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Unitron lens adjustment

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#1 DAVIDG

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Posted 08 January 2021 - 01:32 PM

 I have couple of 60mm Unitron 114  refractors on an  Alt/az mounts. I really like this model and consider it one  of the best 60mm refractors made. The mechanics of the mount are wonderful and the scope just has that "precision" feel when you use it. I use mine fairly often for  white light solar viewing, observing Venus in the daytime and just for quick look around since it is outside in minutes and back in the house in minutes. One of the ones I own unfortunately   has an objective with a fair amount of astigmatism that would require the lens to be refigured. I have plans of one day refiguring but  in the mean time these Unitron  60mm objectives come up for sale on fairly regular bases and one showed up a couple of days ago being sold by a regular member of our Classic group.

   So I purchased it with the hopes that it might have a better figure. It was advertised that one of the screws that hold the retainer ring in place was missing and  I can could see the red paint on the two other screws had been broken so someone had been into it in the past. Not a good sign but who knows what you'll find until you get up on the test bench.

   So the lens showed up yesterday in the mail. It was well  packed and it good shape. So down into the basement to check it out using double pass autocollimation and see what the figure was like. It was BAD, the Ronchi lines were all distorted ! So not to panic since one of the retainer screws was missing so the air gap between the elements was nowhere near uniform. So I removed the lens from the cell and used my monochrome light source to view the interference rings between the element. The good news was they were visible, the bad news they were arcs and not rings that were centered. So the arcs meant that one of the three spacers was different in height vs the other two. The spacers are metal foil and which some type of backing. The foil looks like it was corroded in places. So I pressed  on the spacer opposite the curve in the arc of the interference pattern and  the arc moved inward and turned into rings. Good, that told me that, that spacer was too thick. I carefully shaver it down in thickness by scraping it with a razor blade and then checking the results.

      Now I had nice round rings that were centered. So I reinstalled the lens back in the cell but I only had two screws to hold the retainer in place. I carefully tighten the two screws and was able to keep the interference rings round and centered but it didn't take much to throw them off. I re-centered them and carefully took the lens back down to the basement and back up on the test bench. Good news, nice straight Ronchi bands, the lens is well figured ! 

   So now out to the garage to see if I could find a replacement for the missing screw that hold the retainer ring. I would need all three to keep even pressure on the lens so the  air gap was uniform . The screw  looks to be metric but didn't match a kit I have. Luckily I found one that matched in my collection random screws.  Now I was able to install the retainer ring correctly. Before I did that I placed three taps of Mylar tape on the retainer at 120° centers so the tape would be the points of contacted and clearly defined. The lens was placed back in the cell so the spacers were next to each bolt and the retainer placed so  the taps of tape where over the spacers. Now while observing the interference rings under monochrome light I adjusted the pressure of each screw so the rings were round and centered. Once that was done back down to basement to test the lens again. Straight Ronchi bands. The result is now a correctly adjusted lens that will give me excellent images and it will  replace the bad one in my other Unitron 114.

   Here are some pictures showing the lens fully assembled in the cell. Note that the air gap spacers are next to the bolts that hold the retainer ring. An image of the interference rings, the rings are perfectly round when view directly from above but show as slightly oval in the picture because of the camera angle. An image of the straight Ronchi bands when the lens was correctly adjusted and tested via double pass autocollimation. 

 

                            Happy 2021 !

                             - Dave 

unitron 114 lens.jpg

unitron 114 interference rings.jpg

unitron 114 DPAC test.jpg

 

 

 


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#2 CharlieB

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Posted 08 January 2021 - 01:45 PM

Glad it turned out so nicely!  Who better to get that problem fixed.

 

Charlie



#3 starman876

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Posted 08 January 2021 - 02:16 PM

Nice lens.  It is always amazing how simple some of these fixes are with such excellent results.



#4 pbealo

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Posted 08 January 2021 - 03:18 PM

What's that ugly mass protruding from the top of the lens in the ronchigram?!?!



