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Comparison of 24mm-26mm Eyepieces

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#1 Olivier Biot

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Posted 17 February 2012 - 11:36 AM

Comparison of 24mm-26mm Eyepieces

By William Paolini.

#2 dyslexic nam

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Posted 17 February 2012 - 11:38 AM

Yay

#3 jrbarnett

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Posted 17 February 2012 - 12:16 PM

An absolute gold mine of interesting data.

A few initial observations:

1. Not all "orthos" are created equal.

2. Televue knows edge correction.

3. On a budget? Think UO Abbe Ortho or Sterling Plossl.

4. And, most importantly, Brandon FTW! :grin:

Regards,

Jim

#4 Doug Culbertson

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Posted 17 February 2012 - 12:18 PM

Wow, what an amazingly well done comparison. I must admit that there were some surprises, along with some expected results. A thoroughly enjoyable read!

#5 WarrenM

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Posted 17 February 2012 - 12:32 PM

Wow! Now thats what I call a well done comprehensive review. Well done William!

Warren

#6 Sean Cunneen

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Posted 17 February 2012 - 12:37 PM

Fantastic! A round of applause!

now I am off to Google Sterling...
Sean

#7 BDS316

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Posted 17 February 2012 - 12:48 PM

One take-home message is that you don't have to spend a lot of money to get a great eyepiece. This point was made with STERLING clarity.

Would have been interesting to have seen how well a generic Asian plossl, such as a Synta or GSO would have done...

#8 Jason B

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Posted 17 February 2012 - 12:49 PM

Very well done. The wealth of information is staggering.

#9 coutleef

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Posted 17 February 2012 - 12:56 PM

Well done Bill. We were waiting for that review for a while and it was definitely worth waiting for.

#10 Scott99

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Posted 17 February 2012 - 02:02 PM

great job Bill! That's a lot of work and it's much appreciated.

#11 Starman1

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Posted 17 February 2012 - 03:11 PM

This review is exhaustive and much attempt is made to make comparisons valid.

There are a few quibbles I have with his test procedures however:

1) Comparing a 68 degree eyepiece's edge correction with a 50 degree eyepiece's edge correction would seem to be valid, given that the widefield user will be looking for definition at or near the edge of the field just like the narrower field user. However, if we assume that the edge correction of a 50 degree eyepiece is less than perfect at the edge, while a 68 degree eyepiece is perfect at the same 50 degree point in the field, but deteriorates more at the edge, then which eyepiece actually has the better correction? To give a valid comparison, I would want to see the correction obtained at 20, 25, 30, and 34 degrees off axis for each. It could be the widefield eyepiece yields a better-corrected 50 degree field than the 50 degree eyepiece. If so, the edge correction would have to be "field size compensated" to yield a relative ranking.

2) Bill seems to have some confusion about rectilinear distortion and angular magnification distortion and which aberration causes what in the image. Perhaps a good way to think about these two distortions is to picture map projections. If you reduce a globe to a flat representation, then there will be geometric distortions in the map, worst at the edges. We see this all the time in standard flat maps of the world. This is similar to what we would see as rectilinear (geometric) distortion.
If the globe itself is presented and rotated, a country will come around the edge of the globe, come toward you, and move away from you, changing size as it goes, but it will not be geometrically distorted. yet, it will appear to change in size.
Case one represents rectilinear distortion, while case two represents (it's an inexact analogy, I admit)) angular magnification distortion.

Would we want the planet to appear smaller at the edge of the field than the center? Probably not, though this wouldn't seem to be an issue in a driven scope with the planet's image held in the center. So, in the undriven scope, angular magnification distortion would not appear to be a desirable trait, especially in star fields or for viewing double stars.

But, the wider the field, the more the correction of one aberration leads to more of the other, so an eyepiece corrected for AMD will, necessarily, have RD. Since our eyes tend to have a little barrel distortion (positive RD)(it's a brain thing) when viewing the edge of a field, some amount of negative RD (pincushion) seems to result in an undistorted image to our eyes.
If there is too much RD, the object will appear to stretch and change shape as it nears the edge, similar to that Mercator map projection we talked about.

So what's ideal?
Well, in an undriven scope, a low level of AMD for sure. And for the least distortion visible to our eyes in images, a certain amount of RD. Not surprisingly, many good eyepieces are designed this way.

But what about the person who pans across the field? Well, if there is AMD in any significant amount it would appear as if the image is rolling across the surface of a globe. I have a kaleidoscope that does this--the edge appears farther away than the center because the object changes in size as it crosses the field and our brains tell us we are watching a globe rotating.

