ES82 VS MV68

[icebergahed]

I have been trying to compare these 2 explorer scientific eyepieces which look completely different on the outside.

I was wondering apart from the difference in fov is there anything else the maxvision 68 fall short on as it is considerably cheaper?

 

[Louis D]

The Maxvision 68s were from a cancelled Meade 5000 SWA order when Meade was bought by a Chinese competitor to the Chinese owner of ES.  They are optically identical or possibly better (according to some side by side comparison reports) than the ES-68 equivalents.  Eye relief is better because the eye lenses are flush mounted to the top of the housing rather than being recessed like the ES-68s.

The 68s are clones of the TV Panoptics while the ES-82s are inspired by the various TV Naglers, though none appear to be direct clones.  None are quite as well corrected to the edge as their TV equivalents.  However, they are about 90 to 95% of the way there.  Relative to each other, the 68s are a scaled, positive-only design with decreasing eye relief as the focal length decreases.  The 82s are a negative-positive design with more consistent eye relief across the focal lengths.  In fact, it appears that the 82s are actually several different designs optimized for different focal lengths.

It mostly comes down to a preference issue.  I find the 68s sharper in the center than the 82s.  Even at the edge, the 68s still have sharper stars.  Oh, and the 82s may have a ring of fire and/or edge of field brightening that is absent from the 68s.  Both issues seem to only be associated with negative-positive designs.

[Paul73]

They are both good. What scope are you using? 

The 68° MaxVisions are great value. Strangely, I prefer them to the ES 68°. Not sure of the differences in optics. Coatings maybe?

The 82° Explore Scientifics are (in my view) a step up in quality. Where the longer MaxVisions struggle with field curvature in faster scopes, the ES fuels is better controlled and the extra fov provides a more immersive experience. But, as you pointed out, they are much more expensive. I haven’t heard of, or witnessed, the ‘ring of fire’ with either of these ranges (mentioned by the previous poster). Has anyone else seen this?

Paul

[icebergahed]

3 hours ago, Paul73 said:

They are both good. What scope are you using? 

The 68° MaxVisions are great value. Strangely, I prefer them to the ES 68°. Not sure of the differences in optics. Coatings maybe?

The 82° Explore Scientifics are (in my view) a step up in quality. Where the longer MaxVisions struggle with field curvature in faster scopes, the ES fuels is better controlled and the extra fov provides a more immersive experience. But, as you pointed out, they are much more expensive. I haven’t heard of, or witnessed, the ‘ring of fire’ with either of these ranges (mentioned by the previous poster). Has anyone else seen this?

Paul

Hi Paul 

I have a nextstar 6SE f10 but i have a f6.3 reducer also.

Thanks

[Ben the Ignorant]

The Maxvision 34mm/68° is my favorite widefield eyepiece in the long focal lengths.

 

[Louis D]

8 hours ago, Paul73 said:

Where the longer MaxVisions struggle with field curvature in faster scopes

I have the original 40mm Meade 5000 SWA, and there is no field curvature.  Field curvature has nothing to do with the f-ratio and everything to do with the design of the scope.  Fast scopes of any design can be flat field if designed to be so.

5 hours ago, icebergahed said:

I haven’t heard of, or witnessed, the ‘ring of fire’ with either of these ranges (mentioned by the previous poster). Has anyone else seen this?

It's mostly visible in the daytime or on bright objects like the moon at the edge of field.  Ernest on CN shows the cause of it in this particular eyepiece in this post.  It's actually best referred to as chromatic aberration of the exit pupil (CAEP).  Basically, not all wavelengths of light are focused to the same point.  Some are closer, some are farther.  There is no one pinpoint of light at a given eye relief distance.  The eye relief actually occurs over a range of distances depending on the particular color of the light.  This manifests itself as stars never coming to a pinpoint focus anywhere in the field of view.  Rather, they reach a smallest circular size, with the color changing to blue on one side and red on the other side of best focus.

[Paul73]

At f10 or f6.3, you’ll be happy with either.

Paul

[Paul73]

@Louis D - A good succinct explanation of CAEP. Thank you.