#5 DAVIDG

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Posted 08 January 2021 - 03:31 PM

What's that ugly mass protruding from the top of the lens in the ronchigram?!?!

 That would be the reflection of the carbon based life form  in the optical flat performing the test. lol.gif I keep telling  it to stay out of the picture because  the heat it generates can distort the air by  changing the refractive index and distort the image, but it doesn't listen ! The only thing I have been able to get it to do is hold it's breath while the image is taken. It is smart enough not to hold it too long and pass out but not much smarter then that. 

 

                       - Dave 


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#6 Kasmos

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Posted 08 January 2021 - 03:43 PM

Great info and very timely!

 

Last night I rediscovered this letter from Nihon Seiko on the Unitron History site that mentions some related issues.

It addresses 4 things of interest. 

 

1. I had read some old posts with some folks questioning if the Red Paint was original.

(since so many Unitrons have it, I've always thought it was put there by the maker to keep them adjusted properly)

 

2. NS states that the lenses should be marked to maintain their best adjusted position. Right or wrong, this has also been a topic of debate (and not just for Unitrons).

 

3. The screws should be placed over each spacer.

 

4. The last paragraph explaining how to adjust the lenses is really interesting!!

 

NS-Lens-adj-letter.jpg

This letter and the accompanying drawing can be found under documents. 

https://www.unitronhistory.com/documentation/documents-2/

 

For the record, like many older samples, the screws on my circa '57-'58 114 didn't have the Red Paint. I'm sure mine had been tampered with because they were very tight plus had the lens spacers arranged between the screws. When resetting it, I noticed the screws needed to be fairly loose in order to not pinch the lens. IMO, that's a good reason to have paint or another sealer to keep them in place. 

 

 


Edited by Kasmos, 08 January 2021 - 03:48 PM.

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#7 CHASLX200

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Posted 08 January 2021 - 07:10 PM

Seems the red came along later on around 1970. I don't remember seeing it on the 50's or early 60's made scopes. My 1960 M-152 did not have the red on the cell screws.



#8 DAVIDG

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Posted 09 January 2021 - 10:28 AM

 What I have seen on the Unitron lens that I have worked on and many from other manufactures is that they have  had the retain rings screwed down way to tight. This has  warped the elements and hurt the optical performance of the telescope. I find this puzzling since those that make optics should easily understand how a lens should be mounted in it cell. They should also have  the tools to set them up correctly which just requires a monochrome or semi-monochrome light source. 

   I find the letter from Unitron interesting in that it does not go into any detail on how to adjust the lens. Just  a vague statement to adjust the screws. If one would observe  a terrestrial object  that would not provide the information needed to get the air gap uniform. Observing a star could work  but it would be time consuming and most likely result in what I have seen with the retainer ring way too tight and the elements still out of proper adjustment. 

   My conclusion is that is there was an  information disconnect between those that made the optics and ones that assembled them in the cells. The good news is that with $2 CFL bulb one can adjust the lens correctly and enjoy a better image.

 

              Happy 2021 !

               - Dave 


Edited by DAVIDG, 09 January 2021 - 12:17 PM.

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#9 starman876

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Posted 10 January 2021 - 10:37 AM

As always Dave shows us how simple these fixes can be.  Thanks to Dave's wonderful post and help I got very interested in DPAC and built my set up.  Was first testing in blue because that is what i had that was handy.  Blue is a much more stringent test of an optic as most optics are tested in green.  However  the ones I tested in blue were wonderful lenses.  I now test mostly in green because most lenses are tested that way.   I wonder if commercial lens manufacturers test in all three colors instead of only green?  I notice a number of people have set up there DPAC rig so they can test in red, blue and green.



#10 Terra Nova

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Posted 10 January 2021 - 12:07 PM

 What I have seen on the Unitron lens that I have worked on and many from other manufactures is that they have  had the retain rings screwed down way to tight. This has  warped the elements and hurt the optical performance of the telescope. I find this puzzling since those that make optics should easily understand how a lens should be mounted in it cell. They should also have  the tools to set them up correctly which just requires a monochrome or semi-monochrome light source. 

