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Equipment Discussions >> Eyepieces

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Starman81
Carpal Tunnel
*****

Reged: 03/06/08

Loc: Metro Detroit, MI, USA
Re: ES 25mm 100 AFOV Shipped new [Re: mountain monk]
      #5621141 - 01/13/13 04:16 PM

Thanks for the visuals Tamiji. It definitely looks like a very impressive eyepiece, however the blue fuzzy field stop is quite apparent, even in the photo.

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Shneor
scholastic sledgehammer
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Reged: 03/01/05

Loc: Northern California
Re: ES 25mm 100 AFOV Shipped new [Re: mountain monk]
      #5621159 - 01/13/13 04:26 PM

Got mine yesterday, an hour before I left for Lake Sonoma. Transparency was mediocre, on average, but varied from poor to fair. Getting this eyepiece shortly before I left contributed to my neglecting to swap my regular contact lenses for my observing set (meaning my distance vision was not as good, so fainter stars I could ordinarily see became invisible to me). A fellow observer's 24" barely showed a 15th magnitude galaxy.

Nevertheless, this was first light for my new eyepiece. I was pleasantly surprised by the apparent lack of come - only to realize (as another observer pointed out) that the designers had cleverly used another optical aberration to correct for coma - field curvature. Of course, this is noticeable only when panning the eyepiece, so it's a nice compromise. In my 22" f/4, this eyepiece gives a true field of one degree six arcminutes (1.1°). Even though conditions were poor, this eyepiece let me view several galaxies in the Fornax cluster at the same time, requiring less panning. Contrast was good, but it's hard to determine due to the absence of good transparency. The view of M42 with an NPB filter was stunning - the extended view of the nebulosity was amazing. And some bright galaxy trios looked good. but Markarian's chain looked better in my 20mm ES100°.

Incidentally, I believe the field stop is not internal.

Clears,


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: ES 25mm 100 AFOV Shipped new [Re: Shneor]
      #5621285 - 01/13/13 05:29 PM

Shneor,
This eyepiece is NOT coma-corrected. I only got a chance to look at the Perseus Double Cluster with my 12.5" f/5. The eyepiece performed fine in the Paracorr, and the predominant aberration visible without the coma corrector was.....coma.
That coma may have been combined with field curvature and astigmatism to bloat the amount of visible coma, but there it was.
Eyepieces, except for a couple 50 degree samples from the "80s, are not coma-corrected. And note that an eyepiece can have field curvature without correcting for coma in the scope, and vice-versa.

Edited by Starman1 (01/13/13 05:30 PM)


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Shneor
scholastic sledgehammer
*****

Reged: 03/01/05

Loc: Northern California
Re: ES 25mm 100 AFOV Shipped new [Re: Starman1]
      #5621339 - 01/13/13 05:58 PM

Don, stars were pretty sharp to near the edge. I'm not the only one who saw this. The field curvature was obvious, and it appeared to offset coma to a significant degree.

In general, if it's only my eyes, I won't post. In this case, a very experienced observer noted the apparent correction.

Clears,


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: ES 25mm 100 AFOV Shipped new [Re: Shneor]
      #5621577 - 01/13/13 08:20 PM

What do you mean when you say field curvature offset coma?
Field curvature slowly defocuses the star images, but they stay round. Coma stretches the star images radially into fan shapes.
Field curvature can defocus coma, too, making the comatic image slightly larger and out of focus, but it doesn't reduce coma. It can only make the appearance of it worse.
So what you say cannot be reconciled.
I notice a slight amount of astigmatism near the edge, and depending on which side of focus you sit, this could appear to reduce the radial width of the comatic star (while stretching it circumferentially). On the other side of focus, it would make it worse.
I got a quick glance at the Perseus Double Cluster with the eyepiece in a Paracorr II, and the star images displayed a trace of astigmatism, but no obvious coma or field curvature. Since the Paracorr corrects the last two, it's not surprising what I saw. Without the Paracorr, it displayed the normal amount of coma for an f/5 scope and a 100 degree field, i.e. a lot.
However, if the eyepiece actually corrected coma, it would display coma in the Paracorr, and it did not.
Once again, current eyepieces do not correct for coma. If they did, they'd only be useful in short f/ratio newtonians.

