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Fast newtonians, Paracorr and cheap eyepieces

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#1 Astrojensen

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Posted 29 December 2016 - 03:05 PM

A while ago, I promised someone (but I've forgotten who and where), that I would check out, whether a Paracorr improved cheap eyepiece performance in a f/5 newtonian. 

 

I had a chance to test it last night. I used a 12" f/5 Meade Lightbrigde, a Televue ParaCorr and a 20mm GSO Superview erfle eyepiece. First I checked with the eyepiece alone. As expected, the edges had the typical "seagulls" from the combined coma, field curvature and astigmatism. In the center 50° it wasn't bad at all, and the inner 40° was very sharp, but the edges got exponentially worse after that. I inserted the Paracorr and took another look. Now the stars along the edge were better, but they still weren't sharp at all, due to the field curvature inherent in the eyepiece. If I focused the stars near the edge, they turned into short streaks instead, due to the off-axis astigmatism because the erfle can't handle an f/5 light cone off axis. When the center was in focus, stars around the edge were sizeable blobs. Smaller than the "seagulls", but nowhere near a sharp point.  

 

The overall impression is that it wasn't really worth it. If you're using low-end eyepieces on your f/5 or faster newtonian, don't expect a Paracorr or other coma corrector to save them, because it won't. 

 

I also compared with an 18mm ES82 and the difference was ridiculously large. The performance of the 18mm ES82 *without Paracorr* was vastly superior to the performance of the 20mm GSO *with* Paracorr... 

 

Bottom line: Get a line of good flat-field eyepieces first, THEN a coma corrector. 

 

 

Clear skies!
Thomas, Denmark



#2 orlyandico

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Posted 29 December 2016 - 03:12 PM

what eyepieces are flat-field? are Plossls flat-field? what about the 68 degree eyepieces from the likes of ES?



#3 SteveG

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Posted 29 December 2016 - 03:50 PM

what eyepieces are flat-field? are Plossls flat-field? what about the 68 degree eyepieces from the likes of ES?

Many are well corrected for off axis astigmatism. Televue naglers and Panoptics are the benchline, and the ES line in 68, 82 and 100 degree eyepiece come very close at a substantial cost savings.



#4 havasman

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Posted 29 December 2016 - 04:21 PM

 

what eyepieces are flat-field? are Plossls flat-field? what about the 68 degree eyepieces from the likes of ES?

Many are well corrected for off axis astigmatism. Televue naglers and Panoptics are the benchline, and the ES line in 68, 82 and 100 degree eyepiece come very close at a substantial cost savings.

 

I think Steve has used the more descriptive term. If well corrected is substituted for flat field in Thomas' post then it is more easily understood. 

Still, Thomas' post makes a logical and valid point. I'm glad he did the work to test the concept.



#5 aeajr

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Posted 29 December 2016 - 04:57 PM

A while ago, I promised someone (but I've forgotten who and where), that I would check out, whether a Paracorr improved cheap eyepiece performance in a f/5 newtonian. 

 

I had a chance to test it last night. I used a 12" f/5 Meade Lightbrigde, a Televue ParaCorr and a 20mm GSO Superview erfle eyepiece. First I checked with the eyepiece alone. As expected, the edges had the typical "seagulls" from the combined coma, field curvature and astigmatism. In the center 50° it wasn't bad at all, and the inner 40° was very sharp, but the edges got exponentially worse after that. I inserted the Paracorr and took another look. Now the stars along the edge were better, but they still weren't sharp at all, due to the field curvature inherent in the eyepiece. If I focused the stars near the edge, they turned into short streaks instead, due to the off-axis astigmatism because the erfle can't handle an f/5 light cone off axis. When the center was in focus, stars around the edge were sizeable blobs. Smaller than the "seagulls", but nowhere near a sharp point.  

 

The overall impression is that it wasn't really worth it. If you're using low-end eyepieces on your f/5 or faster newtonian, don't expect a Paracorr or other coma corrector to save them, because it won't. 

 

I also compared with an 18mm ES82 and the difference was ridiculously large. The performance of the 18mm ES82 *without Paracorr* was vastly superior to the performance of the 20mm GSO *with* Paracorr... 

 

Bottom line: Get a line of good flat-field eyepieces first, THEN a coma corrector. 

 

 

Clear skies!
Thomas, Denmark

Thanks for posting this.  

 

I have an F 5.9 Orion XT8i.   Was wondering if there was any major benefit to these coma correctors.

 

 

Did you try the ES82 with the Paracor?  Any significant improvement?



