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A review of the 31 Nagler vs. the 32 Edmund Erfle

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#1 Olivier Biot

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Posted 07 August 2010 - 05:01 AM

A review of the 31 Nagler versus the 32 Edmund Erfle

#2 Ocram

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Posted 07 August 2010 - 05:47 AM

I read this during: :imawake:
My opinion: :goodjob:
What I expect what will happen to the reviewer: :tonofbricks:
What I am doing now: :gotpopcorn:

No serious, this confirms what I have seen more than once as well. You don't need to spend a fortune when picking EP's for your specific scope.
Naglers however are do-it-allways-very-well-in-every-scope EP's.
That's the price you pay for a certain convenience. Got an SCT with a full set or erfles and buy a very fast Dob, get thee some Naglers/Pentax/Ethos to keep your views sharp. Then a Dob becomes a more expensive proposition. When you stick to quality ultra-wide EP's that's not an issue. Nor is it when you only look on-axis...then you can keep your Erfles.

End rant, nice review, albeit a bit short

#3 Alex Gastélum

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Posted 07 August 2010 - 11:10 AM

I wish the review had a slower telescope on the comparison,
the reviews of this eyepiece in excelsis ratings are not bad.
Will the eyepiece suffer from more distortion on a faster scope?

#4 Mark Swanson

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Posted 07 August 2010 - 11:57 AM

I did make this review a bit short, but to answer your question this Erfle performs very well in Newt reflectors down to f4.5 and I have used it in an 94mm f7 Brandon refractor where it is very good across the whole field.

The truth is that it does even better in these latter 2 scope types than in an f10 SCT. That is because of the field curvature inherent in the SCT design.

I did compare this 32mm Erfle to a 32mm Televue Wide field in the 94mm f7 Brandon refractor and I thought the Edmund smoked it in every category. Contrast and sharpness and field of view.

Mark

#5 Alex Gastélum

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Posted 07 August 2010 - 12:21 PM

I have a refractor f6 and a dobsonian f4.5, do you recoment it for those?

#6 Mark Swanson

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Posted 07 August 2010 - 01:54 PM

I just submitted an update to my review that will answer your question. I am sure it will be posted soon. I have used this eyepiece in an f7 94mm Brandon refractor with great results. I would guess it would do OK in an f6, but maybe not quite as well at the edge.

Mark

#7 Mark Swanson

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Posted 07 August 2010 - 01:56 PM

I also used it in an 17.5" f4.5 reflector and it performed very well.You will here why in my update to my review.

Mark

#8 Olivier Biot

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Posted 07 August 2010 - 02:26 PM

I just submitted an update to my review that will answer your question. I am sure it will be posted soon. I have used this eyepiece in an f7 94mm Brandon refractor with great results. I would guess it would do OK in an f6, but maybe not quite as well at the edge.

Mark


The Aug 7 update has been added to the initial review.

#9 eric_zeiner

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Posted 07 August 2010 - 05:58 PM

The only one thing that I noticed is that the Edmund web site say that the AFOV is only 68 degrees as opposed to the 82 degrees listed in this review.

#10 Mark Swanson

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Posted 07 August 2010 - 06:36 PM

Yes, it does say that it is 68 degrees on the website. This was based on a prototype of this eyepiece and not the final product.The Edmund has a slighty larger true field than the Nagler 31.Look at the field stops I have listed under the specifications. Even Edmund says the eyepiece has a 42.5mm field stop. The 31 Nagler field stop is 42mm in diameter. To get the true field at the eyepiece you multiply the field stop diameter by 57.3 and then divide that by the focal length of the telescope in mm. A C8 with the Edmund would have a true field of 42.5 X 57.3 divided by 2032 = 72 arc minutes, or 1 degree and 12 minutes.