With a lot of RD and no AMD? Well, we'd see the object change its shape as it moves across the field, but the field could still appear flat. And if the RD amount is chosen to leave only a tiny bit of AMD in the field, we might see very little distortion in the field.

But, extend the field even more, to 82 degrees, or 90 or 100 or 110 degrees. The curves of solution for AMD and RD diverge even more. Having that *just right* about of RD in the field will result in noticeable AMD. So, a designer could solve for AMD and let RD be a larger amount. Given that most of these eyepieces are deep-sky eyepieces, that is probably the right choice. Used as terrestrial eyepieces, the edge of the field would appear quite distorted because we know how tree trunks and building edges are supposed to appear.
We might even tolerate that, though, if the eyepiece was free from astigmatism and the edge was sharp.

But it does indicate that the demands of designing the perfect astronomical eyepiece and perfect terrestrial eyepiece can be quite different.

I refer you to the discussions of RD and AMD which exist on the web, especially as they relate to binoculars and terrestrial usage.

3) Bill also missed the opportunity to discuss astigmatism and field curvature in the eyepieces. Field curvature's visibility might be related to the scope in which the eyepiece is used, but astigmatism is not. Since most of these eyepieces have narrow fields, where astigmatism should be tightly controlled, this could have been an illuminating comparison. I suppose some of that could be garnered from the comparison with Paracorr used, since, if coma and field curvature are more or less corrected, what's left is astigmatism [excluding chromatic aberrations].

All-in-all, a good step toward trying to be fair in comparisons. It was illuminating to see some highly-touted eyepieces come in down the list, and I applaud Bill's lack of that annoying drone about "tinted" eyepieces.

#12 mercedes_sl1970

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Posted 17 February 2012 - 06:20 PM

An impressive and thoughtful review. Thank you!

Andrew

#13 KidOrion

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Posted 17 February 2012 - 06:50 PM

I'm going to go give my $99 24mm SWA a hug.

Great work, Bill!

#14 Arizona-Ken

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Posted 17 February 2012 - 07:57 PM

An outstanding review.

An excellent attempt at describing the subtle differences among very good eyepieces and enough detail to allow both beginners to understand the differences and also to allow one to understand why some eyepieces are a bit better for different kinds of objects than others.

Here at CN we get good people to provide great insights on equipment but this is an order of magnitude above most efforts.

Congratulations, Bill.

Arizona Ken

#15 Doc Bob

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Posted 17 February 2012 - 11:28 PM

Bill,
Excellent review !! It helps me to now understand what is important when considering purchasing an eyepiece and that my set of Sterlings ain't half bad!!

Regards,
Bob

#16 Jim Rosenstock

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Posted 18 February 2012 - 05:34 AM

A dizzying amount of information that I've just skimmed.

I like the way the information is broken down and laid out. It's instructive how an eyepiece can become a personal favorite, and still perform poorly in categories that aren't a personal concern.

I'll be spending more time with this review....and wondering how it would extrapolate to other focal lengths. Anybody else feeling a 12mm shootout? :question:









:roflmao:

Jim

#17 Joe Bergeron

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Posted 18 February 2012 - 08:53 AM

What happened to that poor Brandon? It looks like someone sanded the anodized finish off the barrel and added some kind of crude label? Almost as jarring as that putting green atop the Meade.

#18 rguasto

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Posted 18 February 2012 - 09:01 AM

The 12mm shoot out results;
(read the 24mm shoot out) ...in conclusion all the 12mm EP's show twice the mag and 1/2 the TFOV :crazy:

#19 csa/montana

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Posted 18 February 2012 - 09:43 AM

Bill, an absolutely amazing Review! :bow:

This review is the ultimate for both beginners and long-time viewers; as it's in terms easily understood.

We all owe you a huge thank you for the time & effort you put into this, to benefit all of us here on CN.

Thank you, Bill! :waytogo:

#20 waskeyc

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Posted 18 February 2012 - 09:45 AM

And Bill hits one out of the park!

Great job Bill, fantastic review with scads of useful information.

#21 Jeff Morgan

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Posted 18 February 2012 - 10:04 AM

What happened to that poor Brandon? It looks like someone sanded the anodized finish off the barrel and added some kind of crude label? Almost as jarring as that putting green atop the Meade.


I was wondering the same. Obviously not current production.

