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Wide field eyepiece for fast scopes - any suggestions?


RobertI

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Having recently posted a topic describing how I have never really been that fussed about eyepieces, I recently tried some rare wide(ish) field viewing with my C8, and when I examined the field of view, I was pretty unimpressed. I was using my F6.3 reducer/corrector and my Hyperion 21mm. Hyperions aren’t famed for their edge correction (so I believe) and the star shapes started to become very distorted about 50% of the way to edge. Not particularly pleasant for viewing large clusters and Milky Way. I swapped with my William Optics 20mm SWA 66 degree eyepieces from my binoviewers, and they were even worse! So I’m wondering, how can I get a nice flat field low power view with my C8 @F6.3?

So far I have narrowed the eyepiece choices down to the UFF 24mm 65 degrees (APM and Altair do these) at around £125 and the Explore Scientific 24mm 68 degree at around £175. My understanding is the UFF gives a flat field, which means no fishbowl effect or ‘field distortion’, but does produce less than pinpoint stars towards the edge. I believe the ES gives the reverse, some fishbowling, but pinpoint stars right to the edge.

So assuming my understanding about these eyepieces is correct, what do people think is a better experience - no ‘fishbowling’ or pinpoint stars to the edge? I’m inclined to say the latter, but interested in people’s experiences.

Also, are there other 24mm 68 degree eyepiece, or thereabouts, which would work well in fast scopes? I can’t afford a Panoptic btw! 🙂

 

 

Edited by RobertI
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I am very happy with an ES 68° 20mm with my C6 at f6.3. I used to have  the ES 24mm and it was flawless but a little on the heavy side for me.

I started with Hyperions and i agree they were abominable in faster scopes. Great at f13 though!

Also ... no fishbowling effect in my ES eyepieces.

Edited by Ags
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8 hours ago, RobertI said:

My understanding is the UFF gives a flat field, which means no fishbowl effect or ‘field distortion’, but does produce less than pinpoint stars towards the edge.

No, these are different things.

Field curvature/flatness is where the focus position is across the entire field. In a flat field scope and eyepiece the focus position in the centre of the field is the same in the centre of the FoV as at the edge and so stars that appear as pinpoints at the centre of the FoV also appear as pinpoints at the edge. In a scope/eyepiece with field curvature the focus position at the centre of the FoV is different to at the edge of the field. Stars appearing as a pinpoint at the centre become a small disk as they drift to the edge, but when they are at the edge you can wind the focuser in or out to make them a pinpoint again. If there is a small amount of field curvature it can often be reasonably overcome by focusing on a star 1/3 of the way from centre so that the image appears reasonably flat across the whole FoV. The UFF designers have prioritised keeping the image plane flat, but this does not tell you anything about any other distortion or aberrations that may exist. From experience I know that the 24mm ES68 has a small amount of field curvature (at f6) if you look for it, but this is quite minor and enough to be a reason to ignore it.

The opposite aberration to field curvature is astigmatism. This is where instead of a star being stretched into a circle by field curvature it is stretched in a manner that is not radially symmetrical. Stars can appear as lines, curves, crosses or seagulls. I do not recall this being noticeable in the ES68. By prioritising a flat field it is possible that the UFF shows some astigmatism at the edge. However, I recall that the UFF is quite a large eyepiece, I think even bigger than the ES68. This could well be the reason for that size: in order to control astigmatism whilst keeping the field flat the designer had to add more elements to the eyepiece. People who have actually used the eyepiece will have to advise on this although from the reputation of the eyepiece it will either be very minor or not noticeable.

When it comes to field distortions there are two distortions. Rectilinear distortion, where straight lines become curves at the edge of the field, and angular magnification distortion, where the magnification in the centre of the field is not the same as the magnification at the edge. One of these increases in the form y = x and the other with y = tan x and so once you have an eyepiece with an apparent field over ~40° you have to have one or the other, or a mix of both. It is impossible to correct for both. If you reduce one you will increase the other. Typically minimising rectilinear distortion is preferred for terrestrial targets and minimising angular magnification distortion is preferred for astronomy. The ES68 has a small amount of RD (and so must also have AMD) and so I have kept a Meade SWA (the predecessor to the ES68) for my spotting scope instead of using a 24 Pan which has lots of RD. I have not used a UFF to know what distortion balance their designer chose but in practice I am not really bothered by either for astronomy as I tend to focus on one target at a time instead of quickly sweeping across large star fields.