#6 rowdy388

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Posted 29 December 2016 - 05:22 PM

My 2 cents: I agree with Thomas that a Paracorr does not *fix* a poorly corrected eyepiece. I've tried the experiment with a GSO 30mm

superview and a Zhumell 16mm 100 and find the view is still unacceptable to me.  A Paracorr in my Meade UWA's and ES 100, 82, & 68's

does, on the other hand, significantly improve the view. 



#7 MitchAlsup

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Posted 29 December 2016 - 05:24 PM

As expected, the edges had the typical "seagulls" from the combined coma, field curvature and astigmatism. In the center 50° it wasn't bad at all, and the inner 40° was very sharp, but the edges got exponentially worse after that. 

The image gets worse cubically (x**3) not exponentially (k**x) going off axis.



#8 turtle86

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Posted 29 December 2016 - 05:31 PM

A while ago, I promised someone (but I've forgotten who and where), that I would check out, whether a Paracorr improved cheap eyepiece performance in a f/5 newtonian. 

 

I had a chance to test it last night. I used a 12" f/5 Meade Lightbrigde, a Televue ParaCorr and a 20mm GSO Superview erfle eyepiece. First I checked with the eyepiece alone. As expected, the edges had the typical "seagulls" from the combined coma, field curvature and astigmatism. In the center 50° it wasn't bad at all, and the inner 40° was very sharp, but the edges got exponentially worse after that. I inserted the Paracorr and took another look. Now the stars along the edge were better, but they still weren't sharp at all, due to the field curvature inherent in the eyepiece. If I focused the stars near the edge, they turned into short streaks instead, due to the off-axis astigmatism because the erfle can't handle an f/5 light cone off axis. When the center was in focus, stars around the edge were sizeable blobs. Smaller than the "seagulls", but nowhere near a sharp point.  

 

The overall impression is that it wasn't really worth it. If you're using low-end eyepieces on your f/5 or faster newtonian, don't expect a Paracorr or other coma corrector to save them, because it won't. 

 

I also compared with an 18mm ES82 and the difference was ridiculously large. The performance of the 18mm ES82 *without Paracorr* was vastly superior to the performance of the 20mm GSO *with* Paracorr... 

 

Bottom line: Get a line of good flat-field eyepieces first, THEN a coma corrector. 

 

 

Clear skies!
Thomas, Denmark

 

No surprise there. Can't make a silk purse from a sow's ear! :grin:



#9 nicknacknock

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Posted 30 December 2016 - 12:56 AM

:funny:



#10 Ernest_SPB

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Posted 30 December 2016 - 02:27 AM

It is true.

 

20 mm GSO Superview (AFOV = 67 deg.) in F5 scope shows 40' aberration spot at edge FOV and 22' on 70% from center to edge (measured by me). Main aberrations are field curvature and astigmatism.

 

The same time angular size (in angular minutes) of coma spot in Newton is easy to calculate: 400*tan(FA)/FN2, where FA (field angle) 67/2=33.5 degree at FOV edge and 0.7*67/2= 23.5 degree on 70% zone, FN (focal number) = 5.

 

So coma will be seen as spot 10.5' at edge FOV and 7' on 70% zone. Compare with 40' and 22' of the EP and you will see that in F5 Newton coma of primary mirror is much lesser then field aberrations of Superview.

 

Paracorr or other coma corrector can remove primary mirror coma, but can not reduce own aberrations of Superview. And impressions from image quality with and without corrector will be almost the same.  


Edited by Ernest_SPB, 30 December 2016 - 02:20 PM.


#11 nicknacknock

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Posted 30 December 2016 - 02:30 AM

A Paracorr is not designed to fix the view when using eyepieces not specifically designed for fast scopes. It will work on the mirror coma and perhaps aid a bit with aberrations in an eyepiece by virtue of the 15% barlowing effect but if a pig is a pig, no paracorr will make it a beauty queen...

 

A Paracorr shines ONLY when used with eyepieces designed to operate in fast scopes as well, with the aim of fixing coma inherent in the mirror, NOT the eyepiece...



#12 precaud

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Posted 30 December 2016 - 12:15 PM

A Paracorr... will work on the mirror coma and perhaps aid a bit with aberrations in an eyepiece by virtue of the 15% barlowing effect but if a pig is a pig, no paracorr will make it a beauty queen...

True, but I've encountered a fair number of eyepieces whose astig and FC behavior are quite noticeably improved by this mere 15% barlow effect.

