23 replies to this topic

[Dan876876]

Hi,

 

New guy, here.
 

I see the mention of "aspherical" on eyepieces, which I read corrects spherical aberrations.
Afaik, refractors (like mine) don't have spherical aberrations.

 

So, is that simply a no-brainer "it is not compatible" kinda deal that everybody know but is never mentioned for the noobs?

 

To me, it sounds like to correct something that isn't there in optics, means to create the problem.

 

Good day night.

 

 

 

(Context: got a 102mm achromat refractor, f=660mm. To keep it under 200x, trying to avoid a cheap barlow, and getting better FOV too than plossl, I'm shopping for something in the 3-5 mm range).

[Astrojensen]

Aspherics, of which there is a literally endless number of different shapes, do a lot more in an eyepiece than simply correct spherical aberration, like avoiding astigmatism in a wide-field eyepiece with few elements, etc.  

 

So they definitely correct very real problems. 

 

Using an eyepiece with an aspherical element on a telescope with perfect spherical correction doesn't change said telescope's correction. 

 

 

Clear skies!

Thomas, Denmark

[Jon Isaacs]

 

Aspherics, of which there is a literally endless number of different shapes, do a lot more in an eyepiece than simply correct spherical aberration, like avoiding astigmatism in a wide-field eyepiece with few elements, etc.

 

In theory, an Aspheric eyepiece could offer an improvement in off-axis sharpness but it seems like it's more advertising than anything. 

 

I've not been impressed with the Baader 41 mm and 36 mm. They in seem to be comparable to the Q-70 eyepieces to my eye.

 

Jon

[ngc7319_20]

I see the mention of "aspherical" on eyepieces, which I read corrects spherical aberrations.

Afaik, refractors (like mine) don't have spherical aberrations.

Well no.  The "aspheric" eyepiece is not trying to correct spherical aberration in the main scope.  It is just another way to design an eyepiece.  Sometimes a lens with an aspherical surface can replace 3 or 4 spherical lenses in a design.    Aspheric lenses are getting cheaper to produce due to technical advancements, and are likely to appear in more designs as time goes by.

 

There are aspheric surfaces used to correct spherical aberration in scopes.  For example the Schmidt corrector plate is an aspheric lens.  Also the Newtonian primary mirror is usually a parabola, which is a form of aspheric.  But these are a different situation than an eyepiece with an aspheric surface.

 

I've not been impressed with the Baader 41 mm and 36 mm. They in seem to be comparable to the Q-70 eyepieces to my eye.

I've been testing some wide field eyepieces for my new 80mm F/5 bino scope.  I'm finding the Baader 36mm Aspheric eyepiece is marginally sharper than the Panoptic 35mm at the field edge in this scope, at least to my aging eye sight (no eye accommodation).  I think in general the Panoptic is a superior eyepiece for edge sharpness, field flatness, and color correction.  But in this particular case there seems to be some lucky cancellation of the field flatness issues, and maybe field edge astigmatism between the Baader Aspheric and 80mm F/5 lens.  There also appears to be less field distortion in the Baader Aspheric during day time use.  So the Baader Aspheric has its place.  Did I mention the Baader Aspheric it costs about half of the Panoptic?

Edited by ngc7319_20, 24 April 2025 - 12:38 PM.

[John Turley]

In theory, an Aspheric eyepiece could offer an improvement in off-axis sharpness but it seems like it's more advertising than anything. 

 

I've not been impressed with the Baader 41 mm and 36 mm. They in seem to be comparable to the Q-70 eyepieces to my eye.

 

Jon

I used to have a Baader 36mm Aspheric, but was not overimpressed with the performance, and replaced it with a (now discontinued) 42mm Vixen LVW, which has much better edge correction, especially in my f7 Refractor.

 

I've not had the chance to compare it with a 41mm Panoptic, but the LVW was considerably less bulky, at 500g was around half the weight of the Panoptic, and at £300 around half the price.

 

John 

Edited by John Turley, 25 April 2025 - 02:26 AM.

[SeattleScott]

There are also the $10 SVBONY aspherics, so simply having a plastic aspheric lens doesn’t automatically make an eyepiece superior to Televue offerings. So I don’t pay much attention to ad copy about aspherics, but instead pay attention to reviews about a particular eyepiece. I understand that there is some optical theory suggesting that aspheric elements can improve eyepiece performance if executed properly. But that clearly doesn’t guarantee premium performance, and the best eyepieces on the market don’t seem to advertise aspherics. Except maybe the Leica zoom, isn’t that advertised as aspheric? One would think Leica would be able to execute well.

[Dan876876]

Thanks to all.

 

Yet another hobby spoiled by marketing evil.

 

And just because we buy one eyepiece that lasts 30 years, it doesn't justify charging 300$ for it.

It's mass produced manufacturing, it's not prescription glasses...

 

That's almost enough to sell everything I just got...

