329 replies to this topic

[deSitter]

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:

 

 

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

[Peter Ceravolo]

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!

 

 

 

[Terra Nova]

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

[oldmanastro]

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

[deSitter]

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

[mikemarotta]

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.

[Peter Ceravolo]

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

 

 

 

 

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.

 

 

 

 

[Peter Ceravolo]

Testing with filters was interesting and quite colorful!

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

 

 

 

 

 

 

 

 

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

 

 

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

[Peter Ceravolo]

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:

 

 

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

[deSitter]

Testing with filters was interesting and quite colorful!

 

 

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

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

[Peter Ceravolo]

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...

 

 

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

[col]

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

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

[Peter Ceravolo]

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

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.

[Peter Ceravolo]

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.

 

 

 

 

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.

 

 

[Peter Ceravolo]

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

 

 

 

 

 

 

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.

[deSitter]

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.

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

 

-drl

[Jumblesayle]

this is from my 5inch 1890,s cooke which suggests the arm with the hole in the end is missing from yours.

this arm allows a cord to pass through the eye and onto the sprung lid allowing you to pull the lid open and time the exposure etc.

[Peter Ceravolo]

D9E67359-596C-4802-8A2A-461523FB3ECF.jpeg

this is from my 5inch 1890,s cooke which suggests the arm with the hole in the end is missing from yours.

this arm allows a cord to pass through the eye and onto the sprung lid allowing you to pull the lid open and time the exposure etc.

Very interesting, thanks for posting Jumblesayle.

 

One can see the same type of arm used in the telescope catalog drawing in Col's post above (post #312).

 

Examining the dew shield and flip cover on my 4" PV, I don't see the mounting holes that would accommodate such an arm. But there are holes in the center of my flip cover that I assume accommodate some kind of post arrangement that might have the same function, but I've not seen any such thing in catalogs or telescope examples... Curious...

[col]

Very interesting, thanks for posting Jumblesayle.

 

One can see the same type of arm used in the telescope catalog drawing in Col's post above (post #312).

 

Examining the dew shield and flip cover on my 4" PV, I don't see the mounting holes that would accommodate such an arm. But there are holes in the center of my flip cover that I assume accommodate some kind of post arrangement that might have the same function, but I've not seen any such thing in catalogs or telescope examples... Curious...

 Hi Peter,

I suspect that the shutter had a cantilever arm which was attached via the two holes and which was slightly cranked upward where it extended past the edge of the cap. A cord running down to the eyepiece from the end of the arm would operate against the spring tension to open the shutter?
I'll try attaching an image from the 1908 catalogue in another reply. It shows a more complex shutter but the arm where the cord presumably attaches is shown.

 

Re the topic of your mount, I tried a search for Dr Hugh Walsham and his mount on the original York Digital library site but without success.

Regards, Col Shepherd

[col]

 Hi Peter,

I suspect that the shutter had a cantilever arm which was attached via the two holes and which was slightly cranked upward where it extended past the edge of the cap. A cord running down to the eyepiece from the end of the arm would operate against the spring tension to open the shutter?
I'll try attaching an image from the 1908 catalogue in another reply. It shows a more complex shutter but the arm where the cord presumably attaches is shown.

 

Re the topic of your mount, I tried a search for Dr Hugh Walsham and his mount on the original York Digital library site but without success.

Regards, Col Shepherd

 

[Peter Ceravolo]

Cooke photo shutter.jpg Cooke Telescope from catalogue drawing showing objective shutter.jpg

Hi Col,

 

The Cooke archives have changed, the Walsham referenced mount is no longer pictured.

 

I did find a number of similar flip covers, but detail is lacking....

 

[col]

Hi Col,

 

The Cooke archives have changed, the Walsham referenced mount is no longer pictured.

 

I did find a number of similar flip covers, but detail is lacking....

 

shutter.jpg

Peter,

I came across this image of a Cooke similar to yours in an earlier CN post.

It doesn't help with the flip cover question but does show there is at least another out there with your mount. 

Col

https://www.cloudyni...d-45-refractor/

[Peter Ceravolo]

Cooke Equatorial-Ref Kunama post on CN.jpg

Peter,

I came across this image of a Cooke similar to yours in an earlier CN post.

It doesn't help with the flip cover question but does show there is at least another out there with your mount. 

Col

https://www.cloudyni...d-45-refractor/

Thanks Col!

That's the first image of a mount that resembles the unit I have. I'll have to contact Kunama for more info.

Peter

[col]

Thanks Col!

That's the first image of a mount that resembles the unit I have. I'll have to contact Kunama for more info.

Peter

Going on the backdrop, I think it might be at the old Mount Stromlo Visitor's Centre. According to the website it is currently closed. I went a there few years ago and don't recall seeing that scope. I'll be interested to learn what you find out.

Col

[mikef0924]

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. 
 

 

 

This is very enjoyable thread. Thanks for sharing!

 

Curious where you obtained the spectral line filters? Also what is their bandwidth ?

 

Thanks