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1899 4" f/16 Cooke Photovisual Apo refractor

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#51 bratislav

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Posted 06 April 2020 - 11:33 AM

I have tried to model Peter's latest acquisition, but it proved to be quite tough. According to Roger Ceragioli's essay on APO's, Taylor's original version used Schott "ordinary/regular" glasses type O.543, O.164 and O.374. Later O.164 got replaced by O.569 because of rapid deterioration of the original middle short flint element. 

Any of those are quite hard to find any information on - try hard as I could, virtually no information could be found that could be used to analyze this triplet.

In the end, it was Peter himself that provided data on some of these glasses, enough to get me started.

Using data provided, combination of O.543, O.569 and O.374 and Peter's measured radii and thicknesses would get close, but nowhere near the satisfactory performance. Then I allowed R6 (which is shallowest) to vary, and then also BFL to vary as well a little. That improved things. The last variation was to allow for some tiny conic/aspherization on R1 (which I think was common in those days. It would also explain zones Peter saw). I think melt variations for Peter's refractor would account for differences in radii and BFL.

The final result is I think quite good for a lens that uses no Fluorite type glass. Its secondary spectrum (C to F) amounts to better than 1/10,000 of the focal length, which puts it squarely into a "real APO" territory. Even with much wider wavelength spread (440 to 700nm) it is still very well corrected. It is also completely flat field for all practical purposes, so any eyepiece will only be limited by its own aberrations. I think a comparison with a more modern (and faster) APO would prove quite interesting.

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Edited by bratislav, 06 April 2020 - 11:44 AM.

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#52 bratislav

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Posted 06 April 2020 - 11:39 AM

Spots, and .zmx file (renamed to .txt to bypass filters)

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#53 Jon Isaacs

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Posted 06 April 2020 - 12:10 PM

Regarding polishing:

 

I was watching one of Jay Leno's car shows and the most valuable classic cars are those that are in decent condition and unrestored.

 

Beautiful scope. Being a mechanical engineer, I love those old designs that relief on mechanical ingenuity rather than electronics.  A modern mount with a separate RA slow motion control would be awesome. This one doesn't need a clutch.

 

Jon



#54 Peter Ceravolo

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Posted 06 April 2020 - 05:20 PM

The gravity drive output is coupled to the worm housing via an adjustable shaft and a couple of right angle gears.

 

IMG_1696.JPG

 

The worm housing is not firmly bolted to the mount, rather it pivots on it's cone tipped screws. The whole worm housing is spring loaded so the worm can be retracted from the sector gear. Because the drive gear is not a complete disk there has to be a quick way to reset the drive for maximum tracking time.

 

IMG_1700.jpg

 

worm spring load 3.jpg


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#55 Peter Ceravolo

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Posted 06 April 2020 - 05:33 PM

here is an image of the worm housing disassembled.

 

IMG_1853.JPG

 

The interesting aspect of this worm housing is the manner in which the worm shaft can be displaced laterally to engage the two right angle gears.

The end of the worm shaft on the left looks like it's threaded but it's not, those are grooves that allow the shaft to rotate but be restrained laterally.

There are 50 grooves per inch for fine lateral motion to properly engage the gears with out the need for clamping collars on the shaft.

 

IMG_1859.JPG


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#56 Peter Ceravolo

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Posted 06 April 2020 - 05:45 PM

The worm looks like it's 32 threads per inch thread, very fine... such fine gears and long OTAs make me nervous...

 

IMG_1712.jpg

 

Unfortunately the worm is corroded beyond repair and will have to be replaced.

 

IMG_1923.JPG


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#57 dave brock

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Posted 07 April 2020 - 03:08 PM

The gravity drive output is coupled to the worm housing via an adjustable shaft and a couple of right angle gears.


The adjustable shaft suggests that the right angle gear it is attached to does actually slide vertically in its housing to adjust the engagement with the second gear (as suggested in the ATS group)? Otherwise why the adjustable shaft.

#58 Peter Ceravolo

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Posted 07 April 2020 - 03:13 PM

Here the RA sector drive gear is clearly visible, as is the big knob that clamps the gear to the RA axis.

 

IMG_1719.jpg

 

The Dec slow motion is a basic tangent arm with the lead screw pushing the arm against a spring-loaded plunger. The Dec slow motion control rod extends all the way down to the observer.

 

IMG_1726.jpg

 

 


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#59 Spectral Joe

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Posted 07 April 2020 - 07:29 PM

The adjustable shaft suggests that the right angle gear it is attached to does actually slide vertically in its housing to adjust the engagement with the second gear (as suggested in the ATS group)? Otherwise why the adjustable shaft.

