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1899 4" f/16 Cooke Photovisual Apo refractor

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#301 deSitter

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Posted 12 February 2024 - 11:22 AM

Hi gang, it's been a while since I worked on the Cooke, but it's finally back together again!

It's not finished yet, but I wanted to get it together, it's been in pieces too long...

My plan is to continue working on it, bit by bit until it's all done. In the mean time I'll be able to observe with it once I make a custom dolly that allows the scope to be rolled out the doorway.

 

Here's a mangled by compression image of it's current state:

 

 

attachicon.gif 4_inch_Cooke_PV.jpg

Peter, why does the latitude yoke seem backward to me? Is it possible for the clock drive to operate with the position reversed? The pivot is centered on the pier so it would not affect CG considerations. But the clearance of the RA circle would be improved at high latitude.

 

-drl


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#302 Peter Ceravolo

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Posted 12 February 2024 - 01:57 PM

Peter, why does the latitude yoke seem backward to me? Is it possible for the clock drive to operate with the position reversed? The pivot is centered on the pier so it would not affect CG considerations. But the clearance of the RA circle would be improved at high latitude.

 

-drl

I thought you were joking... but then I noticed there was no smiley face... :-)

 

Clearly the yoke was designed to work backwards!

 

 

head 2.jpg

 

head 1.jpg


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#303 Terra Nova

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Posted 12 February 2024 - 02:45 PM

Peter, it’s an absolutely beautiful telescope and a Wonderful restoration! Bravo!! waytogo.gif waytogo.gif



#304 oldmanastro

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Posted 12 February 2024 - 03:44 PM

Superb restoration! What a nice precision instrument from earlier times. If you ever take some images with this beauty please post them. 



#305 deSitter

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Posted 14 February 2024 - 06:06 AM

Hi gang, it's been a while!

 

I've not had the opportunity to continue restoration of the 4" Cooke Photo-Visual telescope, but I prepared a talk on the -

 

"Opto-Mechanical Analysis of a 1899 4" Taylor Photo-Visual Objective"

 

for last years Antique Telescope Society virtual convention. You can find mine as well as a host of other talks at this link:

 

https://vimeo.com/user161970464

 

Peter

This was a fascinating lecture. I thoroughly enjoyed that!

 

-drl


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#306 mikemarotta

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Posted 14 February 2024 - 06:24 AM

Fascinating. The work and workmanship are impressive, even humbling.

 

I archived the discussion here and captured a link to the ATS video of this and thie others on that Vimeo page.

 

Congratulations.

 

Mike M.



#307 Peter Ceravolo

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Posted 15 February 2024 - 02:31 PM

I recently acquired spectral line filters of the wavelengths typically used to design refractor objectives:

(in nm) 436 (g), 486 (F), 546 (e), 588 (d) and 656 ( C ). That is; violet, blue, green, yellow and red respectively.

 

You can learn more about the emission lines used in optical design here:

https://www.telescop...t/chromatic.htm

 

 

line filters.jpg

 

 

These filters allow one to see the focus shift of the different colors and quantify an objective's residual chromatic aberration. They are small filters, so I simply rested them on thin double sided tape to hold them in place behind the Ronchi grating. The f/15ish beam made looking into the setup easy and the filters could be quickly swapped out. I rigged a dial indicator to my Foucault/Ronchi tester to measure any focus shift. In this set up the source and grating move together. The testing was done under autocollimation so it's quite sensitive. The objective was bolted to a gimbal type lab mount for easy alignment to the 16" flat.

 

 

color correction measurments.jpg

 

Grating_filter.jpg

 

Taylor_PV_autocollimation.jpg


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#308 Peter Ceravolo

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Posted 15 February 2024 - 02:40 PM

Testing with filters was interesting and quite colorful!

Note that the wavelength imaged and grating position is in the file name.

 

 

546_inside_focus.jpg

 

 

486_outside_focus.jpg

 

 

436_outside_focus.jpg

 

 

One of the first thing I noticed is that the objective was mildly undercorrected in the blue. The longer wave lengths were ideal though.

 

 

486_outside_null.jpg


Edited by Peter Ceravolo, 16 February 2024 - 01:34 PM.

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#309 Peter Ceravolo

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Posted 15 February 2024 - 02:43 PM

Here are the focus shift measurements:

 

wave length / focus shift

656nm / -0.015" (inside focus)

588nm / 0.000

546nm 0.000

486nm 0.000

436nm +0.030"

 

It was difficult to distinguish any focus shift for the central three colors. This testing technique is sensitive and crude at the same time, but it gives one a good idea of what's happening.

 

Here again are the details of the objectives construction:

 

 

Taylor_PV_objective_data.JPG


Edited by Peter Ceravolo, 15 February 2024 - 02:47 PM.

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#310 deSitter

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Posted 15 February 2024 - 05:00 PM

Testing with filters was interesting and quite colorful!

 

 

attachicon.gif 546_inside_focus.jpg

 

 

attachicon.gif 486_outside_focus.jpg

 

 

attachicon.gif 436_outside_focus.jpg

 

 

One of the first thing I noticed is that the objective was mildly undercorrected in the blue. The longer wave lengths were ideal though.

 

 

attachicon.gif 486_outside_null.jpg

Nice - it would be very interesting to rotate the elements relative to each other and retest. The most interesting part of your lecture was how you showed that the elements are all hand-matched by making the easy to make surfaces compensate for the hard to make ones.

 

-drl



#311 Peter Ceravolo

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Posted 16 February 2024 - 12:56 AM

This is a chromatic aberration test, rotating the lens elements will have no effect on the results.

