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Equipment Discussions >> Eyepieces

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Unexpected viewing with the ES 24mm 68° new [Re: great_bear]
      #5941953 - 06/26/13 06:51 PM

Quote:

Quote:


"Seagulls" would be from coma which is from the primary mirror, not the eyepiece.




Coma produces comet-tailed aberrations, not seagulls.
Seagulls are astigmatism - very much an eyepiece issue.



Well, an actual seagull shape can be a combination of coma and astigmatism.
If the sky background is bright, rather than look like a comet, coma may look like a "V" with its point aimed at the center of the field. The "V" will be narrow, though, and not as wide as >
If that "V" is widened until it has a seagull shape, it is likely to be a combination of coma and astigmatism.


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Scanning4Comets
Markus
*****

Reged: 12/26/04

Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5942141 - 06/26/13 09:11 PM

ASTIGMATISM:

http://tinypic.com/view.php?pic=hvuqzr&s=5

COMA:

http://tinypic.com/view.php?pic=6tkyzm&s=5


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csrlice12
Postmaster
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Reged: 05/22/12

Loc: Denver, CO
Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5942186 - 06/26/13 09:38 PM

Thanks Markus. Sometimes a picture is worth a thousand words.

Edited by csrlice12 (06/26/13 09:40 PM)


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5942195 - 06/26/13 09:45 PM

When you combine EYEPIECE astigmatism with coma, it does make "seagulls". TELESCOPE astigmatism produces images that are elongated in one axis that then flip to elongation of the other axis depending on focal position. Slightly inside of focus the astigmatic image will elongate on one axis and on the other side it will elongate on the other axis.

EYEPIECE astigmatism take takes a comet-shaped comatic image from the telescope and spreads out or fans the light of the "tail," forming seagulls and other similar distortions of the comet-like shape. Here are some examples:

EYEPIECE astigmatism:

http://www.telescope-optics.net/images/epa.PNG

TELESCOPE astigmatism:

http://www.telescope-optics.net/images/13d.PNG

TELESCOPE coma:

http://www.telescope-optics.net/images/11d.PNG

You can see how combining the first and third would produce fans or seagulls. Eyepiece astigmatism is common when a less well corrected eyepiece encounters a fast light cone. Most Dobs are fast scopes delivering a fast light cone. Fast dobs also show telescope coma from the steeply parabolized mirror. Telescope coma plus eyepiece astigmatism gives you little fans or seagulls off axis depending on the degree of each aberration and which one predominates. Haven't really checked to see how eyepiece astigmatism, telescope coma and telescope defocus manifest in off-axis SCT images. For me I think the defocus is so pronounced with wide field eyepieces that encourage me to care about off-axis image quality in my SCTs that I can't tell in what other ways the blobby defocused stars are also distorted.

- Jim


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Unexpected viewing with the ES 24mm 68° new [Re: jrbarnett]
      #5942280 - 06/26/13 10:55 PM

Very instructive pics and text, Jim and Markus!

Mike


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Scanning4Comets
Markus
*****

Reged: 12/26/04

Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5942328 - 06/26/13 11:32 PM

Great additions and images there Jim

Cheers,


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5948270 - 06/30/13 03:48 PM

Regarding astigmatism:

Astigmatism means that an optic has two different focal lengths which are at 90 degrees to one another. In the absence of other aberrations, it means that when one orientation is focused, the other orientation will be out of focus, creating a line. "Best focus" is somewhere in the middle.

Anyway I spent the past 4 nights, out under dark skies and did some comparisons and evaluations with the Meade 24mm SWA. Basically it's a good eyepiece.

16 inch F/4.42: Without the Paracorr, coma was visible.. maybe some other stuff. With the Paracorr (F/5.07), I saw no field curvature, the edge correction was about as good as it gets in a fast Newtonian.

TV NP-101. As previously noted, I saw no obvious field curvature but there are some small off-axis aberrations, sufficient to be noticeable but small enough that I could not determine their nature. Near as I could tell it was probably astigmatism. With the NP-101, I expect the off-axis stars to be as sharp and clean as those in the center, this eyepiece, this eyepiece was very good but not that good. In comparison, the sometimes maligned 22mm Panoptic showed perfect tight stars across the field of view.

In another forum, another fellow and I went round and round about the 16mm Series 5000 Meade. He claimed it had coma, I thought it must be astigmatism. I finally purchased a 16mm SWA simply to resolve this question. It did seem like it had coma but that it went away when used with a Paracorr.

It does seem unlikely but I wonder if the reverse coma an coma correct adds could also affect the eyepiece.

