22 replies to this topic

[Princess Leah]

I'm not sure if my memory serves me correct, but I seem to remember cleaning a doublet lens from a Towa or Kenko, that had no spacer - but fitted together so seamlessly, that I felt it was like a cemented doublet in design....

 

Or would there still be a tiny air gap between the elements?

 

Or perhaps i remember incorrectly!

 

Thanks Leah.

[TOMDEY]

As long as the convex radius is longer than the concave radius this can work fine. There are some technical details regarding the glass edges and the sleeve that captivates them... but no need for a spacer, provided the skilled optical and mechanical designers are in agreement. Rudolf Kingslake and I actually discussed these things long ago. He appreciated that I was gratuitously sketching mounts for the lenses he had us designing for homeworks (the other students just left them floating in mid air. minimalist lazy slugs) --- so he offered to mentor me on extra things for "extra credit" --- I later found out what he meant by that. He personally introduced me to the movers and shakers in the world of optics. The rest is history. Lesson in this --- always do more than your teachers or bosses require --- because you sincerely want to... not to suck up or expect something in return. They can tell the difference, and it becomes a win-win a week, month, year, or decade later.     Tom

Edited by TOMDEY, 21 March 2025 - 07:42 PM.

[Kasmos]

I'm not sure if my memory serves me correct, but I seem to remember cleaning a doublet lens from a Towa or Kenko, that had no spacer - but fitted together so seamlessly, that I felt it was like a cemented doublet in design....

 

Or would there still be a tiny air gap between the elements?

 

Or perhaps i remember incorrectly!

 

Thanks Leah.

If it was a Towa, most likely it originally had a ring spacer that was lost. In either case when they are placed together without spacers they can stick together by suction. IIRC, other than some Gotos, generally the Japanese classics didn't come with a contact doublet

Edited by Kasmos, 22 March 2025 - 12:58 AM.

[RichA]

Can't see them allowing glass to glass contact because of a risk of chipping.  Likely all lenses have some kind of spacers, unless cemented.  Also, allowing equiconvex-concave lenses to touch risks crushing silica dust particles between, creating small pits.  Also, ideally, we should not be seeing Newton rings in an objective.  Even though they appear harmless.

Edited by RichA, 22 March 2025 - 12:14 AM.

[Kasmos]

I believe contact doublet objectives only made contact on their edges so were actually air spaced.

[Princess Leah]

I believe contact doublet objectives only made contact on their edges so were actually air spaced.

Brilliant and very quick answers.

Wonderful replies - thanks.

 

This last comment by Kasmos is exactly what I remember and puts my mind at rest.

 

Still can't remember where I came across it before, maybe it was just another thread on CN I read!

 

Leah.

[PawPaw]

Goto Kogaku designed such a lens and applied for a patent in 1955.   Seizo Goto made use of blue plate glass for the crown element in the early years.  Circa late 1960's the blue plate glass likely lost its economic value and he switched to a clear crown glass.  In japan it was known as an "air lens" with contact being made only at the edges.  I own several of these and they are of good quality.  

 

Here is the patent document:

 

 

 

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[deSitter]

Brilliant and very quick answers.

Wonderful replies - thanks.

 

This last comment by Kasmos is exactly what I remember and puts my mind at rest.

 

Still can't remember where I came across it before, maybe it was just another thread on CN I read!

 

Leah.

The inner surfaces have slightly different radii in order to inhibit ghosting. If they had the same radius, some incoming light would be reflected from the concave surface, reflected again from the 2nd convex surface, and then pass through the concave surface, and would arrive at a focus near the actual direct image plane. Bright objects would have ghosts slightly out of focus. By making the radii different, any ghost would be far out of focus at the direct image plane. You also end up with another variable for getting the best performance, at the price of making another surface.

 

-drl

[Terra Nova]

I have to agree with Chris that if it’s Towa (especially post 1963) or if it’s Kenko, you are missing the ring. Air-spaced Fraunhofer achromats are made to take advantage of the refractive properties of air and typically need about 3mils of spacing, (mils not mm).

[TOMDEY]

If it was a Towa, most likely it originally had a ring spacer that was lost. In either case when they are placed together without spacers they can stick together by suction. IIRC, other than some Gotos, generally the Japanese classics didn't come with a contact doublet

When the mating radii are identical, they are (intended to be) cemented, of course and are not coated, because the (properly selected) cement itself renders the interface reflectivity virtually absentee.  When they are CX long, CC short... the surfaces are ARC coated and there is that center axial gap = D²(R₂-R₁)/8R₁R₂ in the middle, which can be noticed and quantified by the Newton's Rings in the middle as seen in monochromatic (red) light. This is the unambiguous way to recognize what the designers intended. It's also possible that some fool removed a spacer --- but close scrutiny will no doubt leave evidence that was originally there. We serviced returned field-use optics in the Navy (opticalmen like our own redoubtable mentor Bill Cook). Me... I worked at Bausch & Lomb and say every imaginable abused returns over the decades. The worst offenders were ham-handed scientists and of course end-users.    Tom

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[davidc135]

It may be that there is a risk of scratches with edge contacted lenses. You may as well separate the lenses with spacers as slow achromat objectives aren't sensitive to actual spacing, within reason. It took +5.0mm spacing to have the same under-corrected SA effect on my 3'' f15 Littrow achromat as +0.1mm did with my 4'' f8 apo fluorite doublet.

