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Vintage Lenses - Spacers, Rings, & Things

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#1 Bomber Bob

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Posted 24 July 2020 - 04:43 PM

Over on the Miscellany thread, I started a discussion about ways to air-space lenses:   https://www.cloudyni...7#entry10361206

 

60 x 800 OTAs

 

Just a couple of things I noticed in testing 60 x 800 Old Towa refractors:

 

- The all black versions had the better star tests; and, the OTAs were heavier than the white & black versions.

 

- Tough to assess age based on the color scheme.  I recall one white & black that had that old & unusual lens cell and dewshield made in just one casting (a single piece).  Its retaining ring was on the eye side, and had 3 tiny grub screws locking it in place.

 

- One thing they had in common:  Except for 2, the Old Towas used a thin brass or steel ring painted flat black rather than 3 foil spacers.

 

Wish I'd had my DPAC rig back then!  My star tests are pretty good, but there's still some subjectivity in interpreting them.

 

So, are there any advantages / disadvantages to spacers vs. rings?  Any performance differences?  What do y'all think, and/or what have you seen?



#2 Bomber Bob

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Posted 24 July 2020 - 04:45 PM

My 1978 Tasco / Towa used 2 vinyl ring spacers -- one between the elements, and the other between the flint and the lip of the aluminum lens cell (a cushion?).

 

I'll defer to Dave G on whether one type is better than another.  I would think the ring spreads out the force on the crown better than 3 foil spacers, but the weights may be so light with the 4" & smaller lenses that it really makes no difference...

 

When I saw the Towa, my first thought was... Cheap!  But maybe not.  I do recall, the brass ring in one of the Old Towas was so thin that the edge was like a razor or scraper blade (the scrapers we bought for cars in North Dakota used brass "blades").



#3 Kasmos

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Posted 24 July 2020 - 05:18 PM

Since it's less work I'm going to copy and paste what I said in Miscellany.

 

Post 1.

It's funny you brought this up since I was just thinking about these spacer rings.

 

I'm not sure but I think they might be steel. It looked like the one in my Crescent was a bit rusty. It also appeared to be uneven in thickness, like it had a tiny uneven lip on it's edge from being stamped out. The one in my '63 60/700 9TE is metal and was not round as in bent. The 60/1000mm Penncrest has two plastic (nylon?), ones. One between the lenses and one that the flint rests on. I also have a 900mm Carton lens that the previous owner put in a Sears cell with a nylon spacer. since Carton didn't use rings, it makes me think it would perform  better if it had three foil spacers. Anyway, I'm begining to think between the two types, the plastic ones might be better.

 

Has anyone ever measured a metal or plastic ring spacer's thickness? I'm sure they are much thicker than the typical foil ones. They seem to space the lenses further apart so that the Newton rings cannot be seen. I once replaced the ring in the Cresent with three foil spacers and it seemed to perform even worse, so switch it back.

 

Maybe this topic could use it's own thread?

 

Post 2.

I think the biggest problem with a ring spacer is, without precise manufacturing, it's likely to have a inconsistant thickness and I doubt that Towa made them that precise. My guess is they were used for mass prodution since it would be much faster to assemble than appling three foil spacers. Afterall, what other decent telescope maker even used them on a Fraunhofer lens?

 

Post 3.

I've had a few turkeys so agree, foil spacers don't necessarily equal good optics. On the other hand there seems to be a ton of members who extol the performance of their Towa made (and a few other makers), ringed spaced scopes, but without a DPAC, or side by side evaluation, or a look for your/myself, the rest of us will never know how good or not they really are.

 

It might be a good or fun stand alone topic to ask and press them on the subject. Though it might get heated.

 

New post

My 60/1000mm Penncrest (Towa with two plastic ring spacers described in Post 1) seems to have fairly good optics. I don't have a DPAC so I won't really know how good it is, or how it rates until I put it up against some my other long refractors. 


Edited by Kasmos, 24 July 2020 - 05:22 PM.


#4 DAVIDG

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Posted 24 July 2020 - 06:04 PM

 Here is test you can do that doesn't need double pass autocollimation. Just put the lens under monochrome or semi-monochrome light and look at the shape of the interference rings between the elements. If the ring is doing a good job the rings will be perfectly round and centered  A CFL bulb will work to show the rings like this picture were I'm using my desk lamp with a CFL bulb.

  As I said in the other thread three points define a plane and the air gap needs to uniform in thickness. That happens when the spacer thickness is the same, they are same size and placed the same distance from the edge of the elements and are at 120° centers. With a ring you have no control of  were the points of contacts are.

 

              - Dave 

 

lens interference pattern CFL bulb.jpg


Edited by DAVIDG, 25 July 2020 - 08:23 AM.

