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Eyepieces with Both 1 1/