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Reduced aperture for pinched optics

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#1 Yourjones

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Posted 28 January 2021 - 08:37 AM

Hi

 

I have the same refractor as the ES 80ED APO f/6. This scope is known to have conspicuous diffraction spikes caused by pinched optics:

 

Castor (Alpha Geminorum)

p.jpg

 

To mitigate the problem, I cut out a card ring to reduce my aperture from 80mm to 74mm. 

 

Screenshot 2021-01-28 at 21.21.19.jpg

Screenshot 2021-01-28 at 21.21.04.jpg

 

The resulting image is indeed better, if not absolutely good:

r.jpg

 

Spikes are still seen, but I can live with that. The reduced aperture turns my scope to f/7. Exposure should increase by about 17%, but looks like it's inevitable.

 

Hopefully this is a reasonable way out.

 

Lu


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#2 junomike

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Posted 28 January 2021 - 08:39 AM

Sad one needs to do that.  The stop-down changes to F6.5.


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#3 mikerepp

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Posted 28 January 2021 - 08:44 AM

You could send it back to ES and have them correct the pinched optics.   My AR152 had pinched optics when it was cold.  Sent to ES right after I received it, back in a couple of weeks and it was perfect.   They did a great job, never an issue since.  Just wish I had more opportunities to use it.  But with a recent back injury it wont be for a while.frown.gif


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#4 KBHornblower

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Posted 28 January 2021 - 08:49 AM

An extreme case of this was a refiguring of the front surface of the 36" refractor at Lick Observatory after polishing out crazing from gradual devitrification of the glass.  For a while they were stopping it down to about to about 22".  Apparently that work was done without the skill Alvan G. Clark had in correcting for small faults in the glass.


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#5 LU1AR

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Posted 28 January 2021 - 09:14 AM

Alexander Conrady (1866 - 1944) was an eminent optical designer, academician, and textbook author. In his classic textbook "Applied Optics and Optical Design"; stipulates that for colorlessness, you want a CI (Chromatic Index) of at least 3.
That is, the f/ratio should be (At least) 3 times the aperture in inches.
True Colorless "Achromatic" refractor, would have a CI of 5 or more.
If the CI is below 3, you will see a fair amount of Chromatic Aberration. If the CI is below 2, CA will be severe.
Your "Masked Refractor, witrh the new 74 mm. objective; will hace a CI of 6.48. Well inside the Conrady specs.
You can do a lot for your "Near Achromatic" refractor, painting the sides of the objective lens doublet (Fiber Marker), and baffling. An economic baffle is a cone of black cardboard inside the tube. Be carefull to blacken inside the focus tube.
Last but not the least: You can made your mask with a 77 mm UV filter in the aperture. The UV filter will have no efect on your image quality; but will absorb the segment below 440 nm. which is a generator of false colors and will always be installed on the mask (which can be removed).
Over that filter, you can screw outdated film photography filters, such as Pale Yellow # 12 which eliminates AC and improves contrast for many objects.
- Lastly, you can use an FLD filter that was used to remove that "Soviet Tint" from film photos, under fluorescent light.
The FLD filter has almost the same effects than those specialized Baader filter for correcting achromatics; but it costs less than the refractor.
Regards.
Edgardo



 


Edited by LU1AR, 28 January 2021 - 09:53 AM.

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#6 Astrojensen

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Posted 28 January 2021 - 09:34 AM

Hi

 

I have the same refractor as the ES 80ED APO f/6. This scope is known to have conspicuous diffraction spikes caused by pinched optics:

 

Castor (Alpha Geminorum)

attachicon.gifp.jpg

 

To mitigate the problem, I cut out a card ring to reduce my aperture from 80mm to 74mm. 

 

attachicon.gifScreenshot 2021-01-28 at 21.21.19.jpg

attachicon.gifScreenshot 2021-01-28 at 21.21.04.jpg

 

The resulting image is indeed better, if not absolutely good:

attachicon.gifr.jpg

 

Spikes are still seen, but I can live with that. The reduced aperture turns my scope to f/7. Exposure should increase by about 17%, but looks like it's inevitable.

