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Lens cell factory defect?

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25 replies to this topic

#1 balticsensor

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Posted 11 January 2020 - 03:31 PM

Hello, I'm looking for an explanation as to why there are no air-spacing inserts on my Fraunhofer achromat. The model is a Skywatcher Evostar 120mm f/8.33. I'm wondering about other owners of this model - have yours got the inserts? I mean, I've seen even the Evostar 150 have those. Could some factory worker simply forget to do it? I wouldn't be so worried if it wasn't for the splotchy paint job on the internal baffles (eyepiece side) and detectable spherical aberration in the star test. The only reason I noticed this was precisely because I wanted to measure and replace those inserts, but lo and behold... I'm quite baffled!

IMG_0022b.jpg

 



#2 aeajr

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Posted 11 January 2020 - 03:36 PM

I can't answer your question, but would suggest you contact the manufacturer and see what they say. 



#3 Bomber Bob

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Posted 11 January 2020 - 03:43 PM

Some lens makers use the translucent vinyl bands rather than 3 spacers.  Usually a fraction of a millimeter thick, and maybe 1mm wide.


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#4 skyward_eyes

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Posted 11 January 2020 - 03:48 PM

Because we don’t use them anymore. They now use a thin plastic spacer ring that supports the two elements 360 degrees and not just three spots. This ring provides the correct spacing but does not extend into the light path.
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#5 Astrojensen

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Posted 11 January 2020 - 03:53 PM

Because we don’t use them anymore. They now use a thin plastic spacer ring that supports the two elements 360 degrees and not just three spots. This ring provides the correct spacing but does not extend into the light path.

It's also virtually impossible to see from the outside, even when you look obliquely at the lens and it should be in view. 

 

 

Clear skies!

Thomas, Denmark



#6 Bomber Bob

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Posted 11 January 2020 - 04:12 PM

Both my ED doublets use the bands.  I can see the one in my AT102ED using a bright green laser pointer -- makes the thin band stand out.


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#7 balticsensor

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Posted 11 January 2020 - 04:26 PM

I see, thanks for the answers!

But now the hard question... How do I adjust the spacing? Correct me if I'm wrong, but I learned that one can correct for spherical aberrations by adjusting the air space? The intra and extra focal diffraction patterns are definitely different.
 


Edited by balticsensor, 11 January 2020 - 04:27 PM.


#8 John Huntley

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Posted 11 January 2020 - 04:32 PM

I see, thanks for the answers!

But now the hard question... How do I adjust the spacing? Correct me if I'm wrong, but I learned that one can correct for spherical aberrations by adjusting the air space? The intra and extra focal diffraction patterns are definitely different.
 

There is no provision for doing this within the normal design of the objective lens cell. If you wish to make such adjustments you will need to remove the objective from the cell and add shims to the spacer ring to adjust the air space between the elements. My guess is that the warranty will be void if it is a new scope though.

 

Adjusting the gap between the lens elements is a delicate process and does not always improve the optical peformance - sometimes the opposite.



#9 balticsensor

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Posted 11 January 2020 - 04:39 PM

Alright, so risky it is. How about a less fiscally risky approach - adjusting the eyepieces. Say if I have an unsophisticated plossl, or an ortho - can there be any adjustments made there to over-correct or under-correct? Starting to need an optical engineer now... grin.gif



#10 Bomber Bob

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Posted 11 January 2020 - 04:42 PM

But now the hard question... How do I adjust the spacing?

 

I wouldn't attempt it, and I've been working on lenses for 40+ years.  Your achro is medium-speed at F8, and is gonna have some SA.

 

How are you going to assess the impact of spacing changes?  Do you have test gear?  I have a DPAC rig, and I have used it to improve performance by rotating the elements for the best alignment.

 

And, the more you handle a bare lens, the more likely you are to damage it.


