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Zeiss Coudé telescope rebfurbish

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#201 Kokatha man

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Posted 14 February 2019 - 01:02 AM

 ......................An opto-mechanical engineer friend suggested the long term stress could have caused cold flow distortions in the lens resulting in astigmatism........................

 

 

Not wanting to start an argument although me & a civil engineer friend of mine got into a "robust" discussion about glass "cold flow" a few years back when I suggested that there was no such thing.

 

https://io9.gizmodo....-496190894?IR=T

 

https://www.cmog.org...does-glass-flow

 

https://www.glassnot...indowPanes.html

 

Whilst older "studies" did keep this notion alive for quite some time I believe that the recent studies have debunked the idea that solid glass can "flow" - but as I said, I don't want to start an argument..! lol.gif


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#202 Peter Ceravolo

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Posted 14 February 2019 - 08:23 PM

Finally! The last of the Zeiss Coudé components has been repainted.

 

The south end of the polar axis, where the massive focuser is bolted, is visible and had to be painted. It was a bit of a challenge making sure all the surfaces that are exposed were repainted. Almost missed a few parts. I'm looking forward to the reassembly this spring!

 

IMG_1689.JPG


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#203 Steve Allison

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Posted 14 February 2019 - 09:49 PM

I just can't imagine Zeiss would produce an object class that was not of the highest quality. Surely something must have happened to the lens somewhere along the way.

 

Or is this just a reality check on the Zeiss legend of superiority?

 

Or was the lens designed to operate in the real world, rather than just on the optical bench?



#204 Peter Ceravolo

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Posted 15 February 2019 - 08:57 PM

Steve I don't think something "just happened along the way". The substrate for the first element was not annealed well. I have no doubt the astigmatism would have been visible at the time of fabrication, they must have just passed it as meeting spec I suspect. Note that in the absence of the internal strain, the lens would have been excellent with a smooth, well corrected wavefront.

 

For most, the astigmatism would probably be lost in the seeing. Roger Ceragioli did mention to me that for most coma corrected objectives the astigmatism can be collimated out by tilting the lens. Perhaps this is the case with this objective.

 

Anyway you can tell, I think, that I am not bent out of shape by a less than perfect lens, even if it is made by Zeiss. They were't perfect, as the link to a Zeiss MakCass in Bratislav's post made clear.

 

The next step with the optics is to better characterize the mirrors, they are not as flat as could be. I'm inclined not to touch any of the optics and preserve the pedigree.

 

To let the cat out of the bag, I have an 8" f/15 semi-apo triplet designed and made for me by Roger that I plan to adapt to the Zeiss mount. I'm inclined to replace the two original Pyrex type mirrors with a fused quartz coated with a non metallic multi-layer dielectric coating by Spectrum Thin Films. It's the same coating as used in the TeleVue diagonals. It's optimized for 45 degrees and apparently lasts for ever according to Tony Pirerra, owner of Spectrum Thin films and an astronomy enthusiast.

 

8.4 Ceragioli objective small.jpg

 

 

The original dome that we purchased with the Zeiss is being refurbished and a new, square base is being fabricated. In the new arrangement the center of motion of the telescope mount will be at the center of curvature of the 3m Observa-Dome, allowing for the longer 8" tube to swing around unfettered.

 

Aperture fever!!!

 

200mm Coude.jpg


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#205 Steve Allison

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Posted 15 February 2019 - 09:57 PM

Thanks, Peter, makes sense.

 

I remember feeling a little heart broken reading about a Clark lens that did not test all that well. But I have always been sort of a romantic...

 

That 8 inch apo on the wonderful Zeiss mount seems like a dream setup!

 

Please keep the posts in this very interesting thread coming!