 

When I first started this thread I heard from Roland Christen who mentioned that his early triplet objectives are direct descendants of the Taylor triplets like this particular one. Interestingly though, Roland does not consider these Taylor triplets, or his earlier objectives, to be apochromats given the dispersion in the violet. They are semi-apos I suspect.

 

I have not looked through the telescope in a long time, but in past sessions looking at Venus I did not notice significant color. I'm looking forward to doing more critical observing when I can get the instrument out doors.

 

One of the things I'll be looking for is spurious reflections caused by the air spaced surfaces with nearly identical radii. These surfaces are notorious for producing problematic ghost images. I investigated a Brashear objective for ghosting effect here:

 

https://www.cloudyni...oley-telescope/

 

The Taylor objective has two sets of potentially problematic air spaced surfaces of near equal radii. The second set of surfaces are not likely to be a problem, mainly because the 5th surface was severely aspherized to compensate for surface figure errors in the previous surfaces. The ghost from this set might just be dispersed enough by the aspheric surface to be invisible.

 

All this investigation is for fun of course, the warranty on this 120 year old objective has expired I think...

 

 

PV_objective.jpg


Edited by Peter Ceravolo, 16 February 2024 - 01:02 AM.

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#312 col

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Posted 21 February 2024 - 07:40 AM

I thought you were joking... but then I noticed there was no smiley face... :-)

 

Clearly the yoke was designed to work backwards!

 

 

attachicon.gif head 2.jpg

 

attachicon.gif head 1.jpg

Hi Peter,

 

The scope looks fantastic, especially considering it's previous condition. Very well done.

 

Regarding the mount and its yoke, this picture of the "Equatorial Mounting Class B' from the1908 catalogue could be yours or a later version of it?

 

Regarding the naming of the Photo-Visual Objective, in both catalogues they are referred to as "Cooke's Patent Photo-Visual Objectives. For Astronomical Purposes. (H. D. Taylor's Patents.)".

 

Regards, Col Shepherd

Cooke Equatorial Mounting Class B-1908 catalogue - resize.jpg


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#313 Peter Ceravolo

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Posted 21 February 2024 - 01:26 PM

Hi Peter,

 

The scope looks fantastic, especially considering it's previous condition. Very well done.

 

Regarding the mount and its yoke, this picture of the "Equatorial Mounting Class B' from the1908 catalogue could be yours or a later version of it?

 

Regarding the naming of the Photo-Visual Objective, in both catalogues they are referred to as "Cooke's Patent Photo-Visual Objectives. For Astronomical Purposes. (H. D. Taylor's Patents.)".

 

Regards, Col Shepherd

attachicon.gif Cooke Equatorial Mounting Class B-1908 catalogue - resize.jpg

Hi Col,

 

There are similarities for sure, but the mount in the catalog is still quite different from the unit I have. The polar pivot area in my unit is canted, the catalog drawing shows that mount head being vertical.

 

I've tried to find similar mounts to mine on this web site of T Cooke & Sons images without success:

 

https://dlib.york.ac...5914&ref=detail

 

Interestingly there are a few mounts pictured that were made for Dr. Hugh Walsham, the original owner of my scope.


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#314 Peter Ceravolo

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Posted 22 February 2024 - 02:08 AM

One of the great things about this antique telescope is that it's a complete example, there are few missing parts of any significance.

But that doesn't mean there are no mysteries.

 

Case in point is the flip up cover the slips into the dew shield. It's a spring loaded device, I assumed that because there were springs in the package, but I could not understand how it functioned.

 

 

flip_cover_6.jpg

 

 

There was no obvious way to attach the now separate springs that either kept the cover open or closed. And I thought the springs kept the cover open.

 

 

flip_cover_1.jpg


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#315 Peter Ceravolo

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Posted 22 February 2024 - 02:21 AM

After discussion with fellow Antique Telescope Society members it was determined that the springs kept the cover closed.

 

 

flip_cover_springs_1.jpg

 

 

flip_cover_springs_2.jpg

 

 

The above mock assembly shows the springs intended position. You can even see the wear marks on the cover from the tips of the long spring arms.

 

But why did they get separated from the cover? There is no obvious signs of disassembly. And how did they survive once separated from the telescope?

 

One knowledgeable ATS member suggested that there was a tall post in the center that guided a string tied to the loop at one side of the cover that ran all the way to the observer. The operator, at the eyepiece, would pull on the string and open the cover. But the there is no obvious way to tie off the string by the focuser. One would think that there would be something obvious to tie off the string given the finesse of the cover design.



#316 deSitter

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Posted 22 February 2024 - 05:38 AM

After discussion with fellow Antique Telescope Society members it was determined that the springs kept the cover closed.

 

 

attachicon.gif flip_cover_springs_1.jpg

 

 

attachicon.gif flip_cover_springs_2.jpg

 

 

The above mock assembly shows the springs intended position. You can even see the wear marks on the cover from the tips of the long spring arms.

 

But why did they get separated from the cover? There is no obvious signs of disassembly. And how did they survive once separated from the telescope?

 

One knowledgeable ATS member suggested that there was a tall post in the center that guided a string tied to the loop at one side of the cover that ran all the way to the observer. The operator, at the eyepiece, would pull on the string and open the cover. But the there is no obvious way to tie off the string by the focuser. One would think that there would be something obvious to tie off the string given the finesse of the cover design.

Would be helpful to photography. Just open the shutter then pull the string slowly.

 

-drl




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