As I understand it, The wider the fov (and the longer the eyepiece mm) the harder it is to get all of the wavelengths focused to the same plane. The 30mm ES 82° is a fine eyepiece but definitely has some CAEP (as shown in the CN post)

What I mean by ‘curvature’, is where the stars look sharp, but as if they are painted onto a concave surface. For the 68° MaxVisions, this is absent from the 16&20mm’s, visible but not too intrusive in the 24mm and quite noticeable in the 28mm (I haven’t used the 35 or 40mm). As a direct comparison, the TV Panoptic 24mm 68° dosn’t exhibit this effect and the ES 82° 24mm shows very very little.

I am not an optics expert, so please forgive my clumsy descriptions.

Paul

[Louis D]

16 minutes ago, Paul73 said:

What I mean by ‘curvature’, is where the stars look sharp, but as if they are painted onto a concave surface. For the 68° MaxVisions, this is absent from the 16&20mm’s, visible but not too intrusive in the 24mm and quite noticeable in the 28mm (I haven’t used the 35 or 40mm). As a direct comparison, the TV Panoptic 24mm 68° dosn’t exhibit this effect and the ES 82° 24mm shows very very little.

Aha, what you meant was pincushion distortion (rolling ball or globe distortion).  It's not an aberration because all light focuses to a point everywhere, just not where you might expect it to be within the formed virtual image.  The Panoptic line is famous for having strong pincushion distortion.  It helps it have better edge correction.

What I thought your were referring to was curvature of field which means that the image plane that the eyepieces focuses on is not planar but curved.  This is fine if it matches the curvature of the image plane of the telescope, but bad if they're opposites.  The net effect is that you have to refocus from center to edge to see everything sharply.

[Stu]

2 hours ago, Louis D said:

Field curvature has nothing to do with the f-ratio and everything to do with the design of the scope.  Fast scopes of any design can be flat field if designed to be so.

My understanding (and experience) is that ED doublets, for instance, suffer more field curvature at shorter focal lengths. I would agree that you can achieve a flat field by design, but this involves more complex designs such as petvals or other quad designs with a flattener and/or reducer doublet at the rear of the scope. I don't think it is possible to produce a short focal length ED doublet with a flat field?

[Ricochet]

3 minutes ago, Stu said:

My understanding (and experience) is that ED doublets, for instance, suffer more field curvature at shorter focal lengths. I would agree that you can achieve a flat field by design, but this involves more complex designs such as petvals or other quad designs with a flattener and/or reducer doublet at the rear of the scope. I don't think it is possible to produce a short focal length ED doublet with a flat field?

Yes, for a doublet the radius of curvature is approximately 1/3 of the focal length so a shorter scope will have more field curvature. Note that this is not the same as a faster scope having more field curvature as a telescope with a larger aperture could be faster whilst still having a longer focal length.

[Louis D]

32 minutes ago, Ricochet said:

Yes, for a doublet the radius of curvature is approximately 1/3 of the focal length so a shorter scope will have more field curvature. Note that this is not the same as a faster scope having more field curvature as a telescope with a larger aperture could be faster whilst still having a longer focal length.

Correct, field curvature is related to focal length, not focal ratio for a given design.

42 minutes ago, Stu said:

My understanding (and experience) is that ED doublets, for instance, suffer more field curvature at shorter focal lengths. I would agree that you can achieve a flat field by design, but this involves more complex designs such as petvals or other quad designs with a flattener and/or reducer doublet at the rear of the scope. I don't think it is possible to produce a short focal length ED doublet with a flat field?

Yes, you need additional elements to achieve a flat field unless you resort to aspherical surfaces.

There may be something to all negative-positive type eyepieces (such as the ES-82s) handling field curvature better than positive-only eyepieces (such as the ES-68s) because they effectively extend the focal length, decreasing field curvature, as seen by the positive section of the eyepiece.  I see something similar when it comes to coma correction in my 10mm Delos.  Stars are pinpoint at the edge with or without a coma corrector at f/6 in my Dob.  Coma decreases with increasing f-ratio in Newts, which is what the negative section of the Delos effectively does, increase f-ratio since the aperture remains the same but the focal length gets longer.

[Stu]

10 minutes ago, Louis D said:

Correct, field curvature is related to focal length, not focal ratio for a given design.

Yes, you need additional elements to achieve a flat field unless you resort to aspherical surfaces.

There may be something to all negative-positive type eyepieces (such as the ES-82s) handling field curvature better than positive-only eyepieces (such as the ES-68s) because they effectively extend the focal length, decreasing field curvature, as seen by the positive section of the eyepiece.  I see something similar when it comes to coma correction in my 10mm Delos.  Stars are pinpoint at the edge with or without a coma corrector at f/6 in my Dob.  Coma decreases with increasing f-ratio in Newts, which is what the negative section of the Delos effectively does, increase f-ratio since the aperture remains the same but the focal length gets longer.

