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Nagler Coma

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#1 Enceladus96

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Posted 03 December 2021 - 03:29 AM

I have a 22 Nagler, 6 Delos and 13 Delite. A 9/10mm is next on the list. The only gripe I have with my eyepieces is the 22 Nagler does suffer from a slight bit of coma. Its definitely my most used eyepiece and It’s not that big of a deal to notice at F5.9 but you can definitely see it a bit especially on the edges. Does the 9mm suffer from that as well? Maybe I should just get another Delite or try a Morpheus? Stellarvue Optimus 9mm? I enjoy the click locks/twist ups from TV which is the reason why I haven’t cared to try anything else!
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#2 Miranda2525

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Posted 03 December 2021 - 04:58 AM

Hi Enceladus96,

 

The 22mm Nagler needs a coma corrector usually more at F/5 and below, but would also benefit from using a coma corrector at F/5.9. I owned the 22mm Nagler and really liked it at first, but after using it for a little while I noticed field curvature, and not coma.

 

Try focusing it out at the edges, and you will see that after doing that, the central stars will become out of focus. If you do the opposite, stars in the central area will be sharp, but stars at about the 60% or 60% mark will be out of focus.

 

I really like the feel of the 22mm Nagler and very wide "porthole into space" feel I got from it, but I could no longer tolerate the field curvature. Younger people can accommodate the field curvature and have a bigger focus shift than older people in their eyes.

 

I sold my 22mm Nagler and bought a 20mm APM 100 degree. It's a lot better for my eyes than the 22mm Nagler.

 

I use a GSO coma corrector with my 20mm APM 100 degree and I am liking it a lot more. The Televue Paracorr II is better than the GSO coma corrector. See if you can borrow one of those two coma correctors and try it out with your 22mm Nagler, you may just like it more!

 

I hope this helps!

 

Clear Skies!


Edited by Miranda2525, 03 December 2021 - 05:00 AM.


#3 Ernest_SPB

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Posted 03 December 2021 - 05:50 AM

It is not clear what models do you mean when specify these 9/10. 9 mm DeLite and 10 mm Delos?

There are a number of eyepieces that represent some coma (usually in a mix with astigmatism).

But both lines (DeLite and Delos) do not show any coma, just a little rest of astigmatism with field curvature + signs of lateral chromatism.


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#4 TOMDEY

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Posted 03 December 2021 - 05:53 AM

Hi, Enc!

 

The coma that you notice is coming from the telescope, not the eyepiece. Your 8" F5.9 Dobsonian needs a Coma Corrector to satiate use with the 22mm Nagler. Newtonian Field Coma (Seidel Lateral Magnitude of) is quadratically-proportional to F# and linearly-proportional to field. The 22mm Nagler Field Stop Diameter is 31.1mm, the 13mm Delite's is 13.8mm, and the 6mm Delos's is 7.6mm. So the coma ratios are Nagler/Delite 2.3x worse and Nagler/Delos 4.1x worse. But that is just revealing the coma of your telescope. Get the Paracorr Type 2 and all three eyepieces will perform magnificently.    Tom 


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#5 Ernest_SPB

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Posted 03 December 2021 - 06:31 AM

How much observer see coma of Newton appearance depends not from linear size of eyepiece field stop, but from its AFOV.

 

So 82-deg. eyepiece in Newton shows 82/72 = 1.14 (14%) more coma appearance comparing to 72-deg. eyepiece. 

Comparing to 62-deg. eyepiece 82-deg. represents 82/62 = 1.32 (32%) more coma of Newton's primary mirror.

 

Note, that an eyepiece shows not only Newton's coma, but can introduce own coma (especially high order coma).


Edited by Ernest_SPB, 03 December 2021 - 08:16 AM.

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#6 TOMDEY

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Posted 03 December 2021 - 08:44 AM

How much observer see coma of Newton appearance depends not from linear size of eyepiece field stop, but from its AFOV.

 

So 82-deg. eyepiece in Newton shows 82/72 = 1.14 (14%) more coma appearance comparing to 72-deg. eyepiece. 