 

Most recent example is an 18mm Paradigm. Without coma corrector, it shows noticeable FC and astig in an F/5 newt. With CC, the FC all but vanishes but leaves some  astig. Put it in an F/6 newt w/o CC, and there is very little FC, astig is the predominant aberration. At F/7.5, astig is still present.

 

And then there are some eyepieces who don't seem to barlow as well as others. Some of the Sterling plossls come to mind. And TeleVue plossls.

 

Wouldn't it be nice if all manufacturers told us what focal length of scope each eyepiece was optimized / corrected for...


Edited by precaud, 30 December 2016 - 12:16 PM.


#13 Ernest_SPB

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Posted 30 December 2016 - 02:27 PM

 

Wouldn't it be nice if all manufacturers told us what focal length of scope each eyepiece was optimized / corrected for...

 

Most (and I even believe all) eyepieces are optimized for a scope with uncertain focal length, ideal image and telecentric off-axial rays.


Edited by Ernest_SPB, 30 December 2016 - 02:29 PM.


#14 precaud

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Posted 30 December 2016 - 05:36 PM

Most (and I even believe all) eyepieces are optimized for a scope with uncertain focal length, ideal image and telecentric off-axial rays.

OK. If so, then what is the meaning when it is said that a particular eyepiece is "well-corrected" for faster scopes?

 

I (perhaps naively) imagined that this meant the designer assumed a steeper angle of of-axis rays, and optimized for it.



#15 havasman

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Posted 30 December 2016 - 06:55 PM

John,

My understanding is the same.



#16 penguinx64

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Posted 30 December 2016 - 09:44 PM

I use a 3x barlow to improve the view of cheap eyepieces in my f/3.9 scope.  It helped some with a 15mm Expanse clone for example.



#17 Ernest_SPB

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Posted 31 December 2016 - 11:11 AM

 

Most (and I even believe all) eyepieces are optimized for a scope with uncertain focal length, ideal image and telecentric off-axial rays.

OK. If so, then what is the meaning when it is said that a particular eyepiece is "well-corrected" for faster scopes?

It means that it's own aberrations are just well corrected. 

 

If EP's aberrations are corrected bad or moderate it can perform on acceptable level only in conditions of slow scope (1:10-1:15). Only well corrected eyepieces can perform good in conditions of fast scope (1:4-1:6).



#18 precaud

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Posted 31 December 2016 - 01:44 PM

Thank you Ernest, that makes sense.

 

Most (and I even believe all) eyepieces are optimized for a scope with uncertain focal length, ideal image and telecentric off-axial rays.

OK. If so, then what is the meaning when it is said that a particular eyepiece is "well-corrected" for faster scopes?

It means that it's own aberrations are just well corrected. 

 

If EP's aberrations are corrected bad or moderate it can perform on acceptable level only in conditions of slow scope (1:10-1:15). Only well corrected eyepieces can perform good in conditions of fast scope (1:4-1:6).



#19 Achernar

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Posted 31 December 2016 - 01:49 PM

Those who expect a Paracorr or any other coma corrector to make up for bad optics, misaligned optics, or poorly corrected eyepieces are doomed to be disappointed. Anyone who has a telescope faster than about F/6 who wants good views across the field of view, or most of it has to pony up the money for eyepieces that are capable of it.

 

Taras


Edited by Achernar, 31 December 2016 - 01:50 PM.


#20 SteveG

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Posted 01 January 2017 - 11:39 PM

Those who expect a Paracorr or any other coma corrector to make up for bad optics, misaligned optics, or poorly corrected eyepieces are doomed to be disappointed. Anyone who has a telescope faster than about F/6 who wants good views across the field of view, or most of it has to pony up the money for eyepieces that are capable of it.

 

Taras

Basically this is it. I think a lot of people end up buying their coma corrector last, after a long chain of first getting their dob, premium mirrors, premium eyepieces, premium collimation tools, etc. All of these components have to be in place, at which the coma correct will show you perfection.



#21 Jon Isaacs

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Posted 02 January 2017 - 02:09 PM

The overall impression is that it wasn't really worth it. If you're using low-end eyepieces on your f/5 or faster newtonian, don't expect a Paracorr or other coma corrector to save them, because it won't.

 

:waygoto:

 

Thomas:

 

At various times I have used a variety of inexpensive 2 inch Widefields in my Newtonians with and without the Paracorr.  The 30mm GSO SuperView, the 26mm Orion Q-70, the 35mm Explore Scientific 70 degree, are some that I recall.  There is some improvement with the Paracorr, in part because of the 15% Barlow effect, or so I suspect.  But in general, it's not a lot, the eyepieces are still sharp in the center of the field and poorly corrected towards the edge.  