Edit:
It just dawned on me that what you are calling field curvature is actually rectilinear distortion (wherein straight lines appear curved near the edge of the field, so ||| becomes )|( in the eyepiece.
Yes, the eyepiece has a lot of that, as do other 100 degree eyepieces.
The correction of RD is impossible in a widefield eyepiece without leaving in tons of Angular Magnification Distortion (AMD), which is considered to be the more deleterious form of distortion in astronomical viewing.
RD is more noticeable when panning quickly, but usually doesn't bother most observers.
AMD, on the other hand, distorts images terribly, so most modern designers opt for RD over AMD.

Field curvature is when the focal plane is curved toward or away from the eye at the edge and it results in out of focus stars. I don't think that was what you were talking about.

Edited by Starman1 (01/13/13 08:26 PM)


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herrointment
Post Laureate
*****

Reged: 03/12/11

Loc: North of Hwy. 64
Re: ES 25mm 100 AFOV Shipped new [Re: Starman1]
      #5621887 - 01/13/13 11:08 PM Attachment (82 downloads)

I'll contribute my usual *BLEEP* photo....

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MRNUTTY
Carpal Tunnel
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Reged: 11/22/11

Loc: Mendon, MA
Re: ES 25mm 100 AFOV Shipped new [Re: herrointment]
      #5622179 - 01/14/13 06:50 AM

Vundabar! Herr Ointment! I'm still waiting...

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herrointment
Post Laureate
*****

Reged: 03/12/11

Loc: North of Hwy. 64
Re: ES 25mm 100 AFOV Shipped new [Re: MRNUTTY]
      #5622830 - 01/14/13 02:59 PM

Ja, sehr gut!

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JMorey
member
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Reged: 11/27/06

Re: ES 25mm 100 AFOV Shipped new [Re: JMW]
      #5622842 - 01/14/13 03:03 PM

Hi Guys,

I had a thought: Being that the ES 30mm 2" 82 degree eyepiece has six elements, and the ES 25mm 2" 100 degree eyepiece has eight elements would there actually be that much of a difference in magnitude because of the two extra elements in the ES 25mm? There true field of view is almost the same. I think it would be interesting to hear a report concerned with both eyepieces, and the differences noted in coatings on the eyepieces. Any thoughts? Thanks much, and clear skies.

Best,
Joe


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herrointment
Post Laureate
*****

Reged: 03/12/11

Loc: North of Hwy. 64
Re: ES 25mm 100 AFOV Shipped new [Re: JMorey]
      #5622860 - 01/14/13 03:14 PM

Mine won't see any action 'till sometime next month by the looks of it.

The weather has been ridiculously uncooperative for longer than I care to recall.


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Astrojensen
Post Laureate
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: ES 25mm 100 AFOV Shipped new [Re: herrointment]
      #5622892 - 01/14/13 03:32 PM

Sounds like around here. Ever since I ordered my ES eyepieces on December 23rd, it's been completely overcast. And the very day I received them, it started snowing. It's been cold and snowy since. Just got another two inches while I was eating dinner...

Those eyepieces must be ridiculously good!!!


Clear skies!
Thomas, Denmark


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Phillip Creed
Idiot Seeking Village
*****

Reged: 07/25/06

Loc: Canton, OH
Re: ES 25mm 100 AFOV Shipped new [Re: Tamiji Homma]
      #5622905 - 01/14/13 03:38 PM

Quote:

I measured TFOV and calculated field stop of ES25-100 along with Nagler 31T5.

41.1mm ES25-100 (last 1% vignetting/fuzzy field stop)
42.2mm Nagler 31T5 (clean to the very edge)

Tammy




I'm a bit surprised that the 31T5 has a bigger field stop. I would have guessed the opposite, even assuming there was a fair bit of pincushion distortion.

Clear Skies,
Phil


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Darren Drake
Carpal Tunnel
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Reged: 10/09/02

Loc: Chicagoland
Re: ES 25mm 100 AFOV Shipped new [Re: Phillip Creed]
      #5622949 - 01/14/13 04:01 PM

Looks like there is some pincushion in the pic. The 9mm looks a little distorted lol.

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herrointment
Post Laureate
*****

Reged: 03/12/11

Loc: North of Hwy. 64
Re: ES 25mm 100 AFOV Shipped new [Re: Darren Drake]
      #5622963 - 01/14/13 04:06 PM

Yes, the 9 looks a bit out of sorts although the exact cause is debatable!

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Starman81
Carpal Tunnel
*****

Reged: 03/06/08

Loc: Metro Detroit, MI, USA
Re: ES 25mm 100 AFOV Shipped new [Re: herrointment]
      #5623118 - 01/14/13 05:33 PM

Jim (herrointment), are you planning on keeping the 20mm now that you have the 25mm? You could probably do without it but then the set would no longer be complete.