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#5 Bomber Bob

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Posted 24 July 2020 - 06:05 PM

I haven't seen any of the "better" vintage fracs with the ring spacers.  My old Mogey not only uses 3 spacers, it has the 3 raised areas on the cell that align with them -- a Zeiss technique?

 

Also, most times, the lenses with ring spacers have no edge alignment marks.  Is the Maker saying... any will do? - OR - our lenses are so well-figured that there's no need to find the best element alignment?  (I tend to think the former rather than the latter applies.)



#6 Terra Nova

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Posted 24 July 2020 - 06:51 PM

I haven't seen any of the "better" vintage fracs with the ring spacers.  My old Mogey not only uses 3 spacers, it has the 3 raised areas on the cell that align with them -- a Zeiss technique?

 

Also, most times, the lenses with ring spacers have no edge alignment marks.  Is the Maker saying... any will do? - OR - our lenses are so well-figured that there's no need to find the best element alignment?  (I tend to think the former rather than the latter applies.)

Some of the Jaegers achromats were Baker design with a fairly wide space that was accomplished by a metal ring.


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#7 Kasmos

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Posted 24 July 2020 - 08:41 PM

 Here is test you can do that doesn't need double pass autocollimation. Just put the lens under monochrome or semi-monochrome light and look at the shape of the interference rings between the elements. If the ring is doing a good job the rings will be perfectly round and centered  A CFL bulb will work to show the rings like this picture were I'm using my desk lamp with a CFL bulb.

  As I said in the other thread three points define a plane and the air gap needs to uniform in thickness. That happens when the spacer thickness is the same, they are same size and placed the same distance from the edge of the elements and are at 120° centers. With a ring you have no control were the points of contact. 

 

              - Dave 

 

attachicon.giflens interference pattern CFL bulb.jpg

One problem. None of the lenses I own with a ring spacer will show Newton Rings. They appear to separate the lenses too much for them to exist. It's also happened with two other lens that have 3 thicker than usual metal spacers.

 

Anyone else experience this?



#8 oldmanastro

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Posted 24 July 2020 - 10:51 PM

I just took out the metal spacer ring from the younger of the 1960s Sears 60mm f/11 refractors and replaced it with foil spacers at 120 degrees from each other. The result was a much improved star test image with clear cut Airy disc and diffraction rings. I tested at 175x just to make sure. The objective had never been removed before so I marked it when it was removed. 

 

The Sears 4454 80mm f/15 had two plastic rings. One was used as a spacer and the other where the flint element rests in the cell. I removed the plastic ring spacer and replaced it with foil spacers. The objective provided an image of a diffraction pattern like the one that Dave showed. I never saw it with the plastic ring spacer. The only telescope where the ring spacers seem to work ok is my older 60mm f/11 Sears refractor. One thing about the metal ring spacer that I just removed from the 60mm, it has rust on it and that by itself can change the surface making it irregular. I think that well placed foil spacers are the best alternative.

 

Guido


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#9 GreyDay

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Posted 25 July 2020 - 04:13 AM

I just took out the metal spacer ring from the younger of the 1960s Sears 60mm f/11 refractors and replaced it with foil spacers at 120 degrees from each other. The result was a much improved star test image with clear cut Airy disc and diffraction rings.

Maybe someone with a camera, a DPAC rig and access to scopes with rings/foil could back this up?


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#10 Kasmos

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Posted 25 July 2020 - 04:27 AM

I removed the ring and replaced it with spacers on one of my under performers (too much CA and soft focus), and it appeared to make things worse, so back in it went. I got tired of tinkering and thinking I might improve it, so I replaced the objective with a much better one. 


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#11 GreyDay

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Posted 25 July 2020 - 06:03 AM

I removed the ring and replaced it with spacers

Getting (knowing) the right thickness foil seems to be a problem. i tried replacing just one foil in an objective, it was practically worn out so i used a few different thickness foils but they all distorted the fresnel rings! so i put the original back in. Luckily it worked!

I sold the scope a few months later, now it's someone elses problem.


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#12 DAVIDG

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Posted 25 July 2020 - 08:51 AM

 As I have said, the air spacing in an objective is an optical element just like the glass ones. The designer uses the fact that air has a refractive index of 1.00 and uses that and the thickness of the air space in the design to achieve the best correction. So if you change the thickness or oil the elements together you change the correction. I'm sure everyone knows this but just in case it is not clear,  if you replace the plastic ring with spacers they need to be of the exact same thickness as the ring. 

