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Coma Correcting Eyepiece

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#26 Shneor

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Posted 13 February 2013 - 03:54 PM

And there is no coma at all visible when using my 9mm ES120° eyepiece in my 22" f/4. Stars are sharp from the center of the field to the edge. Jupiter looks exactly the same in the center of the field as it does at the edge except that it turns white at the edge (bands are still visible, shape is the same).

#27 Jon Isaacs

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Posted 14 February 2013 - 06:57 AM

And there is no coma at all visible when using my 9mm ES120° eyepiece in my 22" f/4. Stars are sharp from the center of the field to the edge. Jupiter looks exactly the same in the center of the field as it does at the edge except that it turns white at the edge (bands are still visible, shape is the same).



Are you seeing Airy disks with diffraction rings?

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#28 iluxo

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Posted 14 February 2013 - 07:14 AM

He's unlikely to see Airy disks or diffraction rings ever with a 22" - the seeing will never be good enough. So I doubt he can see if coma is present or not.

#29 Shneor

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Posted 14 February 2013 - 03:51 PM

He's unlikely to see Airy disks or diffraction rings ever with a 22" - the seeing will never be good enough. So I doubt he can see if coma is present or not.

Come is evident in other eyepieces, such as the ES20mm 100° and the ES25mm 100°. There is no coma visible in the 9mm ES120°. If you can find someone who has one, borrow it and put it in your focuser. You'll be amazed.

Clears,

#30 Jon Isaacs

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Posted 14 February 2013 - 04:48 PM

He's unlikely to see Airy disks or diffraction rings ever with a 22" - the seeing will never be good enough. So I doubt he can see if coma is present or not.

Come is evident in other eyepieces, such as the ES20mm 100° and the ES25mm 100°. There is no coma visible in the 9mm ES120°. If you can find someone who has one, borrow it and put it in your focuser. You'll be amazed.

Clears,


I wonder what would happen if it were in the focuser of a fast "petzval" refractor... There is coma from the mirror, it won't go away by itself. The only way I know of to correct for coma is to add coma of the opposite sign, i.e. a coma corrector or an eyepiece like the Pretoria.

I did the experiment a few years ago, the 9mm Nagler + Paracorr in an NP-101. With the Nagler alone, the field is flat and clean, it's to be expected. With the Paracorr + 9mm Nagler, the reverse coma is easily seen.

Jon

#31 FlorinAndrei

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Posted 17 February 2013 - 07:16 PM

And there is no coma at all visible when using my 9mm ES120° eyepiece in my 22" f/4.


Without a coma corrector, that's physically impossible. What you call "coma" is probably an aggregate of other types of aberration, some due exclusively to the eyepiece, some are global aberrations of the telescope (coma included, but not solely). Perhaps most of them are due to the lesser eyepieces not keeping up to the relatively steep focal ratio. Perhaps the scope is not collimated very well. Perhaps seeing or thermal issues just blur everything to kingdom come, making it impossible to make any relevant performance comparison.

But coma is already present in the real image formed in the focal plane of the telescope, before any eyepiece is involved. See the attached image, it shows coma in a 22" f/4 parabolic reflector at the edge of a 2" field stop, in the real image formed in the focal plane (no eyepiece involved). The little black circle is the Airy disk - the ideal image of a star in the absence of coma and turbulence. The big smear made of red dots is the actual image, bloated by coma. See how large the actual image is compared to the Airy disk?

And all this happens before the eyepiece. All the eyepiece can do is hope to add as little extra damage as possible to this already distorted image. But no eyepiece, even an expensive status-symbol piece of glass such as a 9mm ES120° can magically correct this aberration. For that, you need an actual coma corrector, such as a Paracorr.

Moreover, in reality air turbulence (seeing) would bloat this whole thing even more. A mediocre 1 arcsec seeing would bloat a perfect Airy disk about 5x bigger - see the green circle around the Airy disk.

