329 replies to this topic

[col]

So much for proof reading your own work!!. It's there for eternity now as there appears to be no edit option.

While we're on the subject of "punishment", this is what the Cooke workers had to suffer.

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[col]

Than you for posting that catalog info Col. So if we decide to finish the tube polished brass we won't feel guilty!

 

Peter

Your'e welcome Peter.

 

On the other hand if you look at the photos of the 6" in the observatory on this page:

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

the brass really stands out against the shiny black tube?

 

Col

[col]

Your'e welcome Peter.

 

On the other hand if you look at the photos of the 6" in the observatory on this page:

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

the brass really stands out against the shiny black tube?

 

Col

And for contrast, here's a photo of the focuser end of my 5" showing that vintage brass doesn't have to be dull. I presume the scope was originally used mostly for terrestrial purposes as I have a vague recollection I was told it was first owned by a (retired) ship's captain and consequently the star/sun diagonal did not get a lot of use. The owner before me made his own rotary eyepiece holder and he owned it for approx 50 years so the original diagonal stayed in the scope case.

[Peter Ceravolo]

Your'e welcome Peter.

 

On the other hand if you look at the photos of the 6" in the observatory on this page:

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

the brass really stands out against the shiny black tube?

 

Col

Yes, it does look good. We have not made any final decisions. I'm at the stage of stripping the old paint off the components.

 

I'll be posting next about my dissection of the Taylor triplet Photovisual apo objective.

 

Peter

[Peter Ceravolo]

The Cooke refractor is interesting in that the objective cell is easily detached from the OTA without affecting collimation.

 

 

Back off three screws, twist the objective cell and off it comes to be safely stored out of the elements which, for the Cooke/Taylor triplet photo-visual apochromatic objectives, is a great idea.

 

 

The brass ring the cell bolts to is tipped and tilted for collimation. In the image below the adjustment screws are visible, and the dew shield is in place over the ring.

 

[Peter Ceravolo]

Of course the first thing I did with the lens was test the optical quality - very good!

 

The Ronchi bands are straight, this is a well corrected system. However one can see faint concentric zones, more easily visible in the test than in these images:

 

 

 

What really surprised me was the internal quality of the 120 year old glass, other than a few bubbles the substrate very clean, free of significant striae.

 

 

One can see in the above cross Polaroids image three whitish areas indicating the presence of stress.

[Peter Ceravolo]

Removing the top retaining ring revealed the non conventional lens cell innards.

 

 

Dennis Taylor, the Cooke company's optical designer, invented the first apochromats using a three lens objective. These objectives were very sensitive to lens element alignment, so they needed a non conventional cell that would restrain the lenses even after significant temperature changes changed the cell diameter, causing the lenses to shift relative to each other.

 

In this 4" objective and others of this size range, a metal spring plate was used to apply pressure to the lens edges as the cell expanded and contracted. The image above is in the same orientation as the image of the lens under cross polaroids. One can see that the points of lens/cell contact correspond to the stress points visible under polarized light.

 

I was fortunate to have in my library Taylors book that has a chapter or two devoted to the PV apo lenses, including one on their maintenance:

 

 

[Peter Ceravolo]

Taylor's book suggested doing the usual thing I do when removing a lens from its cell, support the glass on a tall tissue covered tube and lower the cell until it clears the glass.

 

 

I found the force required to slide the cell off the glass to be excessive, I was so concerned that I stopped to think about it some. Although the force was significantly more than I was comfortable with the sliding motion was smooth with no binding, even though I found it difficult to keep the cell square to the glass.

 

It turns out that spring plate imparts a lot of force to the glass. In this picture the plate is in place, just after the lenses came out.

 

 

One can see that the plate at rest is almost in line with the clear aperture of the cell. The objective lens edge is a tenth of inch wider so there is a lot of deflection imparted to the spring plate to make it clear the lenses. As far as I can tell the lugs that project into the cell are there to constrain the spring plate's position in the cell and don't contribute to the force applied to the lens.

 

[ccwemyss]

Interesting that the strip has cuts at the spacing of the elements so they are held independently. Did they catch at all as the edges slid past them?

 

Chip W. 

[Jon Isaacs]

I know of quite a few machine shops in the area, none of which use casting as a regular part of their process. In the 19th century, almost any manufacturing shop could create molds and send them off for casting at a relatively local foundry.  It was the easiest way to get complex shapes, and was cost effective even for very small runs of parts. Today, with CNC machines, it is simpler to start with standard stock, and remove material, especially for short runs. Casting, now, is typically reserved for larger runs, where the higher setup cost can be amortized. 

 

But that does lead to a different style of design. Who, for example, would program a CNC mill to generate raised lettering, rather than engraved lettering, on a part? When making a mold, it's easy and low in cost to raise the lettering. When milling, it means taking a whole surface down to a lower level around the letters. 

 

I was saying that I have an appreciation for these differences in the many small-run parts that one sees in antique mounts that were built from castings that often incorporated aesthetics that appealed to the maker. 

 

Chip W. 