 

Maybe it's more complicated.

 

David

Edited by davidc135, 22 March 2025 - 12:43 PM.

[Princess Leah]

Fascinating contributions.

 

Not only can I not remember the refractor it was that I encountered this - i can't even remember if I encountered it physically or read it on CN!

 

I'm not yet 40 - no excuses!

[Princess Leah]

Goto Kogaku designed such a lens and applied for a patent in 1955.   Seizo Goto made use of blue plate glass for the crown element in the early years.  Circa late 1960's the blue plate glass likely lost its economic value and he switched to a clear crown glass.  In japan it was known as an "air lens" with contact being made only at the edges.  I own several of these and they are of good quality.  

 

Here is the patent document:

This is great information. Many thanks.

 

What is 'blue glass'- not to be confused with 'blue-grass' I assume.

[PawPaw]

This is great information. Many thanks.

 

What is 'blue glass'- not to be confused with 'blue-grass' I assume.

Blue plate glass is essentially window glass also known as "soda glass" in Japan.    Seizo Goto utilized blue plate in his patent primarily because it was readily available after the war and inexpensive.  In the patent document Goto lists the reasons for using this type of glass:

 

'The characteristics of this objective lens are:
a. The material of the first lens is blue plate glass. Blue plate glass is cheap.
b. Tin foil is not used in order to reduce the time and effort in the process.
c. By designing at a long focus, chromatic aberration and spherical aberration of the objective lens can be sufficiently corrected.

 

Goto only utilized this type of crown glass in their F/20 school telescopes.  

Edited by PawPaw, 23 March 2025 - 09:52 AM.

[Princess Leah]

Are the ring spacers in Towa's deemed inferior to foil spacers?

 

I appreciate they are a bit fiddly, but I don't think I have ever had a problem with them.

[Kasmos]

Are the ring spacers in Towa's deemed inferior to foil spacers?

 

I appreciate they are a bit fiddly, but I don't think I have ever had a problem with them.

The distance between the lens elements is more precise and controllable using three pieces of foil than making a ring that is perfectly the same height all the way around. So, technically yes but it's not always noticeable in practice. That said, my Tasco 9TE's images were improved by replacing the ring spacer with 3 thinner foil ones. In that case the CA was really hurting the image, but if a image looks good, why mess with it?

Edited by Kasmos, 24 March 2025 - 10:33 PM.

[RichA]

The distance between the lens elements is more precise and controllable using three pieces of foil than making a ring that is perfectly the same height all the way around. So, technically yes but it's not always noticeable in practice. That said, my Tasco 9TE's images were improved by replacing the ring spacer with 3 thinner foil ones. In that case the CA was really hurting the image, but if a image looks good, why mess with it?

Some photogs have noticed diffraction effects from rectangular metal foil spacers.  i wonder if using half-circles would alleviate that?  Not that it matters with a doublet achromat which won't be used for photography, in-general

[Ken Launie]

The diffraction should only result if the shim intrudes into the ray path. Properly-installed shims shouldn't. When you see them from the outside it should be because you're looking off-axis at them, well outside of the typically narrow field of view.

[RichA]

The diffraction should only result if the shim intrudes into the ray path. Properly-installed shims shouldn't. When you see them from the outside it should be because you're looking off-axis at them, well outside of the typically narrow field of view.

How can they not though?  They are on the lens surface.  If you are using the whole lens surface, they are impeding some of the light.

[davidc135]

How can they not though?  They are on the lens surface.  If you are using the whole lens surface, they are impeding some of the light.

Some of the lens will be within the cell.  David

[Princess Leah]

Some of the lens will be within the cell.  David

If they are within the cell, then whether you are looking off or on-axis is a mute point.... isn't it?

Edited by Princess Leah, 27 March 2025 - 04:53 AM.

[deSitter]

The diffraction should only result if the shim intrudes into the ray path. Properly-installed shims shouldn't. When you see them from the outside it should be because you're looking off-axis at them, well outside of the typically narrow field of view.

They are clearly visible in my telescopes when racked way out of focus. They don't cause spikes because they are primarily longitudinal obstructions, not radial.

 

-drl

[davidc135]

Imagers (I don't know if it is all) insist that prominent spacers will cause unwanted diffraction effects, which wouldn't be noticed visually.

 

David