9 hours ago, RobertI said:

Also, are there other 24mm 68 degree eyepiece, or thereabouts, which would work well in fast scopes? I can’t afford a Panoptic btw!

No, these are the best two options where the choice is "not a Panoptic".

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Can you use 2" EPs? If so, both the 14 and 20mm StellaLyra LER/UWA are excellent. They show no edge of field aberrations in an F/6 scope - sharp to the edge. There are also 1.25" ones in the same range but I haven't tried those.

My favourite wide-field EPs are Baader Morpheus. Don't be put off by your Hyperion experience: these are a very different kettle of fish. Again, clear and sharp to the edge in my scope. YMMV.

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10 hours ago, RobertI said:

So far I have narrowed the eyepiece choices down to the UFF 24mm 65 degrees (APM and Altair do these) at around £125 and the Explore Scientific 24mm 68 degree at around £175. 

I went through a similar process a while ago looking for a maximum TFOV 1.25" EP - I settled on the the UFF in the end and I'm very happy with it, it performs equally well in both my usual fracs at F7 and F5.9. True to the name, the field is extremely flat all the way to the edge, and it seems that most real world tests show that the TFOV turns out to be very close to both the Panoptic and ES despite the AFOV being a few degrees less. The Pan is a bit lighter than the ES and UFF. 

The UFF also has quite long eye relief, so good if you wear glasses - I do not, but in another thread I discovered that I personally need *less* eye relief than some people, and the APM has an M43 thread under the supplied eyecup, so I replaced it with a Baader Morpheus eyecup plus extension. Helpful that you can tweak it if needed. 

The Panoptic has a lot of rectilinear distortion and I'm fairly sure I read the ES has some too - but an owner would have to confirm that. At any rate, the UFF has very little if any. 

One last thing that might be of interest: I have found that I much prefer EPs with large eye lenses from a comfort perspective. The UFF is about 30mm, approx the same as the Morpheus range (which I also really like). 

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2 hours ago, Ricochet said:

No, these are different things.

Field curvature/flatness is where the focus position is across the entire field. In a flat field scope and eyepiece the focus position in the centre of the field is the same in the centre of the FoV as at the edge and so stars that appear as pinpoints at the centre of the FoV also appear as pinpoints at the edge. In a scope/eyepiece with field curvature the focus position at the centre of the FoV is different to at the edge of the field. Stars appearing as a pinpoint at the centre become a small disk as they drift to the edge, but when they are at the edge you can wind the focuser in or out to make them a pinpoint again. If there is a small amount of field curvature it can often be reasonably overcome by focusing on a star 1/3 of the way from centre so that the image appears reasonably flat across the whole FoV. The UFF designers have prioritised keeping the image plane flat, but this does not tell you anything about any other distortion or aberrations that may exist. From experience I know that the 24mm ES68 has a small amount of field curvature (at f6) if you look for it, but this is quite minor and enough to be a reason to ignore it.

The opposite aberration to field curvature is astigmatism. This is where instead of a star being stretched into a circle by field curvature it is stretched in a manner that is not radially symmetrical. Stars can appear as lines, curves, crosses or seagulls. I do not recall this being noticeable in the ES68. By prioritising a flat field it is possible that the UFF shows some astigmatism at the edge. However, I recall that the UFF is quite a large eyepiece, I think even bigger than the ES68. This could well be the reason for that size: in order to control astigmatism whilst keeping the field flat the designer had to add more elements to the eyepiece. People who have actually used the eyepiece will have to advise on this although from the reputation of the eyepiece it will either be very minor or not noticeable.

