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Actually coma IS an attribute of eyepieces

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Newtonians don't ave a flat field,

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What? How so? Please explain.

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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).

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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.

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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.

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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,

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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

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

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

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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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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,

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

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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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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.

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

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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,

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

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

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

Jon

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

Clears,

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