   I find the letter from Unitron interesting in that it does not go into any detail on how to adjust the lens. Just  a vague statement to adjust the screws. If one would observe  a terrestrial object  that would not provide the information needed to get the air gap uniform. Observing a star could work  but it would be time consuming and most likely result in what I have seen with the retainer ring way too tight and the elements still out of proper adjustment. 

   My conclusion is that is there was an  information disconnect between those that made the optics and ones that assembled them in the cells. The good news is that with $2 CFL bulb one can adjust the lens correctly and enjoy a better image.

 

              Happy 2021 !

               - Dave 

It does mention to pay close attention to an attached diagram. Perhaps said diagram provided additional info?



#11 Terra Nova

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Posted 10 January 2021 - 12:12 PM

As always Dave shows us how simple these fixes can be.  Thanks to Dave's wonderful post and help I got very interested in DPAC and built my set up.  Was first testing in blue because that is what i had that was handy.  Blue is a much more stringent test of an optic as most optics are tested in green.  However  the ones I tested in blue were wonderful lenses.  I now test mostly in green because most lenses are tested that way.   I wonder if commercial lens manufacturers test in all three colors instead of only green?  I notice a number of people have set up there DPAC rig so they can test in red, blue and green.

Green is much closer to the center of convergence of the objective’s refracted rays and provides a better signature of the overall correction of the lens’s performance throughout the visible spectrum of electromagnetic radiation.

Attached Thumbnails

  • F4E041AF-0CE3-45CB-A712-F8C8294EE049.jpeg


#12 starman876

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Posted 10 January 2021 - 12:49 PM

Green is much closer to the center of convergence of the objective’s refracted rays and provides a better signature of the overall correction of the lens’s performance throughout the visible spectrum of electromagnetic radiation.

However, we  see all the colors at the eyepiece.  Therefore.  Have the blue and red show straight lines in DPAC will render a much better view at the eyepiece.  The diagram you show is for an achromat. An APO will bring the centers of the blue green and red much closer together.  



#13 DAVIDG

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Posted 10 January 2021 - 12:52 PM

 The reason why you test in green is because of spherochromatism.  Spherochromatism is spherical aberration as a function of wavelengths.  In a  pure reflective system like a Newtonian it has none but a lens has a  range of wavelengths were it is diffraction limited outside that range it has spherical aberration. Most lenses for visual worked are designed to have the best spherical correction in the green and usually at 550nm. At other wavelengths like red and blue they will show spherical aberration.  In deep blue it can easily be 1/2 wave or worse. What many don't understand about achromats and APO is they have both chromatic and spherical aberration even when well designed. You can't  fully correct them. You can get close but not perfect. In a  pure reflective system you can get perfect  correction over the full visual spectral range. 

If a lens tests well in green then it means that the other wavelengths they will have the designed amount of spherical aberration. If it test poorly in green then they will test worse in Red and Blue and have more spherical then the design calls for and also more chromatic aberration as well.  Chromatic aberration and spherical aberration go hand in hand. If one is off the other is off as well. 

 

              - Dave 


Edited by DAVIDG, 10 January 2021 - 03:42 PM.

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#14 DAVIDG

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Posted 10 January 2021 - 01:17 PM

However, we  see all the colors at the eyepiece.  Therefore.  Have the blue and red show straight lines in DPAC will render a much better view at the eyepiece.  The diagram you show is for an achromat. An APO will bring the centers of the blue green and red much closer together.  

 If you see  straight lines in blue or red, that means either the lens was designed for that wavelength and the other wavelengths will show spherical aberration or if it was designed for green and the lines are straight in blue or red the lens is poorly corrected.

