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Mark Swanson
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Starman1]
      #3974601 - 08/10/10 11:45 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


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Larry Geary
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Starman1]
      #3974686 - 08/10/10 12:25 PM

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

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Jon Isaacs
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Larry Geary]
      #3974719 - 08/10/10 12:40 PM

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


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Starman1
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Larry Geary]
      #3974744 - 08/10/10 12:51 PM

Quote:

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.


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Mark Swanson
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Larry Geary]
      #3975454 - 08/10/10 07:08 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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Mark Swanson
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Jon Isaacs]
      #3975476 - 08/10/10 07:19 PM

It was in a 17.5" f4.5 Coulter Dob. I did not have a coma corrector in it at all. I had put on a 2" tectron focuser.

I compared it to a 32mm Televue wide field. It was not one of the smoothside wide fields from Japan, but a later version with rubber eyecups. I can confirm that the later versions were not as good as the smooth side. I have looked through both.I had a smooth side, but sold it to buy the edmund 32 erfle. At a star party I compared someone else's 32 televue to my Edmund 32. It was the rubber eyecup version and the Edmund outperformed it on contrast and sharpness.This Edmund works very well in reflectors and I was pleased with the incredible wide field with the edmund in that dob. I thought the whole field was useable.

Mark


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Jon Isaacs
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Mark Swanson]
      #3975550 - 08/10/10 08:01 PM

Quote:

It was in a 17.5" f4.5 Coulter Dob. I did not have a coma corrector in it at all.




Here is a calculation: The coma free field of an F/4.5 is about 1.6mm. In a 17.5 inch this corresponds to a field of view of 2.75 arcminutes. A 31mm eyepiece would provide about 64x and so the diffraction limited, coma free apparent field of view would be a 3 degree, a pretty small portion of a 82 degree AFoV.

I honestly don't see how anyone can draw conclusions about on-axis sharpnes sharpness in an F/4.5 Newtonian without using a coma corrector, particularly in a Dobsonian when the diffraction limited field is only 3 degrees.

YMMV

Jon Isaacs


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Starman1
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Mark Swanson]
      #3975577 - 08/10/10 08:15 PM

If you thought the whole field of view of a low power Erfle was "usable" at f/4.5 without a coma corrector, then you must have really lax standards about star image quality at or near the edge of the field.
Because the spot diagram of an Erfle at f/5 shows the star images to exhibit severe chromatic aberration:
http://www.telescope-optics.net/eyepiece_aberration_2.htm
and even worse coma from the scope:
http://www.rfroyce.com/mirror_performance.htm
not to mention field curvature from the scope:
http://starizona.com/acb/basics/equip_optics101_curvature.aspx
The last time I tried the 31 Nagler at f/5, without coma correction, the star images at the edge of the field were easily 10X as large as the star images in the center.
Now you have me interested in seeing how good the Erfle is WITH coma correction.


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Mark Swanson
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Re: A review of the 31 T5 vs. the 32 Edmund Erfle new [Re: Starman1]
      #3975704 - 08/10/10 09:20 PM

I had this dob in the early nineties and at one time I did own a 2" Paracor. I may be mistaken on saying that i did not use a coma corrector. I did use this scope without a coma corrector for a while and added the coma corrector later. I could have had it in place when I used the Edmund 32 in that dob. I do remember it being a good view, so I probably did have the Paracor in the scope.

Mark

Mark


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John Rhodes
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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Olivier Biot]
      #3977226 - 08/11/10 03:57 PM

Oliver, Group,
Noting the review and 8/7/10 update that states that the 31mm Nagler isn't sharp on axis in comparison, we must question the report since in his f/10 SCT, the 31mm Nagler has a Strehl ratio on-axis of over 99.9% and no visible color.  Anecdotally we've had no returns or comments at Tele Vue about lack of axial sharpness.  The comment the author makes in this thread regarding the possibility of having to give up axial sharpness for edge field sharpness is one of those hackneyed old tales that needs to be laid to rest.  Kind of like the notion that Barlows degrade the image, you can't make a good zoom eyepiece, and fast refractors can't have good color correction, it ain't so.


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Mark Swanson
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Reged: 03/09/07

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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: John Rhodes]
      #3978120 - 08/12/10 12:24 AM

John,
I never said the Televue 31 Nagler was not sharp on axis in comparison. They are both very sharp on axis. I am certain that the Edmund was slightly sharper. If you don't believe me go out and buy an Edmund 32 Erfle and take the plunge. Compare both eyepieces and then honestly tell me the 31 Nagler is sharper on axis.

Don't rely on Televue's reputation only. Go do the test yourself. I did.

Go to the weatherman web site and read the review by Bob Luffel comparing the 30mm Leitz 88 degree eyepiece in which he said that the 30mm Leitz was in his estimation about 20 to 25% sharper on axis than the 31mm Nagler. He gave the edges to the 31 Nagler.

I am not the only one with eagle eyes.

Mark


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NorthWolf
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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Mark Swanson]
      #3978161 - 08/12/10 01:06 AM

Pentax XW 30mm FTW! 1.63 lbs of Crystal.

88 degrees is awesome though.