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#11 John Huntley

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Posted 11 January 2020 - 04:57 PM

One way to correct the spherical aberration in an F/8 achromat is to use a Chromacor corrector. These are hard to find and expensive but not only reduce significantly the chromatic aberration that such achromats produce but also, if correctly matched to the objective, correct SA as well.

 

When developing the Chromacor it was found by sampling that a large proportion of the chinese achromats that it was being designed for also had spherical abberation of either positive or negative nature. So Chromacors were supplied with a specified SA correction to be matched to the objective. There was also a null version of the Chromacor but I believe that was not the most used.

 

In short, many of these chinese achromat objectives posses a degree of spherical aberration.


Edited by John Huntley, 11 January 2020 - 04:58 PM.


#12 balticsensor

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Posted 11 January 2020 - 04:58 PM

I hear y'all. Won't attempt it. But then again, I did fix the pinch issue and rotated the front element on my previous 90mm f/10 achro. Sure know what caution means. I just have this brain slug, knowing that SA sitting there in my telescope. Perhaps there's a solution on the eyepiece end? Not thinking about the chromacor, really. Would just rather get an APO!


Edited by balticsensor, 11 January 2020 - 05:00 PM.

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#13 John Huntley

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Posted 11 January 2020 - 05:00 PM

.... Perhaps there's an eyepiece-end solution?

See above - the Chromacor.

 

Saving for an ED120 (which tend to be quite well corrected) is probably less expensive though.


Edited by John Huntley, 11 January 2020 - 05:03 PM.

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#14 Bomber Bob

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Posted 11 January 2020 - 05:26 PM

I hear y'all. Won't attempt it. But then again, I did fix the pinch issue and rotated the front element on my previous 90mm f/10 achro. Sure know what caution means. I just have this brain slug, knowing that SA sitting there in my telescope. Perhaps there's a solution on the eyepiece end? Not thinking about the chromacor, really. Would just rather get an APO!

I like my AT102ED so much that I just sold my Classic Vixen FL80S fluorite.  Same weights & lengths, yet that 20mm aperture made a big difference -- especially on globulars.


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#15 Jeff B

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Posted 11 January 2020 - 05:39 PM

"But now the hard question... How do I adjust the spacing? Correct me if I'm wrong, but I learned that one can correct for spherical aberrations by adjusting the air space? The intra and extra focal diffraction patterns are definitely different. "

 

I really hope you are using a green filter (or better yet, two of them stacked) when you say this, otherwise they will appear different.  Also, adjusting the spacing, while improving the SA in green, may very well make it worse in red and blue.  This is where DPAC testing is of great help.

 

Of course the real question:  Are the images sharp at focus (never mind the "CA")?

 

Jeff


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#16 Steve Allison

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Posted 11 January 2020 - 06:45 PM

Just a thought:

 

Couldn't someone design and market a "chromacor" type corrector that just corrects for spherical aberration?

 

I understand one of the problems with producing the chromacor was that exotic glasses were needed to correct for chromatic aberration. Wouldn't a corrector that just dealt with spherical aberration be possible with crown and flint glass or at the most, an ED-type element?

 

I know prism diagonals and certain eyepiece and add a degree of under or over correction, but it would nice if the telescope itself were fully corrected.

 

The real answer, I guess, is to test any telescope prior to purchase to ensure its optical correction is satisfactory. 

 

Again, just a thought,,,


Edited by Steve Allison, 11 January 2020 - 07:18 PM.


#17 stevew

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Posted 11 January 2020 - 07:25 PM

I had an achromat that showed nice even rings inside and outside of focus, I changed the focuser and low an behold the rings were no longer seen on one side of focus [cant remember which side] Turns out that the draw tube on the original focuser was way too long and cut into the light path effectively stopping down the aperture of the scope.

I made an aperture mask 1/8th inch wide that stopped the objective lens down by 1/4 of an inch, and it really improved the inside/outside defocused rings.  

For me the slight loss of light was worth the improved optical performance. Give it a try.