 

Steve



#206 Kokatha man

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Posted 15 February 2019 - 10:22 PM

...I'm a bit heart-broken also Peter - if I interpret your last post the way it reads to me, it is that you are actually going to utilise the Zeiss mount complete with Coudé focus with your new 8" objective & associated mirrors...isn't there some sort of divine retribution for that type of behaviour..?!? lol.gif

 

Of course if I'm correct in my interpretation then you could fashion a porthole in the "new square base" of your observatory & fit the Zeiss objective in said: bigshock.gif that would be several levels of "oneupmanship" on my custom 1/20 wave 16" OOUK mirror* I have hanging on the wall of my studio as a looking glass! wink.gif

 

*Apropos my earlier comments about glass "cold flow" I did in desperation at one stage thoroughly investigate whether the horrific astigmatism in this custom primary was from it sitting face down in its cell for over 2 years looney.sml.gif ...alas, like the myth of cold flow in glass, the primary was simply a lemon I paid through the nose for! frown.gif


Edited by Kokatha man, 15 February 2019 - 10:24 PM.


#207 Peter Ceravolo

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Posted 16 February 2019 - 01:42 AM

...I'm a bit heart-broken also Peter - if I interpret your last post the way it reads to me, it is that you are actually going to utilise the Zeiss mount complete with Coudé focus with your new 8" objective & associated mirrors...isn't there some sort of divine retribution for that type of behaviour..?!? lol.gif

Hey, Kokatha man, the potential for divine retribution for my optical hubris is not lost on me. ;-)

 

I would love to experience the full, original, Zeiss monty.

 

My plan is to do so, at least as far as the optics go initially as major mods always take longer than you think. So the objective and the fold mirrors will be used as is to get the instrument functional sooner rather than later. Despite the new mount paint job, preserving the original optics is key for anyone interested in preserving history. That's why I'd rather replace the fold mirrors completely rather than re polish them to improve their figure. The objective is another matter, but it's at least only the first surface that needs to be touched up.

 

I hate to say, but I have pooched a component, the retainer ring for the objective. As mentioned previously, it is out of round. So much so as to be unusable. Talking about it with Mark, I don't remember dropping it on the cement floor while trying to pry it out of the corroded cell. I'm going to have to devise a work around. Some clips that perform the same function.

 

The original drive electronics scare me, there are a lot of wires with frayed insulation in there. I don't want to plug them in and blow a irreplaceable component.  I'd like to refurbish the electronics but so much has gone into refurbing the mounts exterior that thinking of trying to rescue antique electronics that have a definite shelf life does not make financial sense. I'm looking at adapting a modern servo system, simple for the RA, but a mechanical challenge at this point for the Dec.

 

So, the tug of war between originality and functionality is ongoing...


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#208 bratislav

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Posted 16 February 2019 - 06:01 AM

I'm inclined to replace the two original Pyrex type mirrors with a fused quartz coated with a non metallic multi-layer dielectric coating by Spectrum Thin Films. It's the same coating as used in the TeleVue diagonals. It's optimized for 45 degrees and apparently lasts for ever according to Tony Pirerra, owner of Spectrum Thin films and an astronomy enthusiast.

 

 

Make sure blanks are THICK! Or well oversized (perhaps not possible given the existing geometry). Dozens of dielectric coatings needed can pull in with enough force to bend a large flat. Standard Al with optimized overcoating (for 45 deg) may be safer ... Tony will surely be able to advise.


Edited by bratislav, 16 February 2019 - 06:03 AM.


#209 rolo

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Posted 16 February 2019 - 09:01 AM

For most, the astigmatism would probably be lost in the seeing. Roger Ceragioli did mention to me that for most coma corrected objectives the astigmatism can be collimated out by tilting the lens. Perhaps this is the case with this objective.

 

 

 

 

That may be the case. I recently had a similar issue with a Cooke triplet design. The front element is in a separate adjustable cell and despite using a venire caliper the astigmatism and coma was still there. I decided to do a DPAC test and could easily see the tilt of the front element was off. The tiniest adjustment fixed the problem.