When it comes to field distortions there are two distortions. Rectilinear distortion, where straight lines become curves at the edge of the field, and angular magnification distortion, where the magnification in the centre of the field is not the same as the magnification at the edge. One of these increases in the form y = x and the other with y = tan x and so once you have an eyepiece with an apparent field over ~40° you have to have one or the other, or a mix of both. It is impossible to correct for both. If you reduce one you will increase the other. Typically minimising rectilinear distortion is preferred for terrestrial targets and minimising angular magnification distortion is preferred for astronomy. The ES68 has a small amount of RD (and so must also have AMD) and so I have kept a Meade SWA (the predecessor to the ES68) for my spotting scope instead of using a 24 Pan which has lots of RD. I have not used a UFF to know what distortion balance their designer chose but in practice I am not really bothered by either for astronomy as I tend to focus on one target at a time instead of quickly sweeping across large star fields.

No, these are the best two options where the choice is "not a Panoptic".

Thanks for taking the time to explain, that's a nice simple summary. I think I probably confused myself by reading a review comparing the two eyepieces, and then trying to interpret the conclusions of that review based on insufficient knowledge (and the consumption of two beers). Sounds like you have found the ES68 to be a decent eyepiece.

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2 hours ago, cajen2 said:

Can you use 2" EPs? If so, both the 14 and 20mm StellaLyra LER/UWA are excellent. They show no edge of field aberrations in an F/6 scope - sharp to the edge. There are also 1.25" ones in the same range but I haven't tried those.

My favourite wide-field EPs are Baader Morpheus. Don't be put off by your Hyperion experience: these are a very different kettle of fish. Again, clear and sharp to the edge in my scope. YMMV.

I can use 2" eyepieces in the C8 but I understood that you get vignetting due to the limited size of the internal baffle of SCTs, but I wonder if this would happen with a 2" eyepiece of only 20mm? An interesting proposition as this would give a similar field of view to the 24mm 68 degree eyepieces, but at a slightly higher magnification.

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1 hour ago, badhex said:

I went through a similar process a while ago looking for a maximum TFOV 1.25" EP - I settled on the the UFF in the end and I'm very happy with it, it performs equally well in both my usual fracs at F7 and F5.9. True to the name, the field is extremely flat all the way to the edge, and it seems that most real world tests show that the TFOV turns out to be very close to both the Panoptic and ES despite the AFOV being a few degrees less. The Pan is a bit lighter than the ES and UFF. 

The UFF also has quite long eye relief, so good if you wear glasses - I do not, but in another thread I discovered that I personally need *less* eye relief than some people, and the APM has an M43 thread under the supplied eyecup, so I replaced it with a Baader Morpheus eyecup plus extension. Helpful that you can tweak it if needed. 

The Panoptic has a lot of rectilinear distortion and I'm fairly sure I read the ES has some too - but an owner would have to confirm that. At any rate, the UFF has very little if any. 

One last thing that might be of interest: I have found that I much prefer EPs with large eye lenses from a comfort perspective. The UFF is about 30mm, approx the same as the Morpheus range (which I also really like). 

Thanks, that's really useful, but also slightly annoying as I was erring toward the ES68! 😄 I also like large eye lenses - that is one thing I DO like about the Hyperion. I see that the Altair version of the UFF is quite a bit cheaper than the APM - assuming they are the same eyepiece (they seem to be - I think Celestron do a version too) can you think of a reason for this?

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Another ES eyepiece to consider might be the 26mmf/62° LER. It's lighter (220  vs 340g), and gives with my Heritage 130 P Flextube (f/5) almost the same field of view (2.48° vs 2.51°), and a magnification of 25x vs 27.1x. The price here in Germany is 125€ vs 185€ for the 24/68.

I appreciate that eyepiece; the views are very clear, sharp, (with beginning deterioration at about 80-85% of the TFoV) and without kidney beaning or other faults. It's a very relaxed viewing, with or without glasses,  way better than with my 20 mmf/66° UWA. It somehow reminds me of the older Zeiss binoculars - very sharp on axis, wide field, and the outer deterioration as a quite "natural" impression. And the eye lens diameter is 26 mm (just measured this). Members bomberbaz and greymouser have got one and may chime in. Hth.

https://www.teleskop-express.de/shop/product_info.php/info/p9542_Explore-Scientific-62--LER-Eyepiece-26-mm--argon-purged.html

Stephan

Edited by Nyctimene
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3 hours ago, RobertI said:

Thanks, that's really useful, but also slightly annoying as I was erring toward the ES68! 😄 I also like large eye lenses - that is one thing I DO like about the Hyperion. I see that the Altair version of the UFF is quite a bit cheaper than the APM - assuming they are the same eyepiece (they seem to be - I think Celestron do a version too) can you think of a reason for this?