Is it the case that if the curvature of the eyepiece is the same as the scope then it makes matters worse, if it is opposite curvature then it can cancel out and result in an effective flat field?

I think some of the longer focal length Pentax eyepieces do perform poorly in some scopes, possibly because they are designed to function well in short focal length scopes so that they show curvature in Newts for example?

[Ricochet]

34 minutes ago, Stu said:

Is it the case that if the curvature of the eyepiece is the same as the scope then it makes matters worse, if it is opposite curvature then it can cancel out and result in an effective flat field?

Yes, the field curvature of the eyepiece can add or subtract from that of the telescope. 

34 minutes ago, Stu said:

I think some of the longer focal length Pentax eyepieces do perform poorly in some scopes, possibly because they are designed to function well in short focal length scopes so that they show curvature in Newts for example?

I gather that the long XWs are worse in short scopes with high field curvature. In my dob (fl/radius of curvature = 1218mm) I don't mind the curvature of the XW14. I focus slightly off centre if needed but I don't think it's as bad as the reports had me believe. Maybe as I age and my eyes become more sensitive to it I'll change my opinion but for now the near perfect performance in all other areas makes a slight tweak on the focuser an acceptable compromise. 

[Paul73]

Matching eyepieces to the natural light paths of the scope sounds like a key consideration.

I often wonder about the frac owner recommending x,y,z eyepieces to dob users. Of course, there are those who have experience across a wide range of scopes and eyepiece ranges. A good handful of names spring to mind on both sides of the pond. I won’t embarrass anyone by mentioning names.

The afore mentioned 10mm Delos behaves differently between scopes of differing designs. Great for delving into the guts of a big nebula at x160, open clusters less so in my biggish dob. No probs in a 600mm fl frac.

Great thread - i’m Certainly learning things.

Paul

PS. I’be got to do some proper comparisons now! Boring, but probably quite illuminating.

 

[Louis D]

I'm going from memory, but my recollection is that both refractors and Newts have curved focal planes that curve away from the eyepiece such that you have to move the eyepiece inward to bring the edges to focus.  It's just that refractors have way more curvature for a given focal length than a Newt.

The focal curvatures would have to match to cancel.  If the eyepiece curvature was such that it curves away from the eyepiece, it would better match the natural curvature of refractors and Newts.  If they're opposite curvature, the best you can hope for is focusing 50% of the way out to the field stop to maximize what is in focus.  Only that ring at 50% would be in best focus.  Everywhere else would be either in front of or behind best focus, depending on which side of the circle at 50% you are looking at.

[YKSE]

42 minutes ago, Louis D said:

I'm going from memory, but my recollection is that both refractors and Newts have curved focal planes that curve away from the eyepiece such that you have to move the eyepiece inward to bring the edges to focus.  It's just that refractors have way more curvature for a given focal length than a Newt.

The focal curvatures would have to match to cancel.  If the eyepiece curvature was such that it curves away from the eyepiece, it would better match the natural curvature of refractors and Newts.  If they're opposite curvature, the best you can hope for is focusing 50% of the way out to the field stop to maximize what is in focus.  Only that ring at 50% would be in best focus.  Everywhere else would be either in front of or behind best focus, depending on which side of the circle at 50% you are looking at.

As you know, our memory can fail, better to go from basics

Compare to a refractor, all Newts can treated as virtually flat focal planes, because a Newt has curvature radius as its focal length, e.g. even a small 130mm f5 newt, its curvature radius is to 650mm, that's about to equal a refractor of 2000mm focal length, in another words, much flater focal plane than most refractors on market.

As explained here:

https://starizona.com/acb/basics/equip_optics101_curvature.aspx

Focusing half way to handle FC is a method some prefer, I'm not convinced it's the best method, yes, you see about equally round stars, that, in my world, is not sharp to the edge, it's less sharp everywhere. I'd rather has pin-sharp on-axis view, because as long as the system (scope+ep+diagonal etc) has sphaerical aberration under controll, it's the best quality view you can get. All other aberrations, such as CA or FC from refractor, coma from newts, astigamastism, CA, distortion (Yes, distortion is a kind of aberration too) from EPs, are off-axis aberrations, they donot exist on-axis.