   In geometry we learned that three points define a plane. If you add a forth point, the three that are the highest will define it.  So with three points we know the exact points of contact, add  more and we don't which three will be the ones in contact.  If you have a convex surface placed on top a concave surface and you want an uniform spacing as function of the radius ie at any given radius from the center the distance between the two surface is the same, then that requires three points of contact at exactly 120° centers and exact the same distance from the center and  with the points of contact having  the exactly the same contact area. That is just pure physics and math.  Achromats have been made for over a hundred years and the people that design them and the cells that they are mounted in are pretty smart. So they understand the spacing  issue and this is the method that they have used for years. If a better method was available smart people would have thought of it.

   So why a ring then ?  You have to keep going back to why telescope for the general public are made in the first place. That is money. They are made to make a profit for the people building them.  There is an old saying "that you  pay for what you get and get what you pay for"  So it simply that is cheaper but not better to use a plastic ring.  

 

                    - Dave 


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#13 Bomber Bob

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Posted 25 July 2020 - 10:26 AM

Some of the Jaegers achromats were Baker design with a fairly wide space that was accomplished by a metal ring.

Okay, Terra...  You know I meant traditional Fraunhofer objectives with a very small air gap.  If not, I'm typing that now:  I haven't seen any of the "better" vintage Fraunhofer Achromatic Refractor Objectives with the ring spacers.  My Dakin 4" F10 is a Baker with a 7mm spacer-band -- but at 7mm, it ain't no spacer-ring ala Towa...



#14 Bomber Bob

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Posted 25 July 2020 - 10:29 AM

So why a ring then ?  You have to keep going back to why telescope for the general public are made in the first place. That is money. They are made to make a profit for the people building them.  There is an old saying "that you  pay for what you get and get what you pay for"  So it simply that is cheaper but not better to use a plastic ring.

 

So, given the 3 points of contact vs. a metal or plastic band of (possibly) variable thickness, would it be better to just cement the elements?  For Towa, et. al, would that be cheaper; or, does cementing require even better quality control for the 2 elements?



#15 Bomber Bob

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Posted 25 July 2020 - 10:40 AM

For the new folks who haven't seen parts of my antique Mogey 3" F14, here's the retaining ring:

 

AB3R - Retainer 1 TOP T01.jpg

 

Notice the 3 "raised areas" at 120* separations.  The Mogey is my only frac with this feature.  Makes aligning the spacers a snap!  The ring isn't threaded -- 3 tiny grub screws pass through the lens cell wall to lock it in place.  In 4 years since the restore, no shifts in the lens elements or the ring, so it's a design that lasts.


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#16 DAVIDG

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Posted 25 July 2020 - 10:48 AM

 When you cement a lens you loose a degree of freedom in the design so correction will not be as good. It also takes skill to cement a lens and get the elements centered with no bubbles especially when you don't use a UV curable cement that allows time to get things right.

   So what is the cheapest method to space the lens ? A plastic ring. 

 

                 - Dave 


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#17 Bomber Bob

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Posted 25 July 2020 - 10:48 AM

My 1957 Goto Hy-Score 452 (60x1200) lens has no spacers...

 

GHS452 - Lens Before Cleaning S03.jpg

 

It's a contact doublet.  The alignment mark is original -- and critical.  A slip of just 1mm either direction hurts performance, which is a head-scratcher for me, as Goto uses a brass retaining ring on the eye-side of the lens cell:

 

GHS452 - Lens Post Cleaning S03.jpg

 

No matter how slowly or carefully you thread the ring in, the 2 elements will move... and wreck alignment.  Once I figured that out, I taped the edges -- no more slips!


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#18 oldmanastro

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Posted 25 July 2020 - 10:52 AM

Getting (knowing) the right thickness foil seems to be a problem. i tried replacing just one foil in an objective, it was practically worn out so i used a few different thickness foils but they all distorted the fresnel rings! so i put the original back in. Luckily it worked!

I sold the scope a few months later, now it's someone elses problem.

I have used aluminium foil. it comes in different thickness. On the Sears 4454 80mm I used the thicker foil and it worked. On the 60mm f/11 I used the thinner foil because that's what I had at the moment. It's not the same thickness as the ring spacer but it worked better than the ring. When I say that it improved the image I mean that it went from a messy star test where the Airy disc was distorted and almost embedded with the diffraction ring, to a star test where it is now centered and separated from the first diffraction ring. There is still some distortion that looks like pinching but it can't be. The lenses are not under any undue pressure from the retaining ring. 

 

Guido


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#19 DAVIDG

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Posted 25 July 2020 - 10:54 AM

For the new folks who haven't seen parts of my antique Mogey 3" F14, here's the retaining ring:

 

attachicon.gifAB3R - Retainer 1 TOP T01.jpg

 

Notice the 3 "raised areas" at 120* separations.  The Mogey is my only frac with this feature.  Makes aligning the spacers a snap!  The ring isn't threaded -- 3 tiny grub screws pass through the lens cell wall to lock it in place.  In 4 years since the restore, no shifts in the lens elements or the ring, so it's a design that lasts.

