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Unitron/Polarex 114 and the Ghost Star...

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#1 LukaszLu

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Posted 11 April 2021 - 06:09 AM

I recently bragged about the completion of the Polarex 114 renovation: https://www.cloudyni...n-photo-report/ Since then, I have had the opportunity to direct the instrument only to the stars and Mars, I am still waiting for an opportunity to look at the Moon, which in my opinion is one of the most important tests for this type of refractor.

 

However, during my rehearsals on the stars, I observed a strange phenomenon. Phantom reflection appears near bright stars. It is much fainter in brightness, appears pinpoint at low magnifications, but turns out to be somewhat out of focus (resembles a faint globular cluster or planetary nebula) at higher magnifications. The greater the magnification, the greater the star's distance from the reflection. The reflection does not move with respect to the star due to the movement of the  head or OTA, but changes the angular position in the field of view when the lens cell is rotated, and therefore clearly comes from the lens.

 

Has anyone encountered such a phenomenon? As I wrote in the previous thread, the lens went to a service technician who undertook to clean the heavy fungus infection on the optics. On this occasion, the spacers were removed and reinstalled. The lens has been assembled correctly in accordance with the old spacers marks and the lens is pointing at the sky with the Crown (the element with the narrower side edge).

 

The only thing I'm not sure about is the thickness of the spacers, which may have suffered a bit during the entire operation. At this point, I have to tighten the collimation screws unevenly to avoid a slight coma visible on the diffraction rings with even pressure on the screws, which I think may indicate a minimal misalignment of the lenses or uneven thickness of the spacers. After adjusting the screws pressure, the uneven brightness of the diffraction rings and the rings themselves almost disappear, while a distinct Airy disk remains visible.

 

The image during terrestrial observations seems to be perfect in terms of precision and sharpness.



#2 Bomber Bob

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Posted 11 April 2021 - 07:29 AM

How are the Newton Rings?



#3 starman876

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Posted 11 April 2021 - 07:41 AM

I second JW's question.



#4 LukaszLu

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Posted 11 April 2021 - 07:50 AM

There are no... Note even the slightes trace.

 

And this is how the picture looks like:

 

Attached Thumbnails

  • unitron-ghost-star.png

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#5 DAVIDG

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Posted 11 April 2021 - 09:17 AM

 The only way to really tell what is going on with the lens is to bench test it. Most assume that the lens was well figured but unfortunately what I have seen over 40 years of making and testing optics that has not been the  case. I have tested a handful of 60mm Unitron lens and only 3 were figured correctly, the others had major zones.

   So I recommend that you test the lens using double pass autocollimation. As for the lens spacers, if the thickness is between 0.003" to 0.005" it will make little difference in a lens of this size. What is more important is that the air spacing is uniform in thickness and the best way to determine that and adjust it is by viewing the interference rings between the elements by shining  monochrome light on the lens and observing the fringes.. Then you can adjust the screws to make them round and perfectly centered. If you can not  get them round and centered that means that one or both of the inner surfaces are not figured correctly. If you can not see interference fringes that indicated the lens is not assembled correctly and most likely the front crown element is flipped or the element is wedged very badly and the spacing is now to large to see them.  

  Observing a star and adjusting the pressure on the screw is not a good way to adjust the lens because if you have misalignment in the focuser or the eyepiece your adjusting to remove that and not the correctly adjusting the spacing. 

   Here are pictures of two of my 60mm Unitron lens that I have adjusted correctly. 

 

              - Dave 

unitron lens1.jpg

unitron lens 2.jpg


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#6 Terra Nova

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Posted 11 April 2021 - 09:37 AM

I just looked at your original thread and the original red paint on the fixing screws on the objective cell appears unmarred indicating it had never been taken apart. You had to disassemble it in order to clean and de-fungus the glass. Are you absolutely sure you put it back in exactly the same order and orientation of the two elements and that neither one element or both are flipped?


Edited by Terra Nova, 11 April 2021 - 09:39 AM.


#7 LukaszLu

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Posted 11 April 2021 - 09:52 AM

I've double-checked everything, the lens are properly assembled - the crown on the outside and the more convex side of the crown, containing the traces of the spacers - inside. I noticed that instead of a round rings, in the light of a fluorescent lamp, straight, parallel lines can be seen across the entire surface of the lens. They disappear depending on which collimation screw I tighten.


Edited by LukaszLu, 11 April 2021 - 09:56 AM.


#8 Terra Nova

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Posted 11 April 2021 - 09:53 AM

I've double-checked everything, the lens are properly assembled - the crown on the outside and the more convex side of the crown, containing the traces of the spacers - inside. I noticed that instead of a round rings, in the light of a fluorescent lamp, straight, parallel lines can be seen across the entire surface of the lens. They disappear depending on which collimation screw I tighten.

Interesting. Can you get a picture of that?



#9 LukaszLu

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Posted 11 April 2021 - 09:59 AM

I'll do my best, for now I can show the "inside" of the lens - these are the surfaces that make up the air gap. There are clear, original traces of a spacer on the Crown (on the right).