The adjustable shaft is to allow for latitude adjustment. When you put it back together you want to make sure the two universal joints are properly aligned, at right angles to each other. Otherwise you'll have a serious periodic error.



#60 Peter Ceravolo

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Posted 08 April 2020 - 05:43 PM

The adjustable shaft is to allow for latitude adjustment. When you put it back together you want to make sure the two universal joints are properly aligned, at right angles to each other. Otherwise you'll have a serious periodic error.

I had aspirations of actually measuring the antique mount's tracking accuracy. That dream got toasted with the bad worm.



#61 Peter Ceravolo

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Posted 08 April 2020 - 07:02 PM

One of the other aspects of the Cooke telescope I found exceptionally appealing are the brass with inlaid silver setting circles. And the magnifying lenses are quite useful as we age... :-)

 

Of course I'd never use the things in practice but they sure do looks good!

 

IMG_1687.JPG

 

IMG_1723.JPG

 

IMG_1721.JPG

 

setting circles.jpg


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#62 Spectral Joe

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Posted 08 April 2020 - 08:20 PM

I had aspirations of actually measuring the antique mount's tracking accuracy. That dream got toasted with the bad worm.

Fortunately, the worm looks like one of the easiest parts to reproduce. I was surprised to see the grooved part for controlling axial motion, it's easy to make the grooves on the worm, not so much on the other part. An elegant solution. The drive rate should be pretty stable, once it's set up and the scope is balanced. I've built several weight driven clock drives over the years, and are fond of them. The Cooke drive looks like it has governor weights that pivot to swing horizontally rather than pivoted at the top like pendulums. I haven't seen such an arrangement before.



#63 ccwemyss

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Posted 08 April 2020 - 08:28 PM

That's a beautiful Vernier system. It would get the user to within a second of RA. Assuming a good polar alignment and little cone error, it could be as accurate as a digital push-to system.  

 

Chip W. 


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#64 Peter Ceravolo

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Posted 08 April 2020 - 08:37 PM

Fortunately, the worm looks like one of the easiest parts to reproduce. I was surprised to see the grooved part for controlling axial motion, it's easy to make the grooves on the worm, not so much on the other part. An elegant solution. The drive rate should be pretty stable, once it's set up and the scope is balanced. I've built several weight driven clock drives over the years, and are fond of them. The Cooke drive looks like it has governor weights that pivot to swing horizontally rather than pivoted at the top like pendulums. I haven't seen such an arrangement before.

I along with a few Antique Telescope Society folks are working on a replacement worm now. We're trying to figure out the exact thread pitch and form.

 

Thoughts welcome!

 

IMG_1924.JPG


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#65 TSSClay

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Posted 09 April 2020 - 06:28 AM

Peter,
It looks like a 60 degree thread form. If you know a local machine shop with an optical comparator you could tell for sure. It should be relatively easy to replicate.

Clay
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#66 col

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Posted 09 April 2020 - 07:02 AM

I have tried to model Peter's latest acquisition, but it proved to be quite tough. According to Roger Ceragioli's essay on APO's, Taylor's original version used Schott "ordinary/regular" glasses type O.543, O.164 and O.374. Later O.164 got replaced by O.569 because of rapid deterioration of the original middle short flint element. 

Any of those are quite hard to find any information on - try hard as I could, virtually no information could be found that could be used to analyze this triplet.

In the end, it was Peter himself that provided data on some of these glasses, enough to get me started.

Using data provided, combination of O.543, O.569 and O.374 and Peter's measured radii and thicknesses would get close, but nowhere near the satisfactory performance. Then I allowed R6 (which is shallowest) to vary, and then also BFL to vary as well a little. That improved things. The last variation was to allow for some tiny conic/aspherization on R1 (which I think was common in those days. It would also explain zones Peter saw). I think melt variations for Peter's refractor would account for differences in radii and BFL.

The final result is I think quite good for a lens that uses no Fluorite type glass. Its secondary spectrum (C to F) amounts to better than 1/10,000 of the focal length, which puts it squarely into a "real APO" territory. Even with much wider wavelength spread (440 to 700nm) it is still very well corrected. It is also completely flat field for all practical purposes, so any eyepiece will only be limited by its own aberrations. I think a comparison with a more modern (and faster) APO would prove quite interesting.

Some years ago a friend and I did a side by side visual comparison of my 5" f17 Cooke PV and AP130 f6 EDFS at similar magnifications using modern eyepieces and concluded they were comparable apart from the Cooke image being not quite as bright.


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#67 R Botero

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Posted 09 April 2020 - 07:13 AM

I along with a few Antique Telescope Society folks are working on a replacement worm now. We're trying to figure out the exact thread pitch and form.

 

Thoughts welcome!