Comparing to 62-deg. eyepiece 82-deg. represents 82/62 = 1.32 (32%) more coma of Newton's primary mirror.

 

Note, that an eyepiece shows not only Newton's coma, but can introduce own coma (especially high order coma).

But no, Earnest, that's wrong. The size of the (eyepieces') field stops define their observable limits on TFOV of the telescope objective in object-space, and that objective mirror is the source of the Telescope's coma. These three enumerated eyepieces have negligible coma of their own... the telescope entirely dominates; the Paracorr 2 corrects that.   Tom


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#7 Ernest_SPB

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Posted 03 December 2021 - 09:37 AM

But no, Earnest, that's wrong.

It is not wrong. 

 

Appearance of Newton coma (angular size of its spot - visible by observer's eye) is in linear dependence from eyepiece AFOV.

So short and long focal eyepiece with the same AFOV represent the same level of the Newton coma for observer's eye.

 

It is simple consequence from two facts: (1) linear dependence of coma spot size (measured in mm or micron) from axial distance and (2) the same linear dependence of scope magnification from eyepiece focal length.


Edited by Ernest_SPB, 03 December 2021 - 09:40 AM.

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#8 NC Startrekker

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Posted 03 December 2021 - 09:57 AM

Hi, Enc!

 

The coma that you notice is coming from the telescope, not the eyepiece. Your 8" F5.9 Dobsonian needs a Coma Corrector to satiate use with the 22mm Nagler. Newtonian Field Coma (Seidel Lateral Magnitude of) is quadratically-proportional to F# and linearly-proportional to field. The 22mm Nagler Field Stop Diameter is 31.1mm, the 13mm Delite's is 13.8mm, and the 6mm Delos's is 7.6mm. So the coma ratios are Nagler/Delite 2.3x worse and Nagler/Delos 4.1x worse. But that is just revealing the coma of your telescope. Get the Paracorr Type 2 and all three eyepieces will perform magnificently.    Tom 

You beat me to it Tom and glad you did.  Exactly what I was going to point out to the OP only with more specific data than I would have been able to provide.  



#9 NC Startrekker

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Posted 03 December 2021 - 10:03 AM

But no, Earnest, that's wrong. The size of the (eyepieces') field stops define their observable limits on TFOV of the telescope objective in object-space, and that objective mirror is the source of the Telescope's coma. These three enumerated eyepieces have negligible coma of their own... the telescope entirely dominates; the Paracorr 2 corrects that.   Tom

What Tom is pointing out is easily tested if you have access to a telescope such as the Tele Vue NP-101/127 which offers an essentially flat field due to its Petzal optical design.  There's a reason that Al Nagler and Team use a version of this telescope design to test all of their eyepieces.  In these scopes, if you see coma or field curvature, then it's coming from the eyepiece.  Either are "negligible" as Tom notes when I use my 22mm Nagler T4 in my NP-101.


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#10 Starman1

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Posted 03 December 2021 - 11:56 AM

But no, Earnest, that's wrong. The size of the (eyepieces') field stops define their observable limits on TFOV of the telescope objective in object-space, and that objective mirror is the source of the Telescope's coma. These three enumerated eyepieces have negligible coma of their own... the telescope entirely dominates; the Paracorr 2 corrects that.   Tom

Tom,

You are forgetting that the apparent size of coma is related to field size AND magnification.

The Comatic star is not a mathematical point, but has a size.

So the apparent visual size of coma is related to apparent field.

The linear size of a comatic star at the edge of the field in a 10mm 100° eyepiece is 1/2 the linear size of coma at the edge of a 20mm 100° eyepiece.

But magnification is doubled.  2 x 1/2 = 1.  The apparent visibility of coma is the same for all focal lengths of eyepieces as long as the apparent fields of the eyepieces remain constant.

 

So Ernest is correct--the visibility of coma is related to the apparent field of the eyepiece, not the field stop.

 

There might be a special circumstance if the field stop of the eyepiece is small enough there is no visible coma in the field.  But since the coma-free fields in newtonians are vanishingly small, this doesn't occur.

I see coma in the edge of the field of a 3.7mm eyepiece at f/5 without a coma corrector.  Even though the field stop is small, the magnification is very high.


