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Vintage Lenses - Spacers, Rings, & Things

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#76 PawPaw

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Posted 01 August 2020 - 07:12 AM

I wondered if there may be different thicknesses of foil due to focal length but at .003" it appears to be the same for 900mm as 710mm.

 

I'm not sure how the math works for achromatic elements, my understanding is the 4 surfaces (1&2 crown,3&4 flint) usually have surfaces 2&3 that define the air space between crown/flint as almost identical radii, with surface 1&4 determining the focal length? maybe someone who knows how element design works could confirm or correct?

 

if i'm correct then surfaces 2&3 could be the same across many focal lengths? just guessing i know too little about the design:)

Edmund used .005" air space on the 4 inch objectives they imported from Japan as noted in Sam Browns book All about telescopes on page 177.  When they started designing their own in house with Dr. Rank they changed to .004 air space.  This is the specs on the in house objective originally dated 8-10-78 when designed and updated with Eng changes through 4-24-81.  I find it very interesting that 2/3 down the page they annotate the following:  "All surfaces figured to 1/8 wave + or - 15 rings.

What does "15 rings" mean?  Newton rings?  Maybe one of our resident experts can tell us.  

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  • Edmund 4 achromat-A.jpg

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#77 DAVIDG

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Posted 01 August 2020 - 12:57 PM

"All surfaces figured to 1/8 wave + or - 15 rings. "    One of the critical factors in a refactor are the radii on the surfaces. Their values determine the  spherical  and color correction. So the surface can be well corrected ie optically smooth but if the radius off, then the correction will be off. So they interact with each other and both need to be within spec to have a good image. 

   When you make a lens it has both concave and convex surface. Since convex surface don't focus light you can't do a test like the Foucault test on them to determine the optical figure. So what you do is make a concave test plate. The plate has the exact radius that the convex surface needs and since it can be directly tested via a Foucault test,  it is figured to the optical figure you want on the convex surface. In a majority of the time it is a sphere. 

  So now you place the test plate in contact with the convex surface and under monochrome light you see interference fringes. Just like you see Newton rings between the two inner surfaces of the finished lens when you testing to see if the air gap is uniform. If the surfaces perfectly match in both radius and optical figure you'll get straight fringes. If the surfaces don't match perfectly the lines will turn into arc's and the  more mismatch into  rings . The more rings the greater the mismatch and finally when are way off you won't see any rings at all. 

   So the " 15 rings" is referring to how close the radius on the convex surface matches the test plate. The rings are suppose be perfectly round if the surface match the figure of the  test plate and as I said in most cases it is spherical. So if have you a zone in the figure the ring won't be perfectly round. The spacing between the fringes is always 1/2 a wave, so any departure from the perfectly round or perfectly straight fringes tells you the amount of error as a ratio of the spacing between the fringes vs the departure. For example say a ring has  a bump in it that goes in 1/2  the distance between  the rings. Since the spacing between the fringes is always 1/2 a wave and the error goes in 1/2 this distance, the error is  1/4 wave. 

 

                   - Dave 


Edited by DAVIDG, 01 August 2020 - 07:47 PM.

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#78 PawPaw

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Posted 01 August 2020 - 02:20 PM

Dave.....thanks for the explanation.  I now know why and how.

 

One more question.... can any conclusions be drawn about the quality of a lens by the  circular sharpness and quantity of their interference rings?    I have some lens where the interference rings have nebulous edges and the rings themselves are wide as compared to others where the rings are tack sharp  like drawn with a very fine ink pen.  For example one of my lens has over 30 rings tightly spaced and finely drawn and another > 14 rings widely spaced with mushy edges.  Is this just a function of the amount of air space?

 

Don



#79 Kasmos

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Posted 01 August 2020 - 02:53 PM

Yesterday I did more trial and error work on the Kenko objective.

I made new spacers made from 3 layers of .003" material.

 

The former ones were 1 layer so I wanted to see if a major jump channged anything.

First I stacked the two elements without spacers and it showed off center Newton rings.

I would tend to think that's a bad sign of mismatched curvature or wedge.

 

Kenko-Spacers-1.jpg

It's always difficult to put them evenly to the edge so I made them extend past it.

 

Kenko-Spacers-2.jpg

Then gentily laid it face down and trimed them with a X-acto knife.

Having a concaved face makes doing it safe.

 

The only thing good that came from this was the technique, since it's performnce did not appear to change one bit.