 

Hopefully this is a reasonable way out.

 

Lu

The diffraction spikes you see are not from pinching, but from the lens spacers, and is a common problem. Look at the first image, and you will see three, almost wegde-shaped shadows aiming towards the center of the star, equally spaced 120° apart, and each with two diffraction spikes. This is a tell-tale sign of diffraction from lens spacers. Pinched optics look quite a bit different, with a deformed image, often triangular or elongated. The faint star is perfectly round in the first image, clearly telling that this is not pinching.

 

Another tell-tale sign that this is not pinching, is that stopping the lens down will NOT solve the pinching, unless you stop the lens so far down, that it's no longer visible, because the resolution is now so poor, it's hidden under the spurious disk of the star.  

 

 

Clear skies!

Thomas, Denmark


Edited by Astrojensen, 28 January 2021 - 09:38 AM.

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#7 ngc7319_20

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Posted 28 January 2021 - 09:42 AM

Good solution.  "Only" 1/8 inch off the radius.  Sometimes I will dig around the recycle bin for a proper sized margarine / cottage cheese / yogurt / etc. lid that just snaps on the front of the lens.  These circle cutters they sell at craft stores make nice masks.

 

https://www.walmart....evice/945611006


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#8 ngc7319_20

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Posted 28 January 2021 - 09:54 AM

The diffraction spikes you see are not from pinching, but from the lens spacers, and is a common problem. Look at the first image, and you will see three, almost wegde-shaped shadows aiming towards the center of the star, equally spaced 120° apart, and each with two diffraction spikes. This is a tell-tale sign of diffraction from lens spacers. Pinched optics look quite a bit different, with a deformed image, often triangular or elongated. The faint star is perfectly round in the first image, clearly telling that this is not pinching.

There is a lot of light in a halo around the PSF core.  It could be several effects at the lens edge.  The lens edge could be turned on the glass.  Or the cell diameter is too tight and pressing on the sides of the lens -- this can happen when it is cold and cell contracts faster than glass -- there is a lot of "distance" to contract around the lens circumference / diameter.  Or the retaining ring is too tight.  

 

Pinching a lens is different from pinching a mirror.  Mirror has 3 retaining clips and is probably support on the rear near 2/3 radius (floating cell), so much of the mirror becomes deformed.  But lens has tight fitting circumferential cell / retaining ring / spacers and is only supported at the edge.


Edited by ngc7319_20, 28 January 2021 - 11:01 AM.

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#9 LU1AR

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Posted 28 January 2021 - 10:16 AM

The diffraction spikes you see are not from pinching, but from the lens spacers, and is a common problem. Look at the first image, and you will see three, almost wegde-shaped shadows aiming towards the center of the star, equally spaced 120° apart, and each with two diffraction spikes. This is a tell-tale sign of diffraction from lens spacers. Pinched optics look quite a bit different, with a deformed image, often triangular or elongated. The faint star is perfectly round in the first image, clearly telling that this is not pinching.

 

Another tell-tale sign that this is not pinching, is that stopping the lens down will NOT solve the pinching, unless you stop the lens so far down, that it's no longer visible, because the resolution is now so poor, it's hidden under the spurious disk of the star.  

 

 

Clear skies!

Thomas, Denmark

¡Bravo!
Edgardo



#10 Yourjones

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Posted 31 January 2021 - 03:09 AM

You could send it back to ES and have them correct the pinched optics.   My AR152 had pinched optics when it was cold.  Sent to ES right after I received it, back in a couple of weeks and it was perfect.   They did a great job, never an issue since.  Just wish I had more opportunities to use it.  But with a recent back injury it wont be for a while.frown.gif

Hi Mike

 

I wish you would recover from your back injury the soonest possible. Thanks for your advice, but I think I can live with the scope now. The spikes are more conspicuous when the stars are brighter and when the exposure time is longer. Hence, I won't need to use my aperture reduction ring when shooting targets in a patch of sky without very bright stars.lol.gif

 

Thanks again!