Attached Thumbnails

  • DPAC TOA 130.jpeg
  • DPACTOA130-2.jpeg

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#210 Peter Ceravolo

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Posted 16 February 2019 - 02:07 PM

Make sure blanks are THICK! Or well oversized (perhaps not possible given the existing geometry). Dozens of dielectric coatings needed can pull in with enough force to bend a large flat. Standard Al with optimized overcoating (for 45 deg) may be safer ... Tony will surely be able to advise.

Yes, I am aware of the stress these coatings, with the untold layers, have on the substrate. I need to get a read from Tony if the mirrors can support such a coating.

 

Here is a drawing of the mirrors with dimensions:

 

Fold mirrors dimension.JPG

 

There is no option for increasing mirror thickness, given the cell design. And you can see from the pic below that the surface is pretty close to, if not spot on the, the center of the OTA cube.

 

IMG_2365.JPG

 

And I suspect the cell is of such a design that the pivot point of the adjustments is at the surface.

 

IMG_9337.JPG

 

I often have to do a sanity check on my natural inclination to optimize everything. This telescope is not for photon starved observations, so do 99% reflectivity coatings make sense? But then on the other hand, it's not like it's easy to pull the mirrors out for re-coating so applying a long lasting coating now makes a lot of sense.


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#211 Peter Ceravolo

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Posted 18 February 2019 - 12:06 AM

Well we thought all the re-painting was done. We have several shelves of parts waiting to be reassembled.

 

IMG_0050.JPG

 

Then on a whim, we started positioning things in their place and noticed that the Dec slow motion sub-assembly may not be painted completely.

 

IMG_9215.JPG

 

The curved portion of the mounting bracket, not repainted, looked like they would be visible to the observer. So we looked some of the pre-dissasembly photos to see if we could see if the surface was exposed. Sure enough it is.

 

IMG_2338.JPG

 

Here is an image of the Dec slow motion sub-assembly base plate with the curved part needing to be painted. Such unpainted surfaces would stand out like a sore thumb in the refinished telescope.

 

IMG_0054.JPG

 

It's been brought home on several occasions that I did not take enough photographs, prior to disassembly, to show how the finished instrument should look assembled. You simply cannot have too many photos from all kinds of angles of an instrument like this.


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#212 photomagica

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Posted 18 February 2019 - 02:53 AM

Peter,

I recall from working on Calgary's 1967 Zeiss Jena planetarium projector that the East Germans were having trouble getting quality ball bearings and other materials. We had to replace all of the bearings we could get to. We also had a number of small Zeiss refactors, a 150mm Cassegrain and a 150mm Maksutov. These varied in optical quality as well, in my view from unacceptable for the Cassegrain to very good for the best of the refractors. This lack of access to high quality materials may be an explanation for your astigmatic lens element.

 

Overall we found the Zeiss product design and engineering very good, though sometimes appearing over-complicated. Performance of the Planetarium was ultimately excellent after the materials problems were dealt with.

Bill



#213 figurate

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Posted 18 February 2019 - 09:49 AM

Given the Zeiss inclination to overbuild everything, there may the possibility of machining down a couple of the mating surfaces in that support assembly to give yourself some wiggle room for a slightly thicker flat (which could be easily restored to original spec with shims later on), but unfortunately probably not to the degree you would like.

 

The plan for a 200mm triplet installation using the same light path I think is both sensible and ingenious- while I would like to see the original optics brought back to good functionality, there is no question that what you have in mind could be an extraordinary instrument for any number of astronomical pursuits. Low expansion flats, a stationary ep, rock solid mechanical stability, and everything accessible right there at the viewing position. Sounds like a winner to me, and hopefully worth all the potential trials and tribulations in the end. 

 

Doubtless the annealing on that flint element is to blame here, but I wonder if the tiny elastic deformation in glass substrates under stress that we see using optical means is always perfectly or immediately reversible- whether there can be a lag over a period of time or a range of temperature in the restoring molecular forces.