 

These parallel lines are a pattern on a cotton ball, not the phenomenon described :-)

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  • 20210411_163716.jpg

Edited by LukaszLu, 11 April 2021 - 10:00 AM.

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#10 davidc135

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Posted 11 April 2021 - 10:12 AM

The parallel fringes suggest some wedge in the airspace.

 

The ghost looks like there may be double reflections from the two inner lens surfaces. The offset ghost image supports the wedge idea. When you say collimation screws, do they tighten the front retaining ring or adjust the tilt of the objective as a whole? I'm guessing the former and, if so, would they affect the position of the ghost? It sounds like the spacer thicknesses aren't even as you suggest.

 

With no AR coatings it's brightness would be around 1/25th squared of the real star magnitude, say 6-7 mags dimmer. With damaged coatings maybe 2 mags even dimmer.

 

David


Edited by davidc135, 11 April 2021 - 10:31 AM.

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#11 LukaszLu

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Posted 11 April 2021 - 10:23 AM

That's what I was able to photograph. Look at the bottom-right. The lines thicken or thinner depending on the angle of rotation of the lens cell, and in certain settings they begin to curve, as can be seen in the photos.

Attached Thumbnails

  • linie1.jpg
  • linie2.jpg

Edited by LukaszLu, 11 April 2021 - 10:24 AM.

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#12 DAVIDG

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Posted 11 April 2021 - 10:58 AM

 You have a very large amount of fringes  and the fact that there are so many and straight indicates the spacing between the elements is very large ie too large. With large spacing the sensitivity goes way down  As the spacing decrease the fringes will decrease in number  but the sensitivity will go up and their shape will indicate correct uniformity of the air gap.

 The spacers in your picture show deterioration.  The spacer have most likely swelled and  the glue residue on the elements has increasing their thickness The large air space would cause the ghosting your seeing.

   You need to replace the old spacers with new ones with a thickness between 0.003 and 0.005" . They need to be the same width and length  and placed the same distance from the edge of the element. They also need to be placed at 120° centers and next to the screws that hold the retainer ring in place. 

   Once you remove the older spacers and all the glue, place the elements in contact. Now you should see interference rings like in my pictures and with the new spacers in place and correctly positioned you should be able to adjust the pressure on  the screws so the rings are round and centered. Also the pressure should be very light so if you if gently shake the lens side to side you can feel it move freely in the cell.

 

 

 

                     - Dave 


Edited by DAVIDG, 11 April 2021 - 11:08 AM.

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#13 Terra Nova

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Posted 11 April 2021 - 11:07 AM

The parallel fringes suggest some wedge in the airspace.

 

The ghost looks like there may be double reflections from the two inner lens surfaces. The offset ghost image supports the wedge idea. When you say collimation screws, do they tighten the front retaining ring or adjust the tilt of the objective as a whole? I'm guessing the former and, if so, would they affect the position of the ghost? It sounds like the spacer thicknesses aren't even as you suggest.

 

David

They aren’t collimation screws. They are just retaining or ‘fixing’ screws to hold the elements in place and should not be overtightened. 


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#14 LukaszLu

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Posted 11 April 2021 - 11:12 AM

I did the following experiment in the opposite direction: I increased the thickness of the spacers by adding a layer of aluminum foil or a simple sheet of paper. With the foil, the stripes thickened but did not disappear, regardless of where I added thickness (on 1, 2 and 3 spacers). The lines disappeared after using a sheet of paper, which I also added on 1, 2 and 3 spacers in various layouts.

 

In neither of these layouts I was able to intensify the fringe effect or observe circular rings.



#15 LukaszLu

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Posted 11 April 2021 - 11:13 AM

They aren’t collimation screws. They are just retaining or ‘fixing’ screws to hold the elements in place and should not be overtightened. 

You are probably right, if they were used for collimation, it would be additional screws. That's why I use them very delicately.



#16 DAVIDG

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Posted 11 April 2021 - 11:31 AM

I did the following experiment in the opposite direction: I increased the thickness of the spacers by adding a layer of aluminum foil or a simple sheet of paper. With the foil, the stripes thickened but did not disappear, regardless of where I added thickness (on 1, 2 and 3 spacers). The lines disappeared after using a sheet of paper, which I also added on 1, 2 and 3 spacers in various layouts.

 

In neither of these layouts I was able to intensify the fringe effect or observe circular rings.

 The interference happens between 2 surfaces take place at  integer values of the wavelength of the light you test at. The spacing between the fringes is always 1/2 a wave of the frequency of the wavelength. So if you have large spacing, you will get many interference fringes because the spacing between is many integer spacing thick.  As you decrease the spacing you have fewer number of integer spacing  and the space between the fringes increases  

  So by increasing the spacing you increased the number of fringes and caused them to disappear. You need to decrease the spacing to the correct thickness of 0.003" to 0.005" for this lens and the fringes will now be fewer and sensitivity will greatly increase in determining the uniformity of the spacing. 