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#11 Yourjones

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Posted 31 January 2021 - 03:27 AM

Alexander Conrady (1866 - 1944) was an eminent optical designer, academician, and textbook author. In his classic textbook "Applied Optics and Optical Design"; stipulates that for colorlessness, you want a CI (Chromatic Index) of at least 3.
That is, the f/ratio should be (At least) 3 times the aperture in inches.
True Colorless "Achromatic" refractor, would have a CI of 5 or more.
If the CI is below 3, you will see a fair amount of Chromatic Aberration. If the CI is below 2, CA will be severe.
Your "Masked Refractor, witrh the new 74 mm. objective; will hace a CI of 6.48. Well inside the Conrady specs.
You can do a lot for your "Near Achromatic" refractor, painting the sides of the objective lens doublet (Fiber Marker), and baffling. An economic baffle is a cone of black cardboard inside the tube. Be carefull to blacken inside the focus tube.
Last but not the least: You can made your mask with a 77 mm UV filter in the aperture. The UV filter will have no efect on your image quality; but will absorb the segment below 440 nm. which is a generator of false colors and will always be installed on the mask (which can be removed).
Over that filter, you can screw outdated film photography filters, such as Pale Yellow # 12 which eliminates AC and improves contrast for many objects.
- Lastly, you can use an FLD filter that was used to remove that "Soviet Tint" from film photos, under fluorescent light.
The FLD filter has almost the same effects than those specialized Baader filter for correcting achromatics; but it costs less than the refractor.
Regards.
Edgardo



 

Hi Edgardo

 

Thank you so much for your comment and advice - expertise! It's exciting to know that cutting down the aperture turns out to be beneficial in terms of mitigating chromatic aberration! I've been fancying short focal length refractors and reflectors and was serious considering having an f/2.8 Newtonian on my fleet. Taking this "CI" into consideration, I doubt whether there will be unexpected issues imaging with such scopes ...

 

Lu



#12 Yourjones

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Posted 31 January 2021 - 03:39 AM

The diffraction spikes you see are not from pinching, but from the lens spacers, and is a common problem. Look at the first image, and you will see three, almost wegde-shaped shadows aiming towards the center of the star, equally spaced 120° apart, and each with two diffraction spikes. This is a tell-tale sign of diffraction from lens spacers. Pinched optics look quite a bit different, with a deformed image, often triangular or elongated. The faint star is perfectly round in the first image, clearly telling that this is not pinching.

 

Another tell-tale sign that this is not pinching, is that stopping the lens down will NOT solve the pinching, unless you stop the lens so far down, that it's no longer visible, because the resolution is now so poor, it's hidden under the spurious disk of the star.  

 

 

Clear skies!

Thomas, Denmark

Hi Thomas

 

Thanks for your analysis! I'm pretty sure the spikes are due to the 3 sets of 3 grub screws on the wall of the lens cell (hence 12 screws in total), also spaced at 120 degrees apart from one set to another. I have once opened the lens cell, yes, I opened itlol.gif, and saw nothing between the triplet lenses but 2 thin metal rings, nothing protruding into the optical path.

 

I did try loosening the said grub screws to see the spikes GONE before, but I was so greedy at that time, I unscrewed too much, completely losing the centering of the 3 lenses. Hence, I had to send the scope back to its factory for collimation, centering, calibration - whatever it is called.

 

These pictures in my post were taken upon receiving the fixed scope. I don't have the guts to loosen the grub screws again to eliminate the spikes ...undecided.gif

 

Lu


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#13 Yourjones

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Posted 31 January 2021 - 03:45 AM

Good solution.  "Only" 1/8 inch off the radius.  Sometimes I will dig around the recycle bin for a proper sized margarine / cottage cheese / yogurt / etc. lid that just snaps on the front of the lens.  These circle cutters they sell at craft stores make nice masks.

 

https://www.walmart....evice/945611006

Hi

 

Thanks for your words! I will live with my reduced aperture when imaging very bright stars. I promise I've never seen circle cutters as such. I couldn't even imagine their existence.lol.gif  Will give it a try for cutting out a smoother edge. Thanks a lot!

 

Lu




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