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Mark Swanson
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Loc: Irving, Tx
Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: John Rhodes]
      #3978227 - 08/12/10 02:05 AM

John,

I noticed that you listed every refractor Televue makes and all eyepieces they make as the equipment you use. Therein lies your problem. Instead of relying on Strehil figures, I can't even spell it, go look thru other equipment.The truth is your company has fine products, but nothing that can't be bested by another company. Example: Celestron Ultimas out perform Televue Plossls. I have very little use for pincushion distortion which every Televue eyepiece has.The original Meade series 4000 Ultrawides outperform the type 1 Naglers both during daytime and nightime with their flat fields. The 14mm Meade Ultrawide is a better eyepiece than the 13mm type 1 Nagler.Much easier to view thru and flatter field while still sharp at the edge. I am only referring to these older designs because these are what I have looked thru and compared. I had a 94mm f7 Brandon refractor with a cemented Cristen triplet apo lens that was sharper over most of the field than the original TV Genesis with the 4 element system. It had a flourite element in it.
Amateur Astronomy has been around long before you and I were even born and would be thriving today whether your company ever came along or not.I am frankly tired of all the suckups that rave about your company and it is time for someone to show another viewpoint.

Mark Swanson


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Alex Gastélum
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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Mark Swanson]
      #3978767 - 08/12/10 12:22 PM



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Jon Isaacs
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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Mark Swanson]
      #3978863 - 08/12/10 01:20 PM

Quote:

Example: Celestron Ultimas out perform Televue Plossls.




In 1996 Sky and Telescope did an exhaustive comparison of various Plossls. (Page 41 April 1996 if I recall.) These included various tests on the optical bench as well as field comparisons involving several experienced observers.

The result, they rated the Ultimas and TV Plossls at that top and of equal quality. Meade was probably their biggest advertiser at the time but the Meade series 4000 SuperPlossls, 4 elements by that time, were pretty much in the middle.

When I judge an eyepiece in a particular scope, I pick specific targets and if I want to write about what I see, then I refer to what I saw on those particular targets. For on-axis comparisons of a widefield eyepiece in a fast scope, I typically use the wild duck cluster. It is small and filled with a great number of tiny "pinpricks."

Jon Isaacs


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Tamiji Homma
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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Daniel Mounsey]
      #3978871 - 08/12/10 01:24 PM

Quote:

Quote:

A few quick thoughts:

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


Jon



Agreed




Hi Daniel,

Can you bring your Nagler 31 and Leitz 30 to NUTS star party this weekend? I'll bring my Edmund 32 Erfle, XW 30, something near 30 that I have

Tammy


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Daniel Mounsey
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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Tamiji Homma]
      #3978914 - 08/12/10 01:46 PM

No problem, I'll bring both.

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Tom TAdministrator

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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Mark Swanson]
      #3978943 - 08/12/10 02:00 PM

Quote:

Example: Celestron Ultimas out perform Televue Plossls.




FWIW This is such a broad statement, it's somewhat meaningless. I've had both lines, and Celestron Ultimas and TV Plossls are both fine performers (for my own use I kept the TV plossls and sold the Ultimas).

Quote:


I am frankly tired of all the suckups that rave about your company and it is time for someone to show another viewpoint.

Mark Swanson




As long as there are different types of gear there will be folks who for whatever reason favor one over the other. Just because you like one and someone else likes another does not make either of you wrong in your choice. Everyone is certainly entitled to their own viewpoint but on this board lets keep in mind the need to be tolerant of one another please. Thank you - (That's me speaking as board admin here...)

Speaking as an amateur astronomer, I'd venture to say that you should be aware that John is a company rep and has access to design information. When he states that the strehl ratio of the eyepiece on axis is 99.9%, that means that by design the eyepiece places 99.9% of the light within the airy disk. Since you can't resolve beyond the airy disk (in this case), that's about as good as you can get. Now - I'm not saying you didn't see what you say you saw - I'm just suggesting that the technical details of what's actually going on might be different - ie, transmission (as has already been suggested), contrast, light cone behavior, or something else that might involve user perception. (For example - I can see where certain types of astigmatism might lead one to believe that on axis star images were smaller.) Like was pointed out - I don't think you're really seeing the airy disk in these eyepieces (the magnification is way too low), but then again

Anyway - it's something to think about.

Tom T.


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Jon Isaacs
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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Tom T]
      #3978991 - 08/12/10 02:27 PM

Quote:


FWIW This is such a broad statement, it's somewhat meaningless. I've had both lines, and Celestron Ultimas and TV Plossls are both fine performers (for my own use I kept the TV plossls and sold the Ultimas).




The measurements and observations of the Sky and Telescope report were much more detailed as was the summary. I was merely responding to Mark's blanket claim that the "Celestron Ultima's out perform the TeleView Plossls." Sky and Telescope singled these two series and being a cut above others tested.

Jon Isaacs

Edited by Jon Isaacs (08/12/10 02:28 PM)


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Tom TAdministrator

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Re: A review of the 31 Nagler vs. the 32 Edmund Er new [Re: Jon Isaacs]
      #3978997 - 08/12/10 02:31 PM

Jon,

I remember that article and am in agreement with you. FWIW My comment was directed towards Marks statements - not yours.

Sorry for any confusion.

T


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