 The raised areas on the cell and the retainer ring define the points of contact of the outer lens surfaces and also put even pressure on the spacers so that helps in achieving an even air gap.

  As I have said before I easily modify cells that don't have this feature by place three taps of Kapton tape on edges of the cells that the lens rests on and three taps over the air spacers on the front element. The lens is positioned so the air spacers are over the tape on the bottom of the cell and when the retainer is screwed down it comes in contact with the three pieces on the front of the lens. The  lens then  is checked with monochrome light to be sure the interference rings are round and centered.

 

                 - Dave 


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#20 Bomber Bob

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Posted 25 July 2020 - 10:54 AM

Maybe someone with a camera, a DPAC rig and access to scopes with rings/foil could back this up?

Here's a crazy BB idea:  My Meade 390 has the metal ring.  I could clip out 3 small pieces, and use those for spacers.  I already have the best element alignment via my DPAC.  In theory, I would think the new spacer alignment would be close to the current (with ring) alignment...

 

Or, get out my calipers, measure the ring thickness, and make 3 foil spacers the same thickness, and see how the lens does on the DPAC rig...


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#21 Bomber Bob

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Posted 25 July 2020 - 11:00 AM

BIF:  When I got my Mogey, the lens was missing the spacers.  I read somewhere that Mogey & other Victorian-era lens-makers used postage stamp pieces, so I tried that first.  Results weren't good.

 

So, figuring the Mogey was antique, I used a blank page from our 1872 family Bible (no sacrilege -- it was already falling out).  Much better performance, and gives a good DPAC pattern, too.


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#22 GreyDay

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Posted 25 July 2020 - 11:28 AM

There is still some distortion that looks like pinching but it can't be. The lenses are not under any undue pressure from the retaining ring.

Could be a low spot on the edge of one of the elements causing one of the spacers to be lower than the other two, when i tried replacing just one spacer it was like the cell was pinched causing astigmatism. the fresnel rings were all at one side of the objective and only covering one third. No amount of messing would get the elements spaced evenly, which is why i put the original worn spacer back in. Even then it was touch and go with tightening the retainer the right amount.


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#23 oldmanastro

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Posted 25 July 2020 - 11:41 AM

Could be a low spot on the edge of one of the elements causing one of the spacers to be lower than the other two, when i tried replacing just one spacer it was like the cell was pinched causing astigmatism. the fresnel rings were all at one side of the objective and only covering one third. No amount of messing would get the elements spaced evenly, which is why i put the original worn spacer back in. Even then it was touch and go with tightening the retainer the right amount.

I'll give it a check and let the forum know about it. This has become a very interesting thread!

 

Guido



#24 PawPaw

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Posted 25 July 2020 - 12:42 PM

I have found a micrometer is invaluable in knowing the thickness of foil spacers.   Harbor freight sells these for a reasonable price and it takes all the guessing out of the equation.  As for making your own you may find this post helpful:

  https://www.cloudyni... unitron find

Post # 97 and 101.

 

 

Don


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#25 oldmanastro

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Posted 25 July 2020 - 10:50 PM

I did some measurements on the metal spacer ring using a micrometer. Values varied from .32 to .39mm as I rotated the ring 360 degrees and took several measurements twice just to make sure. Not much precision there. I don't think these rings have ever been very precise spacers. The foil paper spacers that I used were .02mm thick. I decided to replace them with something that would provide more spacing for these lenses. 

 

Sometime ago I bought this roll of thin and gray auto detailing tape at Pep Boys. They were practically giving it away and when I saw it I thought that it could become handy. The tape comes with a backing paper and the adhesive used doesn't barely binds. It's easy to remove. I removed the backing paper and measured the thickness of the tape at .09 millimeters. Then I decided to give it a shot as spacer material for the 60mm lens. It seemed to work well. The image shows the metal ring spacer to the left, the 60mm lens with the new spacers in and the spacer material to the right. You can see concentric diffraction rings. They were not there with the metal ring or foil spacers. 

 

A short break in the clouds allowed me to see Antares at 175x. A much improved star test now with a clearly round Airy disc and just a touch of residual astigmatism that could be from collimation. The pinched optics look was gone. It was a short test interrupted by clouds. It's been raining all day.

 

Anyway that's the way things stand now. i have to test the 60mm objective further. It's definitely not prime optics but I think that it has improved a lot now.

 

Clear Skies!

 

Guido

 

 

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