Edited by figurate, 18 February 2019 - 11:49 AM.


#214 Peter Ceravolo

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Posted 18 February 2019 - 02:24 PM

Peter,

I recall from working on Calgary's 1967 Zeiss Jena planetarium projector that the East Germans were having trouble getting quality ball bearings and other materials. We had to replace all of the bearings we could get to. We also had a number of small Zeiss refactors, a 150mm Cassegrain and a 150mm Maksutov. These varied in optical quality as well, in my view from unacceptable for the Cassegrain to very good for the best of the refractors. This lack of access to high quality materials may be an explanation for your astigmatic lens element.

 

Overall we found the Zeiss product design and engineering very good, though sometimes appearing over-complicated. Performance of the Planetarium was ultimately excellent after the materials problems were dealt with.

Bill

That's interesting Bill, thanks for the input on the ball bearing problem.

 

I'm reconsidering the notion that the instrument was overbuilt, it's institutional quality as opposed to commercial.

I've been in touch with another private Zeiss Coude owner in the UK with an earlier version of the instrument. Some of the spur gears on his are split to take out backlash. We don't see that on ours, it would make a huge performance difference given the drive train. Also the way the RA gear is mounted, in a adjustable rig that allows one to dial out eccentricity and drastically reducing periodic error according to the UK owner.



#215 Peter Ceravolo

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Posted 18 February 2019 - 02:57 PM

Given the Zeiss inclination to overbuild everything, there may the possibility of machining down a couple of the mating surfaces in that support assembly to give yourself some wiggle room for a slightly thicker flat (which could be easily restored to original spec with shims later on), but unfortunately probably not to the degree you would like.

 

The plan for a 200mm triplet installation using the same light path I think is both sensible and ingenious- while I would like to see the original optics brought back to good functionality, there is no question that what you have in mind could be an extraordinary instrument for any number of astronomical pursuits. Low expansion flats, a stationary ep, rock solid mechanical stability, and everything accessible right there at the viewing position. Sounds like a winner to me, and hopefully worth all the potential trials and tribulations in the end. 

 

Doubtless the annealing on that flint element is to blame here, but I wonder if the tiny elastic deformation in glass substrates under stress that we see using optical means is always perfectly or immediately reversible- whether there can be a lag over a period of time or a range of temperature in the restoring molecular forces.

Max in Italy has all original 200mm parts for his Coude. I'm going to have to work up a 30" or so tube for my 8" Ceragioli lens to fit to the mount. And yes the new fold mirrors would be ideal, but I don't think it's worth the trouble to mod things further to get a few extra mm of thickness. We'd love to have the scope operational this summer, so using the original optics initially makes the most sense. but I'm not holding my breath, hoping the Zeiss objective will de-stress and give perfect images.... :-)


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#216 memento

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Posted 05 March 2019 - 03:25 PM

I'm just a fascinated reader of this thread. So great to be able to follow online how this beautiful scope is being restored !! Thomas



#217 photomagica

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Posted 08 March 2019 - 01:04 PM

Peter,

I appreciate your observation about institutional quality. Zeiss had to build its instruments to go into places where little or no maintenance would be performed over long periods of time. East German Zeiss (Jena Instruments) did the best they could towards this goal with the materials they could access. That said I was disappointed by the sample variation among the small Jena telescopes at the Calgary Centennial Planetarium - they were letting through some sub-par optics.

Bill


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#218 Peter Ceravolo

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Posted 10 March 2019 - 01:53 AM

No manufacturer is perfect, batch to batch variations are to be expected, especially with optics since they are sooo finiky. All it takes is for a key person in the shop to retire/move on to have significant effect on product quality.

 

I do not place any particular brand on a pedestal, however I do have a 3.5" Questar (astro-art) proudly displayed on a mantel in the living room. :-)


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