorial Mounting new

new

[Re: Datapanic]
#5417024 - 09/12/12 10:43 AM

Me too, really enjoying this project.
Even more so now that i have used the ignore button for the first time ever.
So much better to read.

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Benach
Pooh-Bah

Reged: 01/24/08

Re: 17" CDK and Equatorial Mounting new

new

[Re: Datapanic]
#5417327 - 09/12/12 01:36 PM

Quote:

Quote:

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

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

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

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Dave O
super member

Reged: 12/21/11

Loc: Sri Lanka

Re: 17" CDK and Equatorial Mounting new

new

[Re: Benach]
#5417439 - 09/12/12 02:43 PM

Quote:

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

Your feedback has been noted. My suggestion would be that if you think it is so important a topic; you move it to its own thread per the OP's request ... place a link here if you want to tie the two together. Dave O

Edit. I am actually quite interested in the topic; and believe it is worthy of its own thread where the experts (and there are many here) can chime in and perhaps even teach a few of us something ... without distracting from the OP's report on his project.

Edited by Dave O (09/12/12 02:57 PM)

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gregj888
sage

Reged: 03/26/06

Loc: Oregon

Re: 17" CDK and Equatorial Mounting [Re: Dave O]
#5420345 - 09/14/12 02:19 AM

Mladen

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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