So... you're justified to take pride in your optics, they are expensive and likely good performers, but don't expect that stuff to change the laws of physics.

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#32 Shneor

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Posted 17 February 2013 - 08:26 PM

And there is no coma at all visible when using my 9mm ES120° eyepiece in my 22" f/4.


Without a coma corrector, that's physically impossible. What you call "coma" is probably an aggregate of other types of aberration, some due exclusively to the eyepiece, some are global aberrations of the telescope (coma included, but not solely). Perhaps most of them are due to the lesser eyepieces not keeping up to the relatively steep focal ratio. Perhaps the scope is not collimated very well. Perhaps seeing or thermal issues just blur everything to kingdom come, making it impossible to make any relevant performance comparison.

But coma is already present in the real image formed in the focal plane of the telescope, before any eyepiece is involved. See the attached image, it shows coma in a 22" f/4 parabolic reflector at the edge of a 2" field stop, in the real image formed in the focal plane (no eyepiece involved). The little black circle is the Airy disk - the ideal image of a star in the absence of coma and turbulence. The big smear made of red dots is the actual image, bloated by coma. See how large the actual image is compared to the Airy disk?

And all this happens before the eyepiece. All the eyepiece can do is hope to add as little extra damage as possible to this already distorted image. But no eyepiece, even an expensive status-symbol piece of glass such as a 9mm ES120° can magically correct this aberration. For that, you need an actual coma corrector, such as a Paracorr.

Moreover, in reality air turbulence (seeing) would bloat this whole thing even more. A mediocre 1 arcsec seeing would bloat a perfect Airy disk about 5x bigger - see the green circle around the Airy disk.

So... you're justified to take pride in your optics, they are expensive and likely good performers, but don't expect that stuff to change the laws of physics.

The only visible aberration is lateral color in perhaps the outer 3-4% of the field - and the actual apparent field is in the neighborhood of 140°. What are you going to believe - your eyes or theory? There's no shape distortion, stars are just as sharp at the edge as in the center of the field. Try one and see.

Clears,

#33 careysub

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Posted 17 February 2013 - 08:59 PM

And there is no coma at all visible when using my 9mm ES120° eyepiece in my 22" f/4.


Without a coma corrector, that's physically impossible. What you call "coma" is probably an aggregate of other types of aberration, some due exclusively to the eyepiece, some are global aberrations of the telescope (coma included, but not solely). Perhaps most of them are due to the lesser eyepieces not keeping up to the relatively steep focal ratio. Perhaps the scope is not collimated very well. Perhaps seeing or thermal issues just blur everything to kingdom come, making it impossible to make any relevant performance comparison.

But coma is already present in the real image formed in the focal plane of the telescope, before any eyepiece is involved. See the attached image, it shows coma in a 22" f/4 parabolic reflector at the edge of a 2" field stop, in the real image formed in the focal plane (no eyepiece involved). The little black circle is the Airy disk - the ideal image of a star in the absence of coma and turbulence. The big smear made of red dots is the actual image, bloated by coma. See how large the actual image is compared to the Airy disk?

And all this happens before the eyepiece. All the eyepiece can do is hope to add as little extra damage as possible to this already distorted image. But no eyepiece, even an expensive status-symbol piece of glass such as a 9mm ES120° can magically correct this aberration. For that, you need an actual coma corrector, such as a Paracorr.

Moreover, in reality air turbulence (seeing) would bloat this whole thing even more. A mediocre 1 arcsec seeing would bloat a perfect Airy disk about 5x bigger - see the green circle around the Airy disk.

So... you're justified to take pride in your optics, they are expensive and likely good performers, but don't expect that stuff to change the laws of physics.

The only visible aberration is lateral color in perhaps the outer 3-4% of the field - and the actual apparent field is in the neighborhood of 140°. What are you going to believe - your eyes or theory? There's no shape distortion, stars are just as sharp at the edge as in the center of the field. Try one and see.