It's probably because of that muddy green 😂

No but seriously, I am not sure why it might be cheaper, but the Celestron is a lot more expensive than any of them, again for reasons unknown. As far as I know, they are all identical aside from cosmetic differences, but I suppose there could be small manufacturing differences, or just possibly import costs - APM are German. 

I personally chose the APM because I prefer the black body of the EP to the other designs, as well as the fact that this design was originally commissioned by APM so I guess just showing my support. 

 

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5 hours ago, Nyctimene said:

Another ES eyepiece to consider might be the 26mmf/62° LER.

This is an eyepiece I have wanted to try for a while. Maybe not quite as optically good as the ES68 24mm but still very good apparently and a great weight and price.

Edited by Ags
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7 hours ago, RobertI said:

Thanks, that's really useful, but also slightly annoying as I was erring toward the ES68! 😄 I also like large eye lenses - that is one thing I DO like about the Hyperion. I see that the Altair version of the UFF is quite a bit cheaper than the APM - assuming they are the same eyepiece (they seem to be - I think Celestron do a version too) can you think of a reason for this?

I have the Altair version (and I like the green). Altair do their own versions of some other EPs, I assume they retail for a bit less as they're not so well known. And Celestron versions often seem to cost more. On the UFF, I have read that they're optically identical but the Altair version is heavier than the APM because the barrels use different metal.

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I have the ES 24mm 68deg eyepiece and it is my most used and favourite. It is quite a lump but also very well made and sealed so that I never worry about condensation. It was the first ES eyepiece that I bought and on the strength of it I have since bought another five. The wow moment for me with the ES 24mm was the much wider field of view and the crystal-clear view. I was coming from a Baader Hyperion Zoom at the time. The ES 24mm has a big eye lens which I agree is an advantage, and personally I find the eye relief, at 18.4mm, to be just right.

I don't have experience of the UFF 24mm 65deg, but 68deg gives a slightly wider field of view and is supposed to be the optimum for your eyes.

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23 hours ago, RobertI said:

Hyperions aren’t famed for their edge correction (so I believe) and the star shapes started to become very distorted about 50% of the way to edge

That’s a lot (50%) of the field to be seeing distorted stars with a Hyperion. I have tried them in both my f6 dobsonian and my f12 Maksutov. Whilst the stars started losing their form in the outer 15-20% of the f6 this was hardly noticeable in the f12. Could it be the collimation of your C8?, or maybe try without the reducer. It’s just that half the field being distorted sounds as though there’s something else at play.

   Ian 

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15 minutes ago, bosun21 said:

That’s a lot (50%) of the field to be seeing distorted stars with a Hyperion. I have tried them in both my f6 dobsonian and my f12 Maksutov. Whilst the stars started losing their form in the outer 15-20% of the f6 this was hardly noticeable in the f12. Could it be the collimation of your C8?, or maybe try without the reducer. It’s just that half the field being distorted sounds as though there’s something else at play.

   Ian 

Yes I have wondered whether something is going on, although my 50% figure is incredibly unscientific and probably exaggerated. Pretty sure collimation is good, out of focus stars are concentric and I was getting some really good detail in Jupiter the other day. The other thing was I was using a 2” SCT diagonal, which has a longer light path and might possibly make the issue worse with the corrector. I was thinking about trying with the 1.25” visual back and prism diagonal just to see if a noticeable difference. But I did read an old observing report where I noted it was not so good with the F6 72ED I used to have, so perhaps just not so good with faster scopes. I’ll also have a go with the F7 frac and F5 newt just to see. 

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Good post from Ricochet at the start of this thread. I’ve been testing all my eyepieces for edge of field star sharpness in an F/6 60ED refractor. The EPs tested included: 

TV Plossls - most of the range.    
TV Panoptics 27 and 24. 
TV Delites 18.2. 11, 7, 4, 3.   
Full set of Kasai orthos.       
Baader asph 36 and MkIV zoom.    
Leica and Zeiss microscope eyepieces.
APM 100 degree 20.    
Pentax XW5.    
Tak TOEs full set.