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#11 Enceladus96

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Posted 03 December 2021 - 12:02 PM

Maybe I’m messing up the vocabulary, as you guys know a lot more than I ever will. I’ve only been doing this for a year haha. But the 22 Nagler is easily my favorite piece but the more I use it the more I notice it suffers from being 100% pinpoint across the field.
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#12 Starman1

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Posted 03 December 2021 - 12:03 PM

As for the 22mm T4 Nagler, I believe it has essentially zero inherent coma, so any visible coma seen will be from the scope.

At f/6, the coma free field is about 3.84mm wide, and the Nagler has a 31.1mm field stop.  Seeing coma from the scope isn't hard.

And a coma corrector makes the coma free field much larger than the field stop in the eyepiece.

 

I came to the conclusion years ago that all newtonian scopes need coma correction, even f/8 and f/9 if you want there to be no coma in the field at all

in a widefield or wider eyepiece.


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#13 Sarkikos

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Posted 03 December 2021 - 12:19 PM

I have a 22 Nagler, 6 Delos and 13 Delite. A 9/10mm is next on the list. The only gripe I have with my eyepieces is the 22 Nagler does suffer from a slight bit of coma. Its definitely my most used eyepiece and It’s not that big of a deal to notice at F5.9 but you can definitely see it a bit especially on the edges. Does the 9mm suffer from that as well? Maybe I should just get another Delite or try a Morpheus? Stellarvue Optimus 9mm? I enjoy the click locks/twist ups from TV which is the reason why I haven’t cared to try anything else!

The 22 Nagler doesn't suffer from coma.  Your telescope does.  You need a coma corrector.

 

Mike


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#14 alnitak22

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Posted 03 December 2021 - 12:20 PM

Maybe I’m messing up the vocabulary, as you guys know a lot more than I ever will. I’ve only been doing this for a year haha. But the 22 Nagler is easily my favorite piece but the more I use it the more I notice it suffers from being 100% pinpoint across the field.

It’s confusing, especially if one hasn’t been at it very long. The short answer is that the 22 Nagler is not the source of your not seeing pinpoint stars, but the mirror in your reflector is the culprit. It doesn’t help that over and over on threads you’ll read someone say that this or that eyepiece “needs” a coma corrector. No it doesn’t. The mirror in the scope needs the coma corrector, not the eyepiece. I have a 22 T4 although my reflector is only able to use 1.25” eyepieces. However in my TV85, stars are sharp except at the extreme edge but that’s due to the field curvature inherent in any 85mm f/7 refractor. I can tweak focus and get sharp stars at the edge which shows its field curvature of the refractor as You can’t focus coma away. In a friend’s f/8 Tak FS102, the field was essentially perfect. In my 6” f/12 Mak, there is no coma seen. If you have a chance to use your 22 N in a different type of scope sometime, you will probably be able to see different results than in your particular scope. And there’s nothing wrong with your scope...it’s a good one. There are just compromises that all designs have, most of which are easily addressed.


Edited by alnitak22, 03 December 2021 - 03:16 PM.

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#15 Enceladus96

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Posted 03 December 2021 - 12:20 PM

What scopes do you typically use the 22 in?

#16 alnitak22

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Posted 03 December 2021 - 12:22 PM

What scopes do you typically use the 22 in?

I use it almost exclusively in my TV85. I have tried it in my 6” Mak and it’s great in that scope as well...though I got the Mak to be a high power lunar/planetary scope and it’s excellent for that. Better than I had hoped.



#17 Sarkikos

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Posted 03 December 2021 - 12:23 PM

These three enumerated eyepieces have negligible coma of their own... the telescope entirely dominates; the Paracorr 2 corrects that.   Tom

Earnest didn't say that these three eyepieces show coma.  He said

 

 an eyepiece shows not only Newton's coma, but can introduce own coma (especially high order coma).

Mike


Edited by Sarkikos, 03 December 2021 - 12:24 PM.


#18 Enceladus96

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Posted 03 December 2021 - 12:24 PM

So pretty much mirrors in general suffer from it?