 

attachicon.gifIMG_1924.JPG

It looks pretty standard. Get a thread gauge from your local DIY/Amazon and measure. Or if you want to be completely sure, thread wires. They tipically come graduated with correct thread depth dimensions. Attached a couple of pictures from my copy of the machinery‚Äôs handbook (1942). 

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#68 R Botero

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Posted 09 April 2020 - 07:14 AM

And some detail on Whitworth dimensions. 

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#69 Peter Ceravolo

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Posted 10 April 2020 - 02:30 AM

I drilled a bunch of holes in a plank of wood and bolted the Cooke onto my old Losmandy mount to get a feel for the objectives performance... Quite pleased!

 

Venus at 300x (modern 5.2mm Pentax eyepiece) was nearly color free, perhaps a slight tinge of red at the limb. Swinging over to Capella I saw beautiful Airy disk and first diffraction ring!

 

One surprise was how nicely the old Huygens eyepieces supplied with the scope worked. They are not marked with fl, but they were high and lower power. Despite the short eye relief, the Huygens offered a pleasant view in the F/16 instrument.

 

IMG_1939.JPG


Edited by Peter Ceravolo, 10 April 2020 - 03:32 AM.

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#70 Peter Ceravolo

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Posted 10 April 2020 - 03:08 AM

Old telescopes are just like old cars, there are different camps when it comes to preservation and restoration. Opinions span the spectrum from "don't touch it!" to "it's yours do what you want". I'm in the middle of the pack, I'm not into resto-mods but at the same time there is a limit to patina.

 

The experience with the packing company prepping the Cooke for transport to the colonies was a perfect example of the hazards old, especially really old looking telescopes are exposed to - they don't get the royal treatment. Because it looked like an old piece of junk, they treated it like an old piece of junk. My bad experience with the Zeiss Coude (the main mount bearing was nearly destroyed in transit by poor crating) made me very sensitive to the little details of packing a delicate instrument. I had to force the company to scrap the first attempt at the crate and redesign it making sure all the delicate parts were protected from vibration. They needed to be reminded that, despite what it looked like, this was a 120 year old instrument that needed to be treated with respect.

 

IMG_1645.JPG

 

crate3.jpg

 

 

 

 


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#71 Peter Ceravolo

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Posted 10 April 2020 - 03:16 AM

Here are some images of the tall cast iron pier. The paint is original and quite worn out. There is no doubt that the pier spent lots of time in the elements.

I can imagine an instrument with this level of finish degradation would probably be scrapped by someone unaware if it's significance. When the finish is this far gone I think one is doing an antique a favor by refinishing it so it is appreciated by the less informed, less likely to be mistreated when the owner passes on.

 

IMG_1650.JPG

 

IMG_1651.JPG

 

 


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#72 luxo II

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Posted 10 April 2020 - 03:53 AM

Peter congrats on  those circles - very similar to the ones on the scope I restored. And indeed they were accurate.

There should be some little bolts inside, somewhere, for adjusting them to get them exactly concentric with the axes.

 

What thoughts re the fish for the pier ? Molybdenum paint ?


Edited by luxo II, 10 April 2020 - 03:54 AM.


#73 Bomber Bob

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Posted 10 April 2020 - 08:15 AM

When the finish is this far gone I think one is doing an antique a favor by refinishing it so it is appreciated by the less informed, less likely to be mistreated when the owner passes on.

 

I agree 100% with that; and, IMO, the Big Scopes really need to be pretty to survive once we're gone.  Questars & other small tabletop scopes have much better survival odds.  After I got all the corrosion & gunk off the brass tube of my Mogey 3, I had it powder-coated Mogey Gray, even though it was originally bare.  And, I'm using the same logic restoring my Tinsley 6" Cass & its massive EQ pedestal mount.

 

Like a lot of folks on this forum, I'm loving your thread.  And, learning as you show us your work, so Thank You!!


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#74 Peter Ceravolo

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Posted 10 April 2020 - 02:24 PM

Peter congrats on  those circles - very similar to the ones on the scope I restored. And indeed they were accurate.

There should be some little bolts inside, somewhere, for adjusting them to get them exactly concentric with the axes.

 

What thoughts re the fish for the pier ? Molybdenum paint ?

lol.gif



#75 Peter Ceravolo

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Posted 10 April 2020 - 02:29 PM

Peter congrats on  those circles - very similar to the ones on the scope I restored. And indeed they were accurate.

There should be some little bolts inside, somewhere, for adjusting them to get them exactly concentric with the axes.

 

What thoughts re the fish for the pier ? Molybdenum paint ?

I read you first post with interest, the green painted mount on the scope you refurbished. Do you have any pics?

Peter




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