#11 eric_zeiner

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Posted 07 August 2010 - 06:57 PM

Yes, it does say that it is 68 degrees on the website. This was based on a prototype of this eyepiece and not the final product.The Edmund has a slighty larger true field than the Nagler 31.Look at the field stops I have listed under the specifications. Even Edmund says the eyepiece has a 42.5mm field stop. The 31 Nagler field stop is 42mm in diameter. To get the true field at the eyepiece you multiply the field stop diameter by 57.3 and then divide that by the focal length of the telescope in mm. A C8 with the Edmund would have a true field of 42.5 X 57.3 divided by 2032 = 72 arc minutes, or 1 degree and 12 minutes.


Thanks for the clarification and like a bonehead I did not even think to look at the field stops. :foreheadslap: How do you think these would perform in an f/5 or an f/6 refractor?

#12 Mark Swanson

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Posted 07 August 2010 - 09:54 PM

If it were a TV Genesis with the field flattener in it I would guess it would do OK. You know the 4 element refractor. I know it did well in an f7 refractor, but it may not be quite as good on edge with an f5 or 6 refractor.

Mark

#13 Alex Gastélum

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Posted 08 August 2010 - 12:16 AM

I was thinking on my triplet 80mm SV apo f6

#14 Mark Swanson

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Posted 08 August 2010 - 09:48 AM

My Brandon 94mm f7 was a Rolend Cristen cemented triplet lens. The Edmund did well in that one. You can pick these used Edmund's up on the internet for under $100. They sell new at Edmund for $225.

Mark

#15 Starman1

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Posted 08 August 2010 - 02:51 PM

Notes:
1) Spot diagrams for the Erfle design (the standard 5-element design, not the modified 6 element design) show:
f/10 on axis, 7 degrees off axis, 10 degrees off axis spots are smaller than the Airy disk.
f/5 even on axis, the spot is larger than the Airy disk.
Critical F/ratio = 8, meaning the eyepiece is not designed to handle f/ratios below f/8 without oblique rays causing astigmatism at the edge of the field.
The design has chromatic aberration on-axis. Red rays are not as focused as other colors. Fortunately, our night vision is insensitive to this color.
2) Spot diagrams for the Nagler design show:
--star images smaller than the Airy disk out well beyond 20 degrees off axis (this assumes the T5 design, on axis, is similar to the original type--Al Nagler told me it's actually smaller than this)
Critical f/ratio = 4

One can't deny what you experienced, but if the star images you saw were smaller in one eyepiece than the other, it may mean one eyepiece was not manufactured to design tolerances.
With such a low power, I wouldn't want to blame seeing conditions or mirror cooling, so the issue of spot size is difficult to address when both eyepieces should have displayed star images no larger than the Airy disk in your scope (which probably isn't visible at low power).

I've been observing a long time, and also have seen similar differences between eyepieces (spot size was one of the reasons I preferred Plossls to Abbe Orthoscopics, for example, and why I loved the Konig designs), even between different samples of the same eyepiece (!). Some of the eyepieces I've owned (and I've had a 31 Nagler for many years) had a noticeable variation from night to night--seeing conditions even affected the quality of my lowest power images. The edge of the field in the 31 Nagler seems to vary in quality for that reason, as well as my ability to accommodate a slightly curved focal plane, the tiredness of my eyes, the ability to achieve a sharp focus (somewhat difficult on short f/ratios), etc.

But I suspect the difference you saw may have been "sample-derived" rather than "design-derived", because the 31 Nagler I own is one of the sharpest eyepieces I've ever looked through, on-axis.

#16 Mark Swanson

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Posted 08 August 2010 - 05:30 PM

Don, I understand what you are saying about the spot size for the 5 element version, but this is a computer designed David Rank eyepiece and it has 6 elements.

On the Weatherman web site there is a review of the 31 Nagler versus the 30mm Leitz by Bob Luffel. I hope I spelled that right. He did a comparison of these two eyepieces and he gave the Nagler 31 the edge on the edges, but said that the Leitz was in his estimation about 20 to 25% sharper than the 31 Nagler in the central area. He ultimately chose the Leitz to keep for reasons that pertained to his own set of eyes. Not for any other reasons I just stated before this last reason. There was something about his own eyes that made the 30mm Leitz easier to view thru.