[eblanken]

Hi (aka Dan876876),

 

First, welcome to Cloudy Night (CN) forums and more specifically to eyepiece forum. 

 

Second, Thanks for posting your question. My answer is that Spherical surfaces are easier and less expensive to make. To "a-spherize" one or more surfaces in the eyepiece is an alternative to adding more spherical lenses, so the lens count can be smaller for about the same performance. All eyepieces are limited by their design tradeoffs: Focal Length (in mm), Apparent Field of View (AFoV in Degrees or Radians), Exit Pupil Location {Eye Relief (ER)}, Field Stop Size {which affects True Field of View (TFoV)}, and Color Correction are just some of the parameters that eyepiece designers trade off when they do their jobs. Cost, Polish, etc. are other factors that are considered . . . 

 

Very Best Regards,

 

Ed

 

P.S. You are welcome to Private Message (PM) me if you want . . .   

Edited by eblanken, 24 April 2025 - 10:48 PM.

[Ernest_SPB]

Aspherics on eyepiece elements are used to...

• control SA in exit pupil (does not have any relation to SA of axial beam)
• correct image distortion (barrel or pincushion)
• sometimes help to control field aberrations like astigmatism

In general, it can reduce number of optical surfaces reducing cost and indirectly - light scattering.

 

But to say honnestly I did not meet upper level astronomical eyepiece with aspherics 

Edited by Ernest_SPB, 24 April 2025 - 11:22 PM.

[RichA]

There are also the $10 SVBONY aspherics, so simply having a plastic aspheric lens doesn’t automatically make an eyepiece superior to Televue offerings. So I don’t pay much attention to ad copy about aspherics, but instead pay attention to reviews about a particular eyepiece. I understand that there is some optical theory suggesting that aspheric elements can improve eyepiece performance if executed properly. But that clearly doesn’t guarantee premium performance, and the best eyepieces on the market don’t seem to advertise aspherics. Except maybe the Leica zoom, isn’t that advertised as aspheric? One would think Leica would be able to execute well.

Some aspheric designs that haven't been made could result in small eyepieces with very good full-field correction with fewer elements than eyepieces with spherical lenses.  This is the case with modern camera lenses.

[Lagrange]

Aspherics on eyepiece elements are used to...

• control SA in exit pupil (does not have any relation to SA of axial beam)
• correct image distortion (barrel or pincushion)
• sometimes help to control field aberrations like astigmatism

In general, it can reduce number of optical surfaces reducing cost and indirectly - light scattering.

 

But to say honnestly I did not meet upper level astronomical eyepiece with aspherics 

I suspect that the technology that allows for widespread use of aspherics in eyeglasses and camera lenses isn't yet able to produce surfaces that are good enough for high-end (by amateur standards) optics for astronomy.

 

Grinding and polishing aspheres can produce lenses with incredible precision, but the methods used must still be too costly for the amateur market.

[Houdini]

Does anybody know an example of an eyepiece layout (with numerical data) containing an aspherical element?

I would love to test this.

Currently, like Ernest noted, there are no premium eyepieces that feature this.

[luxo II]

Baader made Aspheric 36 and 31mm 2" eyepieces, and there are others eg this https://www.amazon.c...y/dp/B0C3RRPT44

The issue is primarily that eyepieces are a low-volume niche product and we're already spoiled for choice with many excellent designs that don't use aspherics, so its not really got a lot going for it per se.

However I suspect the cheap ones may be using moulded plastic lenses, in which case aspherics are easily made. Bear in mind the optics for mobile phone cameras are really tiny, and do indeed have aspheric moulded plastic elements so the same technology could be applied to make very short eyepieces at very low cost. If you don’t mind plastic.

Edited by luxo II, 25 April 2025 - 06:43 AM.

[Lagrange]

With camera lenses there are three (or four) methods used to produce aspheres.

 

The traditional one and the highest quality and most expensive method is to grind and polish them to shape but it's much harder to do than making spherical lenses.

 

The next best option is moulded glass which is heated until it's soft, then put into a precision mould and pressed to shape before it's given a final polish. You need special glass for this and machining marks on the mould (presumably from diamond turning) leaves a circular pattern on the lens surface.

 

A cheaper option again is the hybrid aspheric where a spherical lens is coated with an optical resin that's pressed to shape in a mould and cured.

 

Finally there are plastic moulded lenses which are the cheapest and easiest to make. They work brilliantly in smartphone cameras and are probably used in some cheaper camera lenses, but they don't seem to be as good for larger lenses and high precision optics.

[davidgmd]

 

Afaik, refractors (like mine) don't have spherical aberrations.

 

…

…

 

(Context: got a 102mm achromat refractor, f=660mm…

   
Hi Dan, welcome to CN!

   

You’ve already got good answers to the main thrust of your question. I thought I’d comment on the above.

   
Lenses, and the refractors using them, are subject to spherical aberration (SA).  While apochromats and achromats are primarily designed to reduce chromatic aberration, they also reduce SA through the use of typically 2 (achromats) or 3 (apochromats) lenses with different curvatures in the objective. The original definition of an apochromat included reduction of both chromatic and spherical aberrations.