Could that 3 pound, $1000 hunk of glass also have coma correction features?

#34 Starman1

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Posted 17 February 2013 - 09:10 PM

I've got some on the way to us, so I plan to check it out. If the eyepiece has coma correction, it won't be usable with a Paracorr.
I'll report back when I get them. I'll be using it at f/5.

#35 FlorinAndrei

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Posted 17 February 2013 - 09:37 PM

If that particular eyepiece includes coma correction, which would make it quite unusual, then my conclusions are invalid, of course. Then the eyepiece would be equivalent to a regular eyepiece plus a Paracorr, so to speak.

The theory describing what happens in the focal plane remains correct. But in that hypothetical case, the image would be corrected after the focal plane.

#36 Astrojensen

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Posted 18 February 2013 - 02:59 AM

If the 9mm 120ES corrects coma in a newtonian, the eyepiece would also introduce coma in a refractor, which is *not* a desirable feature. Since we can quite safely assume that ES will also want to sell this eyepiece to owners of large refractors, I think it's pretty safe to assume that they don't have built-in coma correction.


Clear skies!
Thomas, Denmark

#37 Jon Isaacs

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Posted 18 February 2013 - 04:34 AM

The only visible aberration is lateral color in perhaps the outer 3-4% of the field - and the actual apparent field is in the neighborhood of 140°. What are you going to believe - your eyes or theory? There's no shape distortion, stars are just as sharp at the edge as in the center of the field. Try one and see.

Clears,



Shneor:

We all see things differently, have different sensitivities and levels of acceptable aberrations. If I remember correctly, you and I had a go around concerning Markus Ludes clone of the 30mm 80 degree WideScan II. It has been sold by a number of vendors, he sold it as the BW-Optik.

As I recall, you were quite happy with the edge correction in you F/4, I saw loads of astigmatism in my F/4. I also recall seeing comments by you about seeing very little coma at F/4 in eyepieces like the ES-20mm 100 degree eyepiece.

Coma is there, at F/4, the coma free field is only 1.4mm, the field stop of the 20mm ES is about 36mm..

Jon Isaacs

#38 Shneor

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Posted 19 February 2013 - 06:19 PM

Jon, this eyepiece is really, really different. Borrow one and see. Coma in the Widescan did not bother me, but it was present. Several other observers who observed through the 9mm were quite surprised. I was amazed to see that the image of Jupiter did not change shape from the center up to the very edge of the field - but it turned white, although surface detail visible was the same. I am not claiming "coma correction", as I don't know enough about optics. I'm just reporting what I see; and I'm not the only one who has reported that on CN.

Clears,

#39 Jon Isaacs

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Posted 19 February 2013 - 07:31 PM

Coma in the Widescan did not bother me, but it was present.



What the Widescan II had was an abundance of off-axis astigmatism...

Jon

#40 Shneor

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Posted 20 February 2013 - 04:31 PM

Coma in the Widescan did not bother me, but it was present.



What the Widescan II had was an abundance of off-axis astigmatism...

Jon

But that increased the apparent effect of of coma in the 18" f/4.5 I owned at the time.

Clears,

#41 Dave Mitsky

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Posted 22 February 2013 - 10:09 PM

Coma in the Widescan did not bother me, but it was present.



What the Widescan II had was an abundance of off-axis astigmatism...

Jon


As well as lateral color.

Dave Mitsky

#42 Jim Nelson

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Posted 23 February 2013 - 10:28 PM

I am not claiming "coma correction",


If you say there's no coma in the view, then you are claiming coma correction, because your scope has coma.

#43 Shneor

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Posted 25 February 2013 - 02:57 PM

I am not claiming "coma correction",


If you say there's no coma in the view, then you are claiming coma correction, because your scope has coma.

I can only repeat: put one in your focuser and see for yourself. I can only report what me eyes see. I did use it briefly in a 100mm f/5 refractor, and the quality of the view was similar - equally sharp stars from center to edge.

Clears,






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