I'm not sure it would do any good to try it again using 2 layers.

 

Yesterday I also recieved an objective from an always good and tested Manon so it will likely replace it. 

Since the Kenko is a 710mm, and the Manon a 700mm, I'm fairly confident it will fit without any focusing problems. 


Edited by Kasmos, 01 August 2020 - 11:42 PM.

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#80 Pete W

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Posted 01 August 2020 - 04:37 PM

I'll ask, but this might be a stupid question.  How important is it that the newton's rings are perfectly centered in a f/15 scope?    If the rings aren't centered will that cause astigmatism or other optical aberrations?  Slow scopes tend to be much more forgiving if collimation isn't spot on...does this also apply to not-perfectly-centered newton's rings?

 

Just curious.

 

Pete


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#81 DAVIDG

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Posted 01 August 2020 - 04:49 PM

Dave.....thanks for the explanation.  I now know why and how.

 

One more question.... can any conclusions be drawn about the quality of a lens by the  circular sharpness and quantity of their interference rings?    I have some lens where the interference rings have nebulous edges and the rings themselves are wide as compared to others where the rings are tack sharp  like drawn with a very fine ink pen.  For example one of my lens has over 30 rings tightly spaced and finely drawn and another > 14 rings widely spaced with mushy edges.  Is this just a function of the amount of air space?

 

Don

 The only  conclusion that one can make when the spacers are in  place is if the rings are perfectly circular and centered that the  air gap between the surface is uniform. The smaller the air gap the wider and fewer the rings will be. So seeing rings vs not and the number of them is not a direct indication of the quality of the lens. You have to  test the complete assembly  since any radius could be off or a thickness or any surface(s) could have  errors in their figures

 

              - Dave 


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#82 DAVIDG

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Posted 01 August 2020 - 05:12 PM

I'll ask, but this might be a stupid question.  How important is it that the newton's rings are perfectly centered in a f/15 scope?    If the rings aren't centered will that cause astigmatism or other optical aberrations?  Slow scopes tend to be much more forgiving if collimation isn't spot on...does this also apply to not-perfectly-centered newton's rings?

 

Just curious.

 

Pete

It causes on axis astigmatism and lateral color. Lateral color is when a star turns into short spectrum.  You see this when you observer a star close to the horizon and atmospheric dispersion smears it out into  short red and blue spectrum.  In a lens the same thing happens because now the elements are wedged in respect to each other. 

  In a slow lens the elements are weaker then faster a  lens So you can think of them as weaker prisms so for the same amount of wedge error you would get less lateral color in a slow F-ratio lens but if you want the very best performance especially at high power I would eliminate it if I could.

 

                - Dave 


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#83 Pete W

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Posted 01 August 2020 - 07:31 PM

Thanks Dave.   Great explanation.

 

pete


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#84 PawPaw

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Posted 01 August 2020 - 09:10 PM

I am not advocating for another registry.....being a retired IT guy I have an aversion for databases however they are definitely helpful.  I do see advantages for collectors looking for solid information on spacing specs of vintage objectives.   Maybe we can use this thread or a similar thread as a repository for anyone  needing to research the spacing requirements of a Towa, Edmund, Vixen, Goto, Carton, Unitron etc etc etc.  Without a document like we found for Edmund it may come down to the law of averages of collectors measuring spacers and documenting what they find.  Just a thought.

 

Thanks to everyone on this thread and Dave especially for answering questions at least for me that have never been clear.

 

Don



#85 PawPaw

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Posted 02 August 2020 - 08:04 AM

I will start the document here and see where it goes.  Please add your findings and any comments

 

Vintage Refractor len spacing sizes:

 

Manufacturer                                 Lens size               spacing                 material           Lens type                              Comments

 

Edmund  imported                             4"                          .005                      foil                 Fraunhofer                   

Edmund imported                              3"                          .005                      foil                 Fraunhofer

Edmund (Rank design)                       4"                          .004                      foil                 Fraunhofer

Edmund (Rank design)                       3"                          .004                      foil                 Fraunhofer

Unitron                                             4"                          .003                      foil                 Fraunhofer

Unitron                                             2.4"                       .003                      foil                 Fraunhofer


Edited by PawPaw, 02 August 2020 - 08:27 AM.