I have owned several different Edmund 32 Erfles over the years. I would sell and then buy one back. You know how it is in this hobby. The one I have now will never be sold. These are all the same eyepieces I am talking about.

I have a long history of watching this eyepiece destroy one eyepiece after another in the centerfield position.I read a thread on the internet that compared a 5 element 31mm Proxima Erfle to the 6 element 32mm Edmund Erfle and the reviewer said that the Edmund was sharper in the central area and that it would be a great eyepiece for looking at open clusters.

The 31 Nagler was very sharp in the center. I am saying that the Edmund was superhuman sharp in the central area.Tinier pinpricks.

I have a 20 year history of sticking one eyepiece after another in my Diagonal and watching it go down in flames against the 32mm Edmund Erfle in the centerfield area.

Mark

#17 Starman1

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Posted 08 August 2010 - 06:31 PM

You would love the standard Konig designs.
Don

#18 Jon Isaacs

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Posted 08 August 2010 - 06:49 PM

A few quick thoughts:

- I don't think anyone is seeing the Airy Disk with a 3.1mm exit pupil...

- Also, the rear port/rear baffle on an 8 inch F/10 SCT is 38mm (1.5 inches). The has to be some vignetting with a 42mm field stop.

Jon

#19 Daniel Mounsey

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Posted 09 August 2010 - 09:57 PM

A few quick thoughts:

- I don't think anyone is seeing the Airy Disk with a 3.1mm exit pupil...


Jon

Agreed

#20 Starman1

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Posted 10 August 2010 - 01:05 AM

Still, if the star points are smaller with one eyepiece, either one eyepiece has spots larger than the Airy disk on axis, or the eyepiece with larger stars has substantial spherical aberration, or the observer didn't achieve as tight a focus with one eyepiece as with the other.
You don't see the Airy disk as a disk at low powers, but you can definitely see bloated star images in comparison.
It's the why of it that interests me, because both eyepieces should display star images of equal size.

#21 Mark Swanson

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Posted 10 August 2010 - 10:45 AM

Don,

I just submitted info from Edmund to cloudynights that gives stats on spot size, distortion, and false color for this 32 Edmund Erfle. Maybe you can compare it to the 31 Nagler info you have.

Mark

#22 Larry Geary

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Posted 10 August 2010 - 11:25 AM

Perhaps the transmission losses in the Erfle are greater, resulting in dimmer stars which appear smaller.

#23 Jon Isaacs

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Posted 10 August 2010 - 11:40 AM

Another thoughts:

For me, the performance of a widefield eyepiece like the 31mm Nagler is of most importance in a fast scope simply because fast scopes provide the widest fields of view for a given aperture.

So of particular interest is the statement that the Edmunds Erfle eyepiece performs well in an F/4.5 Newtonian. I am curious as to the aperture of the Newtonian used for this test as well as what sort of coma corrector was used.

Jon Isaacs

Jon Isaacs

#24 Starman1

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Posted 10 August 2010 - 11:51 AM

Perhaps the transmission losses in the Erfle are greater, resulting in dimmer stars which appear smaller.

Interesting idea. It could be tested by looking for the faintest stars in the field and comparing those.

#25 Mark Swanson

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Posted 10 August 2010 - 06:08 PM

I don't think so. This eyepiece uses the finest anti-reflection coatings and has fantastic contrast.I would venture to say it may have more transmission than the 31 Nagler.
I think what is going on here is that it is difficult for some people to believe that there could be an eyepiece that is sharper than a Nagler at any position in the field. It should occur to people that to get that sharp of an edge in a nagler, that there might by necessity be a very tiny loss of sharpness at centerfield.The Nagler is very sharp. Just not quite as sharp in the center of the field as the 32 Edmund.

Mark






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