 

One story behind the Paracorr is that Uncle Al looked though a Nagler in a fast Newotnian and realized that what he was seeing was coma and not eyepiece astigmatism, it was time to design a coma corrector.  

 

My general thinking is that you want to start with "well corrected" eyepieces and then to take it to the next level, add the Paracorr.  

 

I do appreciate Ernest's analytical results, I like seeing numbers. :goodjob:  And they support my visual observations so I like that too.  If he were to show his equations and a sample calculation, I would take the time to try to understand it.. 

 

One final note..  My friend TomyKay has the ES-30mm 82 degree.  I have the 31mm Nagler.  Last year (yes 2016 is now last year) he was out with me at our place in the high desert and he had his 10 inch F/4.7 Orion Dob, no Paracorr.  He called me over to take a look at something and I was struck by the level of astigmatism, not a lot but noticeable.  So I grabbed my 31mm Nagler to see what was going on.  The Nagler was significantly cleaner, all I was seeing was coma.  I tried them in my scope, my 16 inch F/4.42, similar result.  

 

Now with the Paracorr, the coma was gone with the Nagler and the views were much improved in the ES 30mm 82 degree, it was much closer to the 31mm Nagler than it had been and quite acceptable to my eye.  Don Pensack makes that comment that when there are two aberrations of similar magnitudes the result can be much more apparent than either one alone.  I think that's what going on here.  

 

Jon



#22 Sarkikos

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Posted 03 January 2017 - 08:53 AM

 

A Paracorr... will work on the mirror coma and perhaps aid a bit with aberrations in an eyepiece by virtue of the 15% barlowing effect but if a pig is a pig, no paracorr will make it a beauty queen...

True, but I've encountered a fair number of eyepieces whose astig and FC behavior are quite noticeably improved by this mere 15% barlow effect.

The Paracorr 2 in my 10" f/4.8 Dob greatly improved the appearance of FC in my XW 20.  Without the Paracorr, the FC was terrible to my eyes, which can no longer accommodate well for the focus difference.  With the Paracorr, the appearance of FC was reduced to an acceptable level by focusing midway between center of field and edge of field.  That trick did not help without the Paracorr.

 

Mike


Edited by Sarkikos, 03 January 2017 - 08:54 AM.


#23 NOLAMusEd

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Posted 05 January 2017 - 08:06 PM

 

A while ago, I promised someone (but I've forgotten who and where), that I would check out, whether a Paracorr improved cheap eyepiece performance in a f/5 newtonian. 

 

I had a chance to test it last night. I used a 12" f/5 Meade Lightbrigde, a Televue ParaCorr and a 20mm GSO Superview erfle eyepiece. First I checked with the eyepiece alone. As expected, the edges had the typical "seagulls" from the combined coma, field curvature and astigmatism. In the center 50° it wasn't bad at all, and the inner 40° was very sharp, but the edges got exponentially worse after that. I inserted the Paracorr and took another look. Now the stars along the edge were better, but they still weren't sharp at all, due to the field curvature inherent in the eyepiece. If I focused the stars near the edge, they turned into short streaks instead, due to the off-axis astigmatism because the erfle can't handle an f/5 light cone off axis. When the center was in focus, stars around the edge were sizeable blobs. Smaller than the "seagulls", but nowhere near a sharp point.  

 

The overall impression is that it wasn't really worth it. If you're using low-end eyepieces on your f/5 or faster newtonian, don't expect a Paracorr or other coma corrector to save them, because it won't. 

 

I also compared with an 18mm ES82 and the difference was ridiculously large. The performance of the 18mm ES82 *without Paracorr* was vastly superior to the performance of the 20mm GSO *with* Paracorr... 

 

Bottom line: Get a line of good flat-field eyepieces first, THEN a coma corrector. 

 

 

Clear skies!
Thomas, Denmark

Thanks for posting this.  

 

I have an F 5.9 Orion XT8i.   Was wondering if there was any major benefit to these coma correctors.

 

 

Did you try the ES82 with the Paracor?  Any significant improvement?

 

At f/5.9, I would invest the money towards better eyepieces in lieu of a Paracorr or other coma corrector. I had an Orion XT8, and it was sharp with all my well corrected eyepieces. I sold my Paracorr T2 even though my current scope is faster than yours (f/4.7), because even though the stars were sharper at the edge, it wasn't worth the extra weight and hassle of changing settings for different eyepeces.




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