 

                - Dave 


Edited by DAVIDG, 11 April 2021 - 11:32 AM.

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#17 DAVIDG

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Posted 11 April 2021 - 11:53 AM

 Here are two pictures I just took of  fringes between a piece of glass that is not very optically flat and a Master optical flat. The  two pieces are just placed on top of each other and there is  a gap between the surfaces just from the air between them.

   In the first picture you see many fringes because the gap is many integer thickness of the wavelength of light I'm testing at. Note while the fringes are not straight they are somewhat the same in shape

    In the second picture all I did was to push down on the  top piece and that squeezed out some of  the air. Now the gap is less and there is fewer integer spacing of wavelength of light between them. Now there are fewer fringes and they are farther apart. Note they now show better the distort of the surface of the glass.

   This is what is happening in your lens. The air spacing is too large to correctly show the fringes and that large gap is also causing the ghosting your seeing. It is also affecting the optical performance of your lens. The paper and foil spacer  you tried are much too thick to allow the fringes to be seen. 

 

                  - Dave

fringe large spacing.jpg

 

fringer small spacing.jpg


Edited by DAVIDG, 11 April 2021 - 11:57 AM.


#18 starman876

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Posted 11 April 2021 - 12:12 PM

are there thee spacers?  In the pictures I only can see two.



#19 clamchip

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Posted 11 April 2021 - 12:13 PM

The last time I tested a lens with a unknown spacer thickness I just tried

various paper in the house.

A micrometer is perfect for this.

Remember 0.003" - 0.005" inches not a metric as Dave mentioned should

be about right for your 60mm lens.

Robert

 

IMG_9868.jpg


Edited by clamchip, 11 April 2021 - 12:14 PM.

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#20 Terra Nova

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Posted 11 April 2021 - 12:22 PM

3-M makes (or used to make) an all metal 3 mil thickness aluminum duct tape.  



#21 LukaszLu

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Posted 11 April 2021 - 12:54 PM

I have removed the old spacers and am trying to use foil or paper. The fringes seem to be gone, but so far no ring traces ...

 

@starman876: yes, there have been 3 original spacers but they seem a little to stiff when removed - perhaps new glue has penetrated them and has actually increased in thickness?


Edited by LukaszLu, 11 April 2021 - 01:41 PM.


#22 DAVIDG

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Posted 11 April 2021 - 02:12 PM

 Use no spacers and place the elements in direct contact.  The surface must be extremely clean. any glues residue will cause them not be seen. 

    If you still can't see them it is  your light source and it's  not monochrome enough.  Try filtering the light source. You can just look through a green filter and you don't have to filter the light source. A number 58 eyepiece filter will work well and also a yellow or red one.

 

                  - Dave 



#23 LukaszLu

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Posted 11 April 2021 - 02:56 PM

Dave, unfortunately, I am not able to clean the glass perfectly from the traces of the original spacers, I know that the service technician who previously cleaned the lenses did not manage to do so either. But I tried to place lenses separated by only a single layer of baking foil - it's very thin, so thin that when I tried to apply it to another lens, the Newton's rings responded to each touch, showing that the lenses were practically touching each other. And nothing - no sign of the rings ...

 

I recently replaced the spacers on a Carton 60/900. Unfortunately, I do not have a precise tool for measuring the thickness of the foil, but then I experimented with the version that allowed me to get the correct Newton rings and an excellent image. I repeated the same. The longitudinal stripes have disappeared, but Newton's rings have not appeared. The artificial star test still shows the same secondary reflection, rotating as the lens rotates.

 

As far as I can measure the thickness of the original spacers, they are approximately 0.1 mm (0.04 '') thick. It is worth noting that this is 10 times greater than 0.003-0.005 '', which is always the case when discussing the air gap.

 

Out of curiosity, I checked the thickness of the plastic rings separating the lenses in the 60/900 Towa refractor. Turns out they are over 0.5mm (0.2 '') thick! This is over 50 times more than the value recommended in the discussion. Does this value of 0.003-0.005 result only from the lens diameter, or are there any other factors influencing it?



#24 davidc135

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Posted 11 April 2021 - 04:08 PM

If there is a residual contamination under the original spacer positions change to new spacer positions if the glass surfaces are clean there, keeping the 120 deg separation of course.

 

If you reproduce the interference fringes as straightish lines try and follow one line across the objective. If a straight-edge connects one end of a single 'fringe' to the other then the number of fringes it cuts cross, divided by two, will be the difference in curvature of the two radii in waves of light. It was hard to see in post#11but they looked very close so probably not much point except for fun.

 

Another approach would be to correlate the ghost star angle and separation with the spacer positions and microscopically shim one or two spacers until the ghost and it's parent star coincide. Maybe new, clean glass positions will do the trick.

 

David



#25 LukaszLu

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Posted 11 April 2021 - 04:23 PM

Dear friends, on my next attempt to replace the spacers, I just dropped the lens and chipped off the entire side. This is how my project of renovating this beautiful telescope ended. Thank you for all your help and time.




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