Chip:

 

In your first post, you wrote:

 

"What I appreciate about this time was how commonly available casting was as a manufacturing technique. Creating a model and a mold is an art that's nearly lost by comparison to today."

 

Casting techniques today are far more sophisticated than they were 120 years ago. Machine shops are not foundries. The machine shop I worked with for 30 years would occasionally send out for castings.  Googling Foundry in San Diego, there are quite a number of them.  I am not sure what sort of production run is necessary but some seemed geared towards small size runs.

 

I think casting is doable if one wants but cast parts are generally not as strong as machined parts and most cast parts will need to be machined anyway so with modern machining it comes down to both cost and strength/quality. 

 

I think if you want something cast, it can happen but it will be more expensive.. 

 

Anyway, this thread is about Peters scope and mount.  Back then, things were simpler.  The parts were cast and then machined.  And they took the time for things like engraving and raised lettering. 

 

Jon

[Peter Ceravolo]

Interesting that the strip has cuts at the spacing of the elements so they are held independently. Did they catch at all as the edges slid past them?

 

Chip W. 

 

Great question Chip!

 

The lens elements sport rounded edges to avoid binding when being fitted into the cell. I first noticed this decades ago in a diagram published in a 1970's Sky and Telescope article on the Taylor PV objectives.

In the pic below I placed a machinists square against the widest lens element, the middle one, to show the rounded edge. Even the narrow edged first and third element of the triplet have rounded edges.

Even though the lens edges are ground round, they are still precision sized, within a few thousandths of an inch of each other. But as can be seen in the picture, the rounded edge is not centered on the lens edge - no worries, it still worked!

 

 

Edited by Peter Ceravolo, 18 April 2020 - 02:32 AM.

[Peter Ceravolo]

When lenses are fabricated, there may be a variation in thickness that has a tendency to produce oblong images. This wedge can be "dialed out" by rotating the lenses against each other until the optimum orientation is achieved. The lens elements are then marked by pencil, or in the case of the Cooke/Taylor triplets, by indents ground into the lens edges and spacers for permanence.

 

In his book Taylor stresses the importance of keeping these marks aligned:

 

 

 

 

After extracting the lenses from the cell I was careful not to touch them and photographed the orientation of the lenses elements. Well... As can be seen in the image below the last lens is way out of alignment with the first and middle lens. The alignment marks are circled in hard to see red.

 

 

 

 

Even the alignment of the spacer and the last element is not what it should be:

 

 

 

All this might suggest that the lens has been serviced since it left the Cooke workshop 120 years ago. I can't imagine Dennis letting this go by... but then, there is more...

 

[col]

Taylor's book suggested doing the usual thing I do when removing a lens from its cell, support the glass on a tall tissue covered tube and lower the cell until it clears the glass.

 

IMG_1784.JPG

 

I found the force required to slide the cell off the glass to be excessive, I was so concerned that I stopped to think about it some. Although the force was significantly more than I was comfortable with the sliding motion was smooth with no binding, even though I found it difficult to keep the cell square to the glass.

 

It turns out that spring plate imparts a lot of force to the glass. In this picture the plate is in place, just after the lenses came out.

 

IMG_1789.JPG

 

One can see that the plate at rest is almost in line with the clear aperture of the cell. The objective lens edge is a tenth of inch wider so there is a lot of deflection imparted to the spring plate to make it clear the lenses. As far as I can tell the lugs that project into the cell are there to constrain the spring plate's position in the cell and don't contribute to the force applied to the lens.

 

IMG_1773.JPG

Referring back to my 2008 image of my Cooke 5" PV from this topic link, we removed the lens edge retaining spring plate by pulling it up with needle pliers (obscured in the image) before separating the elements from the cell.:  https://www.cloudyni...elescope/page-2

Possibly less stressful than forcing the elements out against the spring pressure?

 

 

[Peter Ceravolo]

Col, yes, in hindsight it would have been less stressful to remove the plate first with needle nose pliers, but Taylor made no mention of that option in his instructions. In fact he suggested doing it the usual way which in the end was fine. The only reason it was stressful is that there was a lot of friction, I have since learned that other cells are not as tight as mine.

 

I just tried removing the plate with pliers, much easier, but for the last bit I used my thumbs to bow out the plate while extracting it fully to avoid it rubbing on the polished surface.

 

If you have more pics of the extraction process with your lens I'd love it if you posted them.

 

Peter

[Peter Ceravolo]

Col's link to the 2008 thread; https://www.cloudyni...elescope/page-2

shows Richard's image of his - now mine - 4" lens with the retainer removed.

 

If you look closely there is a spot on the lens, the image is reproduced here at higher resolution and with the spot circled in red:

 

 

I had a look at the spot in a microscope!:

 

[Peter Ceravolo]

It's amazing what kind of damage bug guts can do to a sensitive glass surface. There is no way to know how long the bug had been on the glass.

Here is another picture with the bug cleaned off and the remaining surface damage, the magnification and orientation is much the same as the other image for comparison.

 

 

 

 

I was warned by one of the Antique Telescope Society members that the Cooke-Taylor triplets are notorious for having a sensitive middle lens element whose surfaces degrade over time. and this is not new, it was written up in a scholarly journal back in the early 1900's.