#22 russell23

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Posted 18 February 2012 - 10:12 AM

I found Bill's comments about 24mm Panoptic fully in line with what I found with the 27mm Panoptic. There is no doubt the Panoptic is sharp right to the edge with very tight pinpoint stars. However, as Bill noted in comparing the 24mm Pan to the 24mm ES and Meade, it gives a dimmer view. I found the same with the 27mm Panoptic in the sense that I felt like I was loosing light compared to other eyepieces.

Dave

#23 russell23

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Posted 18 February 2012 - 10:14 AM

What happened to that poor Brandon? It looks like someone sanded the anodized finish off the barrel and added some kind of crude label? Almost as jarring as that putting green atop the Meade.


Wasn't there some discussion early in the thread in "eyepieces" forum that the Brandon being used was modified in some way for a zoom eyepiece adapter of something?

Dave
I was wondering the same. Obviously not current production.



#24 rdandrea

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Posted 18 February 2012 - 11:14 AM

Excellent work. And a great example of why I keep coming back to Cloudy Nights each day.

#25 BillP

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Posted 18 February 2012 - 01:12 PM

1) Comparing a 68 degree eyepiece's edge correction with a 50 degree eyepiece's edge correction would seem to be valid, given that the widefield user will be looking for definition at or near the edge of the field just like the narrower field user. However, if we assume that the edge correction of a 50 degree eyepiece is less than perfect at the edge, while a 68 degree eyepiece is perfect at the same 50 degree point in the field, but deteriorates more at the edge, then which eyepiece actually has the better correction? ...

2) Bill seems to have some confusion about rectilinear distortion and angular magnification distortion and which aberration causes what in the image. ...


3) Bill also missed the opportunity to discuss astigmatism and field curvature in the eyepieces. Field curvature's visibility might be related to the scope in which the eyepiece is used, but astigmatism is not. ....

All-in-all, a good step toward trying to be fair in comparisons. It was illuminating to see some highly-touted eyepieces come in down the list, and I applaud Bill's lack of that annoying drone about "tinted" eyepieces.


Hi Don. Appreciate the comments...including the critical ones. :) For #1 you are correct, a 100 deg AFOV eyepiece that shows only 60% of its FOV sharp is indeed corrected better than a 50 deg AFOV eyepiece that shows 99% of its FOV sharp. So out to the same TFOV position the 100 deg EP is better. I chose not to evaluate the eyepieces like this because I feel it makes no sense from the human perception level and how much a user will be satisfied with their eyepiece. So from a practical standpoint, IMO most observers relate how they like or dislike their eyepiece's performance based on how well it is handling its FOV. Most people will not be very satisfied with a 100 degree EP that has only 50% of its FOV sharp, but will be satisfied very much with their 50 deg AFOV EP that is sharp to the edge. So IMO it is more important to judge this aspect of performance "relative" to whatever the FOV the EP shows. This is why I did it this way. Plus IMO it is more intuitive from the visual observation standpoint as most don't look at FOVs and register TFOV points. Anyway, this is the rationale for the approach I used.

For #2 you may well be correct. I have flipped on this aspect several times. I used to hold your contentions, but then people right here on CN argued it was incorrect and I moved to a different understanding. What is the correct one? I am not sure. I was actually reluctant to make assessments on what drivers were on distortions or aberrations as in the end, we must guess because we can't separate them to assess individually since other distortions and aberrations are present. What is important though, is when you observe with these did the targets distort their shape? Did they distort their positions and angles? In truth I think that we can never separate the causes and in the end the more correct position would be to say that the eyepiece contains a mix of RD and AM which distorts the target to a moderate level or minor level, etc. The observational impact is what is important, more than the exact cause, at least this is my position as a 100% visual observer. In any reviews I do in the future, I will probably combine these distortions as potential drivers for the distortions observed. It is edifying to let others know what could be the drivers, but is of no real value IMO trying to figure out if it was 80% AMD and 20% RD, etc.

For #3, I purposely left this out as observationally what is important is is my image clear and sharp. So I simply provided what I saw, a sharp or non sharp image. I did quality in the narratives and the charts if FC was the only driver since this we can fix with a turn of the focus knob. But in the end, I didn't really feel it was important for observs to have an assessment of the degree of astigmatism I saw as in the end what is important to know is was my off axis sharply rendered, and if not could I correct it, and how did this behavior change with different focal length scopes. So this is what I strove to capture, the practical results.

I will keep these things in mind for next comparisons.

Thx.






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