   Here is fan plot that may help explain what is going on. It is for an achromat. Note the plots on the left, the green line is on the X axis and almost perfectly straight. This shows no spherical aberration. Now note the red and blues lines, they are at an angle and they have slight amount of curve to them. The fact they are on an  angle show they don't come to focus were green does but since they are on top of each other that red and blue focus at the almost the same point  but not were green does. Also the red and blue lines are slightly curved this because they have spherical aberration. 

   Now look at upper right graph labelled Longitudinal spherical Aberr.  Again you see that the green line falls  exactly on the vertical axis and is straight so no  spherical aberration. The red and blue fall to the right so they focus behind green, that is a measure of chromatic aberration but again they are not perfectly straight which shows they have spherical aberration.   What this all shows is  a well designed achromatic has both residue color and spherical aberration that is a  function of wavelength which is called spherochromatism.

    You test in green light since that is were the lens is designed to have none. If it does then all the other wavelengths will be off in both color correction and spherical aberration. 

 

                   - Dave 

7f15fanplot.jpg



#15 starman876

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Posted 10 January 2021 - 01:48 PM

always appreciate the education Dave.  What you say makes sense. However, I thought that a well designed triplet was to cure some of these problems so everything focuses at the same point?



#16 DAVIDG

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Posted 10 January 2021 - 04:13 PM

always appreciate the education Dave.  What you say makes sense. However, I thought that a well designed triplet was to cure some of these problems so everything focuses at the same point?

 The definition of an Achromat is that it brings two wavelengths to the same focal point,  and in most cases that is red and blue but not at the same focal point as green. An APO means it brings three or more to the same focal point but again not at green.  So you have a range of focus and range of color correction. Your eye  naturally  focus at the average of all those. 

   Here is  a twist on a typical achromat, the Clarks designed some of their lens so Green and Red focused closer then blue. They did this because they believed the planets natural colors were more toward the range and mixture of colors between  red and green.

Here is a plot of the color correction of one of their lenses.  You can also have APO photographic lens that will test terrible for visual use because they were designed for the spectral response and resolution  of the film.  You don't need diffraction limited optics when the film or sensor can't resolve that high. The resolution only needs to be better then the resolution of the sensor. The bottom line when testing a lens you need to have a idea of what it was designed for.

 

              

 

                   - Dave 

 

clarkcolorcorrection.jpg



#17 pancho61

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Posted 10 January 2021 - 05:06 PM

Hi David,

 

Thanks for the informations, what is your lamp type and the wavelenght?

 

It seems in the past sodium lamp was mostly use by japan manufacturer for setting the doublet so my question is if you use green lamp with different wavelenght you should modify the specifications from factory by shifting the chromatic settings. Does it is right?


Edited by pancho61, 10 January 2021 - 05:06 PM.


#18 pancho61

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Posted 10 January 2021 - 05:08 PM

A colleague open a thread about using lamp for fixing spacing problem ( sorry it is in french)

 

https://www.webastro...doublet-bk7-f2/



#19 DAVIDG

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Posted 10 January 2021 - 06:48 PM

Hi David,

 

Thanks for the informations, what is your lamp type and the wavelenght?

 

It seems in the past sodium lamp was mostly use by japan manufacturer for setting the doublet so my question is if you use green lamp with different wavelenght you should modify the specifications from factory by shifting the chromatic settings. Does it is right?

  If you are referring to the source I use to  adjust the air gap to be uniforn it  uses a wavelength  at 546nm but it doesn't make any difference what the wavelength is for setting the air gap. The spacers define that. The shape of the interface rings tell you if the air gap is uniform. The pattern would be the same as either correct  with a uniform pattern of round and centered rings or distorted,  no matter if I used a sodium source in the yellow or one in the green. 

   

    Here is  a picture of the interference rings of  a well centered lens using a my desk lamp which uses a  cheap  CFL bulb. A CFL bulb is a gas discharge type so it has many monochrome emission lines to make  white light and these are separated enough to make semi monochrome  light  and show an  interference pattern. The pattern would be the same if I used my mercury source or a sodium source. It is the shape that counts.  

 

                 - Dave 

 

lens interference pattern CFL bulb.jpg


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