 

Here is a snip of one the the pages illustrating the lens degradation:

 

 

[col]

Col, yes, in hindsight it would have been less stressful to remove the plate first with needle nose pliers, but Taylor made no mention of that option in his instructions. In fact he suggested doing it the usual way which in the end was fine. The only reason it was stressful is that there was a lot of friction, I have since learned that other cells are not as tight as mine.

 

I just tried removing the plate with pliers, much easier, but for the last bit I used my thumbs to bow out the plate while extracting it fully to avoid it rubbing on the polished surface.

 

If you have more pics of the extraction process with your lens I'd love it if you posted them.

 

Peter

No I'm afraid not Peter. The only images I can find are those posted in the older topic. Besides, I would have had to climb into the laminar flow cabinet to get a better angle. The strip was eased out by alternately working from its opposite corners to move it a little at a time till it came free. I guess we had the advantage of Steve's 30+ years in Amateur and professional Astronomy at the time and on not having Taylor's book so we had to work it out for ourselves .

On a separate point, the lens elements were all still aligned to their respective marks so it had either never been out or had only ever been cleaned expertly sometime in its first 50 years. Cheers, Col

[col]

No I'm afraid not Peter. The only images I can find are those posted in the older topic. Besides, I would have had to climb into the laminar flow cabinet to get a better angle. The strip was eased out by alternately working from its opposite corners to move it a little at a time till it came free. I guess we had the advantage of Steve's 30+ years in Amateur and professional Astronomy at the time and on not having Taylor's book so we had to work it out for ourselves .

On a separate point, the lens elements were all still aligned to their respective marks so it had either never been out or had only ever been cleaned expertly sometime in its first 50 years. Cheers, Col

Peter, I did some searching and found a few more pics of my 5" objective being cleaned but this is the only other one which appears to relate to the extraction. I can't recall what he was doing but it looks a bit scary. Col 

 

[bratislav]

Is that Steve Lee ? He'd know what he was doing 

Edited by bratislav, 19 April 2020 - 07:44 AM.

[Peter Ceravolo]

Col that spring plate looks familiar!

 

I don't recall when or where I heard this statement... "treat it like glass and it will surely break!" :-)

 

When messing around with this older equipment I've gotten into the habit of taking a lot of pictures from different angles.

 

What was the driver for your PV disassembly? How dirty was it?

[plyscope]

Is that Steve Lee ? He'd know what he was doing 

It looks like Steve Lee to me. I remember his ATM pages and actually met him once over 20 years ago when I was fortunate to have a tour of the AAT at Siding Springs.

 

Interesting video here. https://www.aao.gov....e-and-the-stars

[Peter Ceravolo]

One of the reasons I wanted to acquire the Cooke telescope was to dissect the early apochromat objective for historical reasons. Although there are accounts of the objectives being reworked over the decades, not much detailed constructional information has been documented or published.

 

This 4" telescope's history is not well documented, it was found in a derelict observatory on a property that was undergoing renovation. The objective, #69, was traced to 1899:

 

 

 

 

No other information is known, for example, has this lens been re-polished to fix a damaged middle element? So after the lenses were extracted from the cell out came the spherometer!

 

 

 

 

Handling the antique glass to measure all the dimensions that one can is a bit nerve wracking. After all the easily measurable parameters were documented, I modeled the lens in simple 2D CAD software to determine the dimensions I could not easily measure, like the center thickness of the middle biconcave element. I even measured and modeled the edge chamfers.

 

 

 

 

Everything in the drawing had to add up, for example the objective's total thickness, that included the lens spacers, but it didn't. I eventually traced the problem to the realization that the last surface was a very weak concave, that meant that I was not measuring the total objective thickness properly. So I remeasured with the first element down, then everything added up properly.

 

 

 

 

 

[col]

Is that Steve Lee ? He'd know what he was doing 

Yes, It's Steve.

[Kunama]

re Dr. Hugh Walsham:

 

http://articles.adsa...IEW&classic=YES

[col]

Col that spring plate looks familiar!

 

I don't recall when or where I heard this statement... "treat it like glass and it will surely break!" :-)

 

When messing around with this older equipment I've gotten into the habit of taking a lot of pictures from different angles.

 

What was the driver for your PV disassembly? How dirty was it?

Yes the plate is a very simple and effective means of restraining the lenses.

 

I trust my memory less an less so increasingly take images of most things I work on for future reference. Especially now with phone cameras. Back when we did the Cooke it was still a novel practice for me.

 

Steve has "handled" all types optics up to the 3.9 metre AAT so I'd trust him with any scope.

 

When I bought the Cooke the lens was clean other than for a small spot of fungus on the centre element. Knowing how it can permanently damage glass I took it to Steve at my first opportunity. It cleaned off ok with isopropyl alcohol but left a small etched spot.

 

I suspect that unlike your's Peter, my Cooke has always been valued by its owners and has spent most of its life in its original case. That's the same today as it's weight and size makes it difficult to take out for casual observing and I currently don't have the space to have it on display. One day I hope. Col

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