Interestingly, the only EPs that maintained sharpness across the FOV, controlling field curvature and astigmatism, without noticeable distortion, were the Delites (all superb, though a small bit of FC with the 18.2), the Pentax XW (superb, though with a touch of false colour at the edge), and the TOEs (all perfect in every way). Most of the Plossls and orthos were good, though not perfect, despite their limited AFOV. Good widefields like the Panoptics showed fuzzy stars at the edge, but they could be sharpened with a change in focus (proving that the problem was field curvature rather than astigmatism). However, they also show significant pin cushioning. Every eyepiece is a compromise. Doing these experiments proves once again that expensive eyepieces don’t necessarily perform well in all telescopes.

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Yes, correct.  If the scope has significant field curvature, like a 60mm F/6 that is not field flattened, a flat field eyepiece won't necessarily be the best image quality across the field.

| + ) = )

In narrow 45-60° fields, field curvature is less of a problem that when the apparent field gets wider.

The field stop widths of the eyepieces in question:

Panoptic 24mm--27.0mm

ES 24mm x 68--27.2mm

APM 24mm UFF--27.3mm

Having used all 3, it seems to me that if all 3 were stopped to 27.0mm, they'd all be sharp to the edge.

But the APM 24mm, though the most comfortable one of the 3 to use in the field, has the most issue with star image quality right at the edge.

I think that can be ignored because you don't seriously look at any object, even a double star, right at the very edge of the field.

It isn't as sharp at the edge as the Panoptic, but it does have noticeably less pincushion distortion.

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Did a bit more observing with the Hyperion 21 last night, with the C8 and F6.3 reducer/corrector. The first thing to note is that the previous time out I was using the double cluster to do the testing - I realise now this was a poor choice as the two clusters are spread out over the FOV and exaggerates any problems and not typical of most clusters I look at where the object sits in the middle of the FOV. So when I observed some other clusters tonight the effects were not so obvious, and the experience was much better.

So doing some closer examination this time, I found that the stars started losing definition from about 50% out from centre, but could be focussed with a small adjustment (implying field curvature?). When getting to the extreme outer edge, lots of seagulls appeared which changed shape when focussing (astigmatism?), but only at extreme edge and not noticeable in normal use. I think I could detect barrel distortion when panning, but not a problem for normal viewing of objects. I did try swapping from my 2" SCT diagonal to a 1.25" prism diagonal and if anything the issues were slightly worse, certainly no better. 

I changed the eyepiece to the 102ED F7, and the results were better, with the stars losing definition from further out from the centre (about 60-70%). Forgot to check the extreme edges. Will try with an F5 scope and see what happens.

So it was interesting to do these tests in more depth, and armed with a bit more knowledge from this thread, to really understand what's going on.

I now have a used ES68 24mm on the way hopefully, and will do a comparison with the Hyperion. :)   
 

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16 minutes ago, RobertI said:

could be focussed with a small adjustment (implying field curvature?). When getting to the extreme outer edge, lots of seagulls appeared which changed shape when focussing (astigmatism?)

Yes and yes. You've done some good testing there and should now be in a better position to appreciate the differences between the Hyperion and the incoming ES68 when you want to look at wider objects, or if you want to put the object off centre. 

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I use a TSFLAT2 substituting for the nosepiece of my GSO 2" Dielectric diagonal.  Since the diagonal has SCT threads, I use an SCT to M48 thread adapter, plus 15mm of M48 tube extension, to attach the M48 threaded TSFLAT2 in front of the diagonal.  This effectively flattens the field of view to unnoticeable levels visually.  I couldn't stand using my 72ED refractor for low power, wide field viewing until I figured out this hack.

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1 hour ago, Louis D said:

I use a TSFLAT2 substituting for the nosepiece of my GSO 2" Dielectric diagonal.  Since the diagonal has SCT threads, I use an SCT to M48 thread adapter, plus 15mm of M48 tube extension, to attach the M48 threaded TSFLAT2 in front of the diagonal.  This effectively flattens the field of view to unnoticeable levels visually.  I couldn't stand using my 72ED refractor for low power, wide field viewing until I figured out this hack.

Would love to see a couple of pictures of this setup Louis! 

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