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#86 DIYDarren

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Posted 03 August 2020 - 11:45 AM

Good afternoon everyone-

 

Not trying to hijack this thread, but I find it interesting and I have a relevant situation. Tasco 152x scope- very old Royal Astro specimen- 60mm x 910mm. When I got it, the objective was all whacked out- reversed crown AND reversed entire objective. I have since corrected those issues and performance is "much better."

 

I finally just got a CFL lamp, and was able to inspect the Newton Rings this morning. They are very circular and there are LOTS of them - I could probably count 50 rings if I wanted to (but don't think this has any bearing on anything), but alas they are NOT centered. I tried to take a picture but my phone just couldn't get a good pic.

 

So, is this purely a function of tilt of the Crown relative to the Flint? Or should I also consider the rotation of the two relative to each other? There are no orientation marks on the lenses, and since someone cleaned and replaced incorrectly, i'm sure whatever factory orientation was already jumbled. (If there were orientation marks, they may have been cleaned off by former cleaner).

 

I guess question is:

  • 1. How do I consider rotational orientation in the process? Or is this even something to consider at this stage? Is there a bench test for this? (I read some articles a while back about a trial-and-error approach, but can't find these now).
  • 2. To "center" the newton rings, I should consider placement of the foil spacers, moving one or more in from the edges to adjust the tilt?   (This was a question I had on another thread- I guess the gap between crown and flint is not perfectly uniform, meaning that placement of spacers in from edges may affect the tilt.?)

 

Thank you!!!

 

Darren



#87 PawPaw

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Posted 04 August 2020 - 07:22 AM

Darren,

What is the model # of your Tasco?  It sounds like your objective has foil spacers?  If so they have probably degraded if you cannot get the rings centered.  I find making new ones to exactly the same thickness is the best option for me in these cases.  You have several options on making or purchasing the foil spacers.....I personally make mine but it requires a micrometer to make sure the thickness is uniform and correct.    I do not know what the Tasco specs was for air space on your lens but maybe one of our Tasco experts will chime in.

 

Cheers

 

Don



#88 DIYDarren

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Posted 04 August 2020 - 08:58 AM

Hi Don, 

Thanks- appreciate the thoughts, and first place is to look at the spacers themselves. I have not "messed around" yet. Yes they are the old foil spacers. Its an un-model-numbered early scope- the "152x" as it is referred to here. Bomber Bob and those guys know about them. 

 

I do not have any micrometers, but maybe will pick some up. Meanwhile I will take the lenses out of the cell and look to see how much movement it takes to get the rings centered, and maybe i will notice something amiss with one or more of the spacers. 

 

Thanks- 

Darren



#89 Kasmos

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Posted Today, 03:17 AM

Monday night I did more experimenting on the 9TE. I removed the soda can spacers and replaced them with spacers made from two layers of .003" adhesive foil, making it .006" which looked thinner than them. It was late and a bit cloudy but I wanted to test it out.

 

The clouds kept rolling by interfering with Jupiter and Saturn, but as the scope settled in I did get a couple of pretty good views of Jupiter. It seemed like the CA wasn't too bad once properly focused, but if it was a little off, it really showed. Considering how bad the conditions were, Saturn was kind of iffy. It's smaller size seemed to make it more affected by color and the clouds just wouldn't let up.

 

The eastern part of the sky was clearer, but more turbulent. I could see some slight shading on Mars but the turbulence and flickering made it frustrating to look at. While it was fairly wavy, the Moon looked good. I'd viewed it before with this scope and wonder why it looked so color free? I'd thought it's brightness would enhance the CA, especially on it's rim, but it really didn't.

 

Overall it was hard to really know if changing the spacers had done much. I figured trying it during the day on my normal target would tell me more.

 

On Tuesday I had other things to do so only did some quick solo testing. While it still probably has more CA than the Mayflower, I noticed that when focused on a partcular detail on the power pole, the objects in the background, just slightly out of focus were noticeably more color free than they had been with the previous spacers or the original ring spacer.

 

This made me wonder should I go down to just a .003" spacer? I didn't have time to do it,  but I wondered, what would happen if I used a spacer thicker than the original ring? What would it show, was I really going in the right dirrection? The quickest and easiest thing to do was to leave the foil ones in and place the ring on top of it. The result, lots of yellow and purple fringing!

 

So, as much as I hate to do it again, I suppose I have to try it one last time using one layer of .003" foil.


Edited by Kasmos, Today, 03:21 AM.



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