 
Theoretically, an aspheric lens could be used in an objective to reduce SA, though I don’t know if anyone is doing that. Aspherics are more difficult and expensive to produce than spherical lenses.
 

[davidgmd]

Just thought of an example where an aspheric objective is used. It's not for an astronomical telescope, but an indirect ophthalmoscope. That’s the instrument where the eye doc wears a funny-looking headpiece and holds a lens an inch or two from your eye while shining a blindingly bright light in your eye to observe your retina.

  
That single element objective lens held close to your eye is an aspheric, designed to reduce spherical aberration. It gives a sharper image of retinal details than a spherical lens.

[Tangerman]

If you're looking at the SVBony (and other brands may sell these as well) set of 4mm, 10mm, and 23mm aspheric eyepieces, unfortunately these aren't very good, though they are very cheap. The 4mm is essentially useless. I've yet to see a report of anyone getting anything remotely resembling a sharp image with it, and I certainly didn't succeed. I found the 10mm to be fine in isolation, but when compared to a basic GSO Plössl, I realized it was soft on-axis, which no eyepiece should be. I stopped using the 23mm before I could really compare it to anything else, but it seemed serviceable while I used it, though I wouldn't be surprised to find it's soft on-axis like the 10mm if I were to compare it to another eyepiece.

[j.gardavsky]

The Leica WW Aspheric zoom is an example of the premium eyepiece with the aspheric technology. Certainly not a plastic, as the Leica APO Televid has been designed according to the military optics specificatios.

 

The Carl Zeiss (West Germany) W-Pl microscopy eyepieces, also repurposed for the astronomy telescopes, have one aspheric surface.

 

DOCTER Company has developed their proprietary (OEM) aspheric surfaces technology, applied to the optical systems manufactured on contract.

 

The aspheric surfaces have been sometimes found in the other tactical optics eyepieces, designed for angular measurements in a wide field. The technology has been developed for the orthoscopic eyepieces designs (Abbe, Richter).

 

Best,

JG

[RichA]

There used to be four kinds of aspheric production technology:

1.  Molded plastic lenses.  The cheapest, found in some low-end eyepieces now and cheap camera lenses.

2.  Molded glass lenses.  More expensive, but not hyper-accurate.

3.  Hybrid plastic on glass molded lenses.  Used in some camera lenses.

4.  Ground glass aspheric lenses, most expensive.  Nikon Noct Nikkor lenses and Leica Noctilux camera lenses used one aspheric surface of this type:  $4000 and $10000, resp.

Edited by RichA, 25 April 2025 - 03:43 PM.

[j.gardavsky]

Here is a video of SCHOTT manufacturing the aspheres in Switzerland,

 

https://www.schott.c...gsaahvuealw_wcb

 

Enjoy the Swiss pronounciation of English,

JG

[Lagrange]

Here is a video of SCHOTT manufacturing the aspheres in Switzerland,

 

https://www.schott.c...gsaahvuealw_wcb

 

Enjoy the Swiss pronounciation of English,

JG

That's a good video. The tech is very clever but those production methods look like they'd be too costly to produce eyepiece lenses for the amateur market.

[havasman]

Yet another hobby spoiled by marketing evil.

Only if you let it be "spoiled".

In studies those most aware of and sensitive to advertising's manipulation are often found to be least manipulated by it. So there's another reason to chill out about it.

 

It's mass produced manufacturing, it's not prescription glasses...

Often high end eyepiece's lenses are better made than the lenses in prescription eyeglasses. Certainly not always but often.

And mass production is a relative term. The whole of our hobby's equipment demand would not make a day's production run under some definitions of it. Eyepiece production does not meet many definitions of mass production. An assembly line does not assure mass production. A well practiced group of Mexican grannies will use an assembly line to make tamales.

Edited by havasman, 25 April 2025 - 04:43 PM.

[j.gardavsky]

That's a good video. The tech is very clever but those production methods look like they'd be too costly to produce eyepiece lenses for the amateur market.

I agree with you,

but the eyepieces won't need any nanometer surface precission, unless the refractor lenses or the telescope mirrors have it.

 

Thank you for watching the video,

JG

[Jon Isaacs]

I agree with you,

but the eyepieces won't need any nanometer surface precision, unless the refractor lenses or the telescope mirrors have it.

 

Thank you for watching the video,

JG

 

Even then they probably don't need such precision.  They are only magnifying the image.. 

 

"It is not usually made clear, that these elements, objective and eyepiece, are by no means comparable in importance. The astronomer's hopes are almost wholly tied to the size and quality of the objectve. The objective of even the smallest telescope, because of its larger dimensions, the severe optical requirements it must meet, and the difficulty of its construction, completely overshadows the eyepiece."

- from "How to Make a Telescope," by Jean texereau, page 1, paragraph 2.

 

Jon