Darren, isn't that 'barrel distortion' when the image distorts in a concave sort of way at the edge?


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: ES 25mm 100 AFOV Shipped new [Re: Starman81]
      #5623163 - 01/14/13 06:04 PM

Barrel Distortion (Negative rectilinear distortion):

||| becomes (|)

Pincushion Distortion (Positive rectilinear distortion):

||| becomes )|(

Most eyepieces produce pincushion. The picture of the ES eyepieces has barrel distortion.


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herrointment
Post Laureate
*****

Reged: 03/12/11

Loc: North of Hwy. 64
Re: ES 25mm 100 AFOV Shipped new [Re: Starman81]
      #5623169 - 01/14/13 06:08 PM

For me, these eyepiece amount to an insane extravagance. My employer decided to keep us busy to the tune of 400+ hours of overtime last year and these Eps are my reward.

If I buy something I tend to hang on to it and so far I'm plenty happy with the 9, 14 and 20.



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thrawn
scholastic sledgehammer


Reged: 03/14/10

Loc: Los Angeles, CA
Re: ES 25mm 100 AFOV Shipped new [Re: herrointment]
      #5623202 - 01/14/13 06:33 PM

Someone put a tiny piece of tape on that stop near the field lens, and check if it's in focus (or almost in focus) looking through the eye end. Then we'll know for sure if the field stop is external or internal.

I'm not on CN much anymore btw so I may not notice the answer.


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Jobryant
sage


Reged: 01/13/09

Re: ES 25mm 100 AFOV Shipped new [Re: thrawn]
      #5624390 - 01/15/13 01:17 PM

Is the rubber eye guard permanently attached or can it be taken off either by unscrewing it or just pulling it off?

So from the reports I've read so far this EP would not be suitable for daytime use. How would it compare to the Ethos 17 when it comes to pincushion distortion or the Nagler 31? I found the Ethos 17 fine to use as a daytime EP. My current daytime EP is the 88deg 30mm Leitz and I love it. I do see distortion(fishbowl effect) but it doesn't bother me. After months of side by side comparisons I actually choose the Leitz over the Pentax XW40 for my daytime viewing to give you idea of my tolerance to pincushion distortion. That being said would the ES25 100 still be considered a bad choice to use as a daytime EP for a guy like me that isn't too sensitive to the distorted fishbowl effect?


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: ES 25mm 100 AFOV Shipped [Re: Jobryant]
      #5624657 - 01/15/13 03:35 PM

Quote:

Is the rubber eye guard permanently attached or can it be taken off either by unscrewing it or just pulling it off?



It pulls right off.
Quote:


So from the reports I've read so far this EP would not be suitable for daytime use. How would it compare to the Ethos 17 when it comes to pincushion distortion or the Nagler 31?




Note that TeleVue's philosophy of design tends to reduce Angular magnification distortion to a minimum in their eyepieces. That means that rectilinear distortion (pincushion is one example) will be present in proportion to the width of the field of view. That means a 31 Nagler would have less than a 17 Ethos. It appears the 25mm has also solved for AMD because RD is noticeable when you look for it. That's good for astronomy.
Quote:


I found the Ethos 17 fine to use as a daytime EP.



So long as the 25mm doesn't produce too big an exit pupil for your daytime eye, you would find it similar.
Quote:


My current daytime EP is the 88deg 30mm Leitz and I love it. I do see distortion(fishbowl effect) but it doesn't bother me. After months of side by side comparisons I actually choose the Leitz over the Pentax XW40 for my daytime viewing to give you idea of my tolerance to pincushion distortion. That being said would the ES25 100 still be considered a bad choice to use as a daytime EP for a guy like me that isn't too sensitive to the distorted fishbowl effect?



Well, RD isn't usually a problem unless you look at fields of view with a lot of linear features (such as trees), or pan the eyepiece rapidly. Of course, AMD would be bad in panning as well because every object would appear to start distant at the edge of the field, get closer in the center and then grow more distant again.
That both forms of distortion would result in something antithetical to distortion-free daytime observing is, in my purview, the primary reason to stick to slightly narrower fields of view in the daytime--the percentages of distortion at the edge of the field would be less.
However, if the distortion doesn't bother you (and for stationary viewing it seldom is obtrusive), then go for the wider field of view.


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