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#18 balticsensor

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Posted 12 January 2020 - 03:32 AM

Of course the real question:  Are the images sharp at focus (never mind the "CA")?

How would I know if that's all I observe with? But I did a side-by-side with my 90 f/10 before selling it. I could definitely see more planetary detail with the 120 f/8, but the views were not as clean and sharp. There's the seeing-vs-aperture dynamic, seeing-vs-focal ratio dynamic and different ammounts of chromatic abberation between the scopes. It's impossible to measure the quality of optics amongst all these variables. 
 

I made an aperture mask 1/8th inch wide that stopped the objective lens down by 1/4 of an inch, and it really improved the inside/outside defocused rings.  

For me the slight loss of light was worth the improved optical performance. Give it a try.

I might try to mask that edge off indeed. Will try!



#19 Jon Isaacs

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Posted 12 January 2020 - 05:00 AM

Couldn't someone design and market a "chromacor" type corrector that just corrects for spherical aberration?

 

 

Valery Deryuzhin who designed and manufactured the Chromacorr, also designed and manufactured the Saffix.  

 

https://www.cloudyni...aries-safix-r48

 

Jon


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#20 Steve Allison

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Posted 12 January 2020 - 06:02 AM

Thank you, John.

 

I assume they are no longer available...



#21 Rutilus

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Posted 12 January 2020 - 03:21 PM

This is the spacer ring taken out of my 120mm f/8.3 Synta scope. The spacer ring

from my scope measured 0.2478mm in thickness.

 

If your scope (OP) did not have the spacer ring fitted, you would see a very

obvious Newton ring pattern in the lens and chances of it being in the centre of the

lens are near to zero, for as you tighten the retaining ring the two lenses move 

and the Newton ring pattern moves off centre, for the two lenses are nesting together

i.e. touching in the middle. I know for I did it as an experiment with my scope.

 

 

 

 

 

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#22 Sol Robbins

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Posted 12 January 2020 - 03:42 PM

The Safix was an interesting concept in that it could be used in different telescope designs. Mine is loaned to a nearby college and is used in one of their 16" SCTs. It only corrects for SA, so other aspects of optical quality remain. I don't think I have ever seen being on the used market.

 

I don't know how old your scope is, or if you recently purchased it from a reputable vendor. If the latter, maybe you can work out an exchange if the SA is really bad.



#23 balticsensor

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Posted 13 January 2020 - 05:54 PM

This is the spacer ring taken out of my 120mm f/8.3 Synta scope. The spacer ring

from my scope measured 0.2478mm in thickness.

 

If your scope (OP) did not have the spacer ring fitted, you would see a very

obvious Newton ring pattern in the lens and chances of it being in the centre of the

lens are near to zero, for as you tighten the retaining ring the two lenses move 

and the Newton ring pattern moves off centre, for the two lenses are nesting together

i.e. touching in the middle. I know for I did it as an experiment with my scope.

So are you suggesting to try and adjust the pressure of the retaining ring?



#24 Steve Allison

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Posted 13 January 2020 - 06:27 PM

The Safix was an interesting concept in that it could be used in different telescope designs. Mine is loaned to a nearby college and is used in one of their 16" SCTs. It only corrects for SA, so other aspects of optical quality remain. I don't think I have ever seen being on the used market.

 

I don't know how old your scope is, or if you recently purchased it from a reputable vendor. If the latter, maybe you can work out an exchange if the SA is really bad.

Thanks, Sol.

 

Fortunately, my refractors are all beautifully corrected. Well, except for my Orion 120mm, which has some SA.  I was basically just thinking out loud.

 

Steve



#25 Nippon

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Posted 14 January 2020 - 01:16 AM

I second the green filter test. If it's an f/8 achro and has crisp rings one side of focus and on the other side the ring pattern is far less distinct. This is caused by the color error. If the outer ring looks about the same brightness on either side I think your SA correction is okay. 




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