Jon

Edited by Jon Isaacs (06/30/13 03:51 PM)


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5948407 - 06/30/13 05:32 PM

Quote:

Regarding astigmatism:

Astigmatism means that an optic has two different focal lengths which are at 90 degrees to one another. In the absence of other aberrations, it means that when one orientation is focused, the other orientation will be out of focus, creating a line. "Best focus" is somewhere in the middle.

Anyway I spent the past 4 nights, out under dark skies and did some comparisons and evaluations with the Meade 24mm SWA. Basically it's a good eyepiece.

16 inch F/4.42: Without the Paracorr, coma was visible.. maybe some other stuff. With the Paracorr (F/5.07), I saw no field curvature, the edge correction was about as good as it gets in a fast Newtonian.

TV NP-101. As previously noted, I saw no obvious field curvature but there are some small off-axis aberrations, sufficient to be noticeable but small enough that I could not determine their nature. Near as I could tell it was probably astigmatism. With the NP-101, I expect the off-axis stars to be as sharp and clean as those in the center, this eyepiece, this eyepiece was very good but not that good. In comparison, the sometimes maligned 22mm Panoptic showed perfect tight stars across the field of view.

In another forum, another fellow and I went round and round about the 16mm Series 5000 Meade. He claimed it had coma, I thought it must be astigmatism. I finally purchased a 16mm SWA simply to resolve this question. It did seem like it had coma but that it went away when used with a Paracorr.

It does seem unlikely but I wonder if the reverse coma an coma correct adds could also affect the eyepiece.

Jon



John,
If the coma corrector corrects the coma from the objective, the image provided to the eyepiece will have no coma.
If you saw coma without a coma corrector and no coma with it, it isn't because the coma corrector canceled the coma in the eyepiece--it's because it canceled coma in the objective.
It's not totally impossible for an eyepiece to produce coma, but it is usually a characteristic of extremely simple lens designs, which the 16mm does not qualify for.

I've tested this eyepiece, and it has mild field curvature, a mild-to-moderate edge of field astigmatism, minimal lateral chromatic aberration, and mediocre suppression of off-axis or out-of-field scattered light, but decent suppression of peripheral light reflection off the top of the eyepiece. Its best performance wasn't in the NP101, but in my f/12.8 Maksutov. It was out-performed by a 16 Nagler, but at 3X the price.
If a coma corrector improved the images in this eyepiece and it was not coma you were talking about, it would likely to be the mild field flattening of the Paracorr at work. Or the mild f/ratio change affecting the induced astigmatism in the eyepiece from the light cone.


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5948860 - 06/30/13 11:11 PM

Quote:

John,
If the coma corrector corrects the coma from the objective, the image provided to the eyepiece will have no coma




Don:

That's how it seemed to me but consider this:

Imagine that you do have an eyepiece with coma. You also have a moderately fast Newtonian and a Paracorr. Now I believe that if the relationship between the Paracorr and the focal plane is not correct, then either too much negative coma is added or not enough negative coma is added. This would seem to depend on which side of the "proper relationship you are."

Now rather than adjusting the distance between the focal plane and the Paracorr so that it provides the appropriate amount of negative coma to correct only the mirror, you experimentally adjust the relationship between the Paracorr and the eyepiece so it minimizes the coma of the mirror + eyepiece.

Of course there are some assumptions about how the Paracorr works but it does seem at least plausible that the Paracorr could be adjusted to so that it adds more negative coma than is necessary to correct the mirror.

If that were the case, then correcting for coma in an eyepiece would seem to be possible.

Jon


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Scanning4Comets
Markus
*****

Reged: 12/26/04

Re: Unexpected viewing with the ES 24mm 68° [Re: Starman1]
      #5948869 - 06/30/13 11:16 PM

Quote:

Dave:

What scope(s) are you seeing the field curvature with?

I own the 24mm Meade version of this eyepiece and I have used with a number of scopes, most quite fast. It's not a perfect performer in the NP-101 (101mm F/5.4 Petzval), there is some slight residual what I remember to be astigmatism, it's not a clean as the 31mm Nagler, but it is a very good performing eyepiece, there aren't many better.

I spent one evening comparing the 24 mm Meade SWA to my 20mm Type 2 Nagler in a 16 inch F/4.42 + Paracorr, it was close... it's a good eyepiece in a fast scope.

Jon




Jon,

I have owned the 28mm Meade SWA and also the 28mm ES 68. You'd think that both were similar performers. However, in my fast scope, the Meade 28mm SWA showed a lot less coma than the ES 68 which is quite similar. I found that weird.

There must be something quite different in the two brands, (ES 68 / Meade SWA Series).

Cheers,


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