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Clave Not As Sharp On Axis

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#1 Kent10

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Posted 20 June 2022 - 06:39 PM

Does anyone have more information regarding on axis sharpness in a Clave?  This excerpt below was taken from Chris Lord’s “Evolution of the Astronomical Eyepiece” page 31.  He states that the sharpest imagery occurs 30% towards the edge of the field of view.  Has anyone noticed that?  I did last night on M5 with a Clave 6mm.  I was a little surprised to see the dense area of M5 tighten up when viewing that area slightly off axis.  Then I recalled reading something about on axis sharpness and found this.  I intend on checking again tonight.  My Zeiss 6mm Mono and TMB 6mm Mono were very sharp on axis in comparison.

 

“However, unlike the Abbé Orthoscopic and its derivatives, where longitudinal spherical correction is zero on axis, the assymetric form of the Plössl leads to a zonal correction and the sharpest imagery does not occur on axis but some 30% towards the edge of the field of view. At low to medium powers this is of no consequence, but it is noticeable at high powers (exit pupils less than 1.5mm).”


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#2 Kent10

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Posted 21 June 2022 - 09:06 AM

I saw the same last night.  I started with double stars and when viewed slightly off-axis with the 6mm Clave, the stars appeared brighter and with tighter airy disks.  I think I am onto something in understanding how best to use my Clave at higher power.  I will have to test on the planets when they are back.


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#3 SandyHouTex

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Posted 21 June 2022 - 10:04 AM

I think what Chris mentions is true.  He knew a lot about eyepieces and ray-traced most of them.

 

He is sorely missed.



#4 ngc7319_20

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Posted 21 June 2022 - 10:37 AM

What scope are you using them in?  How high was the target in the sky?


Edited by ngc7319_20, 21 June 2022 - 10:38 AM.


#5 Kent10

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Posted 21 June 2022 - 10:56 AM

Tec 200FL.  The objects ranged from lower in the sky, Iota Cancri and Algieba to higher in the sky, Cor Caroli, M3 and M13.  M5 last night.  I could easily see differences comparing Delos/Ethos and minimum glass eyepieces as well.


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#6 lylver

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Posted 21 June 2022 - 01:26 PM

Tec 200FL. 

Bad choice.

Clave plössl was calculated and confirmed to be used at f/14 on "the standard SAF scope 200mm f/7" with the Clavé barlow for planetary and f/7 (without the barlow) for deep sky.

The standard A.Koenig formula was for the Zeiss B triplet objective (>=f/15) or Zeiss A doublet (f/18)


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#7 Kent10

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Posted 21 June 2022 - 01:51 PM

Bad choice.

 

smile.gif  I still enjoy using my Clave.  Now, I understand them better.


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#8 CrazyPanda

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Posted 21 June 2022 - 02:47 PM

I saw the same last night.  I started with double stars and when viewed slightly off-axis with the 6mm Clave, the stars appeared brighter and with tighter airy disks.  I think I am onto something in understanding how best to use my Clave at higher power.  I will have to test on the planets when they are back.

Seems very unusual for an eyepiece to be sharper off-axis than on-axis.


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#9 Kent10

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Posted 21 June 2022 - 03:01 PM

Seems very unusual for an eyepiece to be sharper off-axis than on-axis.

I think it is and that is what surprised me when I 1st noticed it.  Then I recalled reading something about it and tracked it down.  Never thought about it before the other night and I use them a lot.



#10 SteveC

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Posted 21 June 2022 - 06:02 PM

Bad choice.

Clave plössl was calculated and confirmed to be used at f/14 on "the standard SAF scope 200mm f/7" with the Clavé barlow for planetary and f/7 (without the barlow) for deep sky.

The standard A.Koenig formula was for the Zeiss B triplet objective (>=f/15) or Zeiss A doublet (f/18)

My eyepieces work equally well in my F5.6 TEC110,  f7 TEC140, and f15 Mak. I guess the Clave is fine if you anticipate viewing slightly off axis, but I think it's still a head scratcher in that regard.



