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1.25" wide field between 27mm and 29mm?

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#1 Eddgie

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Posted 07 January 2013 - 11:39 AM

There seems to be only a couple of eyepeices in 1.25" format between 24mm and 32mm. I think there is a 1.25" ED eyepeice that is 25mm, but only about 60 degree AFOV, and I think the Flat Field 27mm is only about 53 degree afov...

Is there something in the 27mm to 30mm range with a bit wider than this?

I would think that if 24mm is limited to about 68 degrees, and 32mm is limited to about 52 degrees, then somewhere inbetween, there should be something around 28mm that is 60 degree AFOV.

But I can't seem to find it.. LOL.

If anyone knows of something I am missing, please let me know.

#2 Jon Isaacs

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Posted 07 January 2013 - 11:49 AM

Eddgie:

The math says a 28 mm eyepiece would have an maximum "effective" AFoV of about 56 degrees.

I suspect that most just figure it's not worth the effort to design a widefield eyepiece just to get a few extra degrees over a Plossl.

Jon

#3 BillP

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Posted 07 January 2013 - 11:55 AM

If I recall correctly, the AFOV of the old Edmund 1-1/8" (28mm) eyepiece is approx 58 degrees. It has no field stop and uses the barrel. Noticably wider AFOV than a 28mm RKE.

#4 oldtimer

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Posted 07 January 2013 - 12:20 PM

This is also true of the 30mm Rini (no field stop). A pretty nice eyepiece if you can find one.

#5 Starman81

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Posted 07 January 2013 - 02:05 PM

I think the Pentax XL 28 with its 55* AFOV would fit the bill.

#6 Eddgie

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Posted 07 January 2013 - 04:22 PM

Ah, the 28mm Edmund. That is interesting. I did not know about this eyepiece if it is not the RKE. Will check into it.

I want lower power in my binoviewer than the 24mm Hyperions give, but I want to maintain a bit wider apparent field than a 32mm Plossl would give.

The problem with the Hyperions is that the magnification is high enough that wen seeing is less than great, stars appear somewhat bloated.

I used to have a 27mm Pan, and when seeing was not great, I would drop to that for sharper stars. I sold the Pan when I bought the Binoviewers, but now find that I miss this ability.

I may though just drop down to 32mm with some Plossls, but again, I thought if I could get half way while still maintaing a bit wider apparent field, it would be a good compromise.

Thanks. Perhaps there are some other options as well.

#7 Eddgie

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Posted 07 January 2013 - 04:34 PM

I did not know about the Pentax, but it is apparently no longer being made...

I could not even find a price.

#8 Scanning4Comets

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Posted 07 January 2013 - 05:10 PM

http://www.cloudynig...1265016/Main...

#9 BillP

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Posted 07 January 2013 - 05:13 PM

Ah, the 28mm Edmund. That is interesting. I did not know about this eyepiece if it is not the RKE.


Its a war surplus eyepiece that I believe Edmund sold as far back as 1958. Taken apart it is an Asymmetric Plossl (true Plossl?).

#10 GlennLeDrew

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Posted 08 January 2013 - 02:46 AM

It seems that the largest AFoV obtainable at given field stop diameter and which does not result in bad edge-of-field darkening is what the doctor ordered?

#11 Jaimo!

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Posted 08 January 2013 - 07:52 AM

Ah, the 28mm Edmund. That is interesting. I did not know about this eyepiece if it is not the RKE.


Its a war surplus eyepiece that I believe Edmund sold as far back as 1958. Taken apart it is an Asymmetric Plossl (true Plossl?).


The innards of the Edmund 1 1/8"

Posted Image


I was also able to find it in a 1" version:

Posted Image

Jaimo!

#12 BillP

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Posted 08 January 2013 - 09:48 AM

WoW...a 1" version!!! How cool. Never saw one. Great find :bow:

#13 Jaimo!

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Posted 08 January 2013 - 04:24 PM

The 1" is VERY similar to the 1 1/8"... 25.4mm vs. 28mm.

Bill, if you are interested send me a PM, I think the eyepiece could be available to go on sebbatical to the "Old Line State" for a couple of weeks...

Jaimo!

#14 faackanders2

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Posted 26 January 2013 - 12:38 PM

Why such a narrow 2mm range at low power? Is this for 1.25" binoviewers, or a 1.25" telescope?
I believe 24mm 68AFOV panoptics rate themselves widest TFOV for 1.25".

#15 Starman1

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Posted 26 January 2013 - 01:04 PM

I've owned two of those old 1-1/8" Edmund Plossls, both without filter threads.
First, the edge correction is not good and there is no sharp field stop. It really needed a field stop iris at the focal plane.
Second, the apparent field was 54.5 degrees (measured it several times, though the stars didn't simply vanish past a sharp field stop), but the last several degrees wasn't worth having. The eyepiece REALLY needed to be stopped down.

In contrast, the 35mm Ultrascopic/Elite/Gold Series/Ultima was a lot nicer.
A 29mm field stop (larger than the Edmund), though not fully illuminated at the edge, and a sharp edge to the field, and the last few degrees were quite usable. It has a 49 degree field.

Here are a few you might find out there:
Antares W70 25mm 65 degrees
Celestron XCel LX 25mm 60 degrees
Meade HD60 25mm 60 degrees
Long Perng Plossl 25mm 55 degrees
Meade S5000 26mm Plossl 60 degrees
Siebert Ultra 28mm 58 degrees.

#16 Starman1

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Posted 26 January 2013 - 01:10 PM

Also, I count 123 different 1.25" eyepieces between 25 and 30mm on the Buyers Guide to eyepieces. Many are discontinued and only available used, and many don't have a field larger than 50 degrees. But there is a larger number of eyepieces available in that size range than you think. For one thing, nearly every Celestron and Meade comes with an eyepiece in that size range.
I grant you that the pickings get slim if your requirement is at least a 55 degree field (see my previous post).






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