#19 Bener

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Posted 03 December 2021 - 12:26 PM

Totally out of my league here, but I wonder if perhaps the aberrations the OP is seeing may possibly be a function of collimation?



#20 alnitak22

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Posted 03 December 2021 - 12:28 PM

So pretty much mirrors in general suffer from it?

Yes, mirrors are the source of most coma by far. And especially mirrors from f/6 and lower. If your XT8 were say, an f/8 scope, the mirror may not produce coma that you could see. But again, you have a great scope, especially for a first one. Either just ignore the coma you see or get a Paracorr and be done with it! 


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#21 Sarkikos

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Posted 03 December 2021 - 12:30 PM

What Tom is pointing out is easily tested if you have access to a telescope such as the Tele Vue NP-101/127 which offers an essentially flat field due to its Petzal optical design.  There's a reason that Al Nagler and Team use a version of this telescope design to test all of their eyepieces.  In these scopes, if you see coma or field curvature, then it's coming from the eyepiece.  Either are "negligible" as Tom notes when I use my 22mm Nagler T4 in my NP-101.

But, how much of the coma produced by a telescope which has coma, is seen by the observer is determined by the AFOV of the eyepiece.

 

Mike


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#22 Sarkikos

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Posted 03 December 2021 - 12:35 PM

I just wish observers would stop saying that this or that eyepiece produces more or less coma.   Coma can come from an eyepiece, but that is not usually the case.   Coma comes from the primary mirror.

 

There was a poster on CN - now gone - that would tie himself into knots on this point.  He was constantly on the search for an eyepiece that would produce less coma in his Newt.  I think it was his way of avoiding the purchase of a Paracorr.  grin.gif

 

If you really want an eyepiece that shows less of the coma that is coming from the primary mirror - and you don't want to buy a coma corrector - then you need to get an eyepiece with a narrower AFOV.  The coma will still be there, but you will see less of it.  Maybe Plossls with 50 degrees or less?

 

Mike


Edited by Sarkikos, 03 December 2021 - 12:38 PM.

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#23 alnitak22

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Posted 03 December 2021 - 12:46 PM

Totally out of my league here, but I wonder if perhaps the aberrations the OP is seeing may possibly be a function of collimation?

Certainly possible. But if he doesn’t see collimation errors at higher power, probably not.



#24 alnitak22

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Posted 03 December 2021 - 12:51 PM

I just wish observers would stop saying that this or that eyepiece produces more or less coma.   Coma can come from an eyepiece, but that is not usually the case.   Coma comes from the primary mirror.

 

There was a poster on CN - now gone - that would tie himself into knots on this point.  He was constantly on the search for an eyepiece that would produce less coma in his Newt.  I think it was his way of avoiding the purchase of a Paracorr.  grin.gif

 

If you really want an eyepiece that shows less of the coma that is coming from the primary mirror - and you don't want to buy a coma corrector - then you need to get an eyepiece with a narrower AFOV.  The coma will still be there, but you will see less of it.  Maybe Plossls with 50 degrees or less?

 

Mike

Ah, our friend Scanningforcomets. Whom I renamed Scanningforcomments!  But he was far from alone in saying such and such an eyepiece “needs” a coma corrector. Even Roland chided commenters on the old Astromart forums for trying to assess eyepiece performance in an f/5 reflector without a Paracorr. Which was the only coma corrector around then. And I’m painfully aware some posters have no idea who Roland is.


Edited by alnitak22, 03 December 2021 - 01:10 PM.

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#25 25585

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Posted 03 December 2021 - 02:55 PM

As for the 22mm T4 Nagler, I believe it has essentially zero inherent coma, so any visible coma seen will be from the scope.

At f/6, the coma free field is about 3.84mm wide, and the Nagler has a 31.1mm field stop.  Seeing coma from the scope isn't hard.

And a coma corrector makes the coma free field much larger than the field stop in the eyepiece.

 

I came to the conclusion years ago that all newtonian scopes need coma correction, even f/8 and f/9 if you want there to be no coma in the field at all

in a widefield or wider eyepiece.

Even Maksutov Newtonians? I thought their meniscus lens eliminated coma.




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