#11 Kent10

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Posted 21 June 2022 - 07:26 PM

I have a Clave barlow as well as other barlows.  I'll have to try them and see how that affects the image.


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#12 stevenwav

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Posted 22 June 2022 - 07:51 AM

Wow - This is great and very timely information! I just purchased a full set (Gen 2 or 3 - pretty awesome) with the intention to use these at f/8 and higher - I will be sure to use the barlow for planetary and without the barlow for dso, with an eye off-axis. I will post a new thread or attahc an image here when recieved. - hopefully in a week or two.



#13 Kent10

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Posted 22 June 2022 - 08:16 AM

Congratulations on your set, Steve.  I look forward to hearing what you think about the on-axis views.  It is a subtle difference that I hadn't noticed until recently.  I do a lot of eyepiece comparisons.


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#14 stevenwav

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Posted 22 June 2022 - 08:31 AM

Thanks Trent - I will compare with my Zeiss monos as well. What a nice project for us - we are fortunate to be able to conduct these comparos.



#15 SandyHouTex

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Posted 22 June 2022 - 08:30 PM

Congratulations on your set, Steve.  I look forward to hearing what you think about the on-axis views.  It is a subtle difference that I hadn't noticed until recently.  I do a lot of eyepiece comparisons.

Maybe you should write a CN Report on your comparisons, since we lost BillP.  It would be helpful for someone considering certain eyepieces.



#16 Kent10

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Posted 23 June 2022 - 01:40 AM

Thanks, Sandy, for the inspiration.  All I need is time.  I miss BillP too and was wondering what he would say about this thread.  I wonder if he will ever come back.


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#17 Kent10

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Posted 23 June 2022 - 01:44 AM

I tried a couple barlows tonight including the Clave and on-axis seemed the same as slightly off-axis.  I used the 12mm with the barlow and the 6mm without where once again with the 6mm on double stars I could see a difference between on and off axis.  Later I enjoyed views of globulars with the 12mm and barlows but the seeing wasn't good enough to make any judgements.



#18 ngc7319_20

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Posted 23 June 2022 - 08:46 PM

I tried two different Clave 6mm in a TMB 105/650 (F/6.2) APO triplet.  I just don't see this effect.  At the field center both samples show a tight Airy disk and single diffraction ring.  At 50% of the field radius off-center I do see traces of astigmatism and maybe coma.  I'm viewing with a 5x24 scope between the eyepiece and eye to magnify the image.  And I'm setup in a lab on an optical bench with a 16" diameter collimator, so there's no seeing effects, no atmospheric dispersion, etc.

 

Hard to understand.  Is it possible the OP has slight eye astigmatism, or slight astig in the scope, which is being canceled by slight off-axis astig in the eyepiece?  Or atmospheric dispersion being cancelled by eyepiece aberrations?  It seems like there are several references to the effect, but balanced against that, it is hard to say how many people didn't see it and therefore didn't report it...   Hmmm....



#19 Kent10

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Posted 23 June 2022 - 09:33 PM

I tried two different Clave 6mm in a TMB 105/650 (F/6.2) APO triplet.  I just don't see this effect.  At the field center both samples show a tight Airy disk and single diffraction ring.  At 50% of the field radius off-center I do see traces of astigmatism and maybe coma.  I'm viewing with a 5x24 scope between the eyepiece and eye to magnify the image.  And I'm setup in a lab on an optical bench with a 16" diameter collimator, so there's no seeing effects, no atmospheric dispersion, etc.

 

Hard to understand.  Is it possible the OP has slight eye astigmatism, or slight astig in the scope, which is being canceled by slight off-axis astig in the eyepiece?  Or atmospheric dispersion being cancelled by eyepiece aberrations?  It seems like there are several references to the effect, but balanced against that, it is hard to say how many people didn't see it and therefore didn't report it...   Hmmm....

Thanks for trying this out.  One thing I need to check is to be sure the center of my 6mm Clave is clean.  And then the next step is to try other Clave, 4, 5, and I have other 6mm I believe including a 2nd generation chrome.  Perhaps it is just this one 6mm I have been using and it is subpar.  A rare night of rain here at the start of the monsoon season so I may not be able to try tonight.  But I can go check to see if the 6mm I have been using is clean in the center.



#20 Kent10

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Posted 23 June 2022 - 09:48 PM

I checked the 6mm I have been using.  I shined a flashlight in the bottom and couldn't see anything too bad.  There was eyelash oil on the entire surface which I cleaned.  I get that a lot and clean the Clave frequently.  I don't think that would affect only the center, however.  But I will keep testing and checking when I can.  And try different focal lengths. 



#21 ngc7319_20

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Posted 23 June 2022 - 10:12 PM

I checked the 6mm I have been using.  I shined a flashlight in the bottom and couldn't see anything too bad.  There was eyelash oil on the entire surface which I cleaned.  I get that a lot and clean the Clave frequently.  I don't think that would affect only the center, however.  But I will keep testing and checking when I can.  And try different focal lengths. 

Try placing the flashlight 20 feet away, and then look at the flashlight through the eyepiece in the normal way.  This should easily show any bad stuff in the eyepiece.  You should just see a uniformly bright field of view.  But it is possible to see dust spots, smudges, bubbles in the glass, failing balsam / adhesive, etc.



#22 Kent10

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Posted 23 June 2022 - 11:00 PM

I did your 20 feet flashlight test.  I like it.  Thanks.  It was easier to see any dirt.  There were light bits everywhere as I would expect from an older eyepiece.  This is probably one of my cleaner Clave.  Nothing dark or big right in the middle.  So the next step will be to test my other 6mm and other focal lengths.  Do you think it would be easier to see any differences in a larger scope such as my 200mm compared with a 100mm.  I do have some 100mm scopes I could eventually try.  But I mostly use my 200.

 

BTW where I see a difference is just slightly off axis.  It is not 50%.  Maybe 25% to the edge from the center.  But I haven't been careful to measure.  I just start on axis and then move off axis and I tried different directions and I see 2 brighter stars (airy disks) when looking off axis.  The 1st night with M5 I saw more stars in the core when doing this.  They just popped out closer to what I saw in my monos.  But yesterday I couldn't get the same affect due to seeing I believe.

 

I do have an ADC which I could try and wasn't using for these tests.  Iota Canri in particular was quite low.



#23 Kent10

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Posted 23 June 2022 - 11:03 PM

Also something to consider. . .  I do have some astigmatism but I am also very near-sighted and view without my glasses or contacts.  So my eyes may be "unusual" compared with others.



#24 ngc7319_20

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Posted 24 June 2022 - 01:06 AM

Also something to consider. . .  I do have some astigmatism but I am also very near-sighted and view without my glasses or contacts.  So my eyes may be "unusual" compared with others.

Where do you look in the field of view when you say it is sharper off-axis?  Do you look both left and right of the field center?  What about above and below the field center?  If it was cancelling some astigmatism, I would expect maybe the same "better sharpness" left and right of field center.  But then worse images above and below the field center.  Or vice versa, or some similar combination where opposites sides from the center give same result, and 90 degrees to those gives worse result.



#25 ngc7319_20

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Posted 24 June 2022 - 01:14 AM

 Do you think it would be easier to see any differences in a larger scope such as my 200mm compared with a 100mm.  I do have some 100mm scopes I could eventually try.  But I mostly use my 200.

 

I am not sure if differences would be easier to see in 100mm vs 200mm scope.  The size of the Airy disk (in say fractions of a mm at the eyepiece) will depend only on the F-ratio of the scope.  Larger F-ratio will make larger Airy disks.  So I would expect any errors in the eyepiece to be more apparent in faster scopes (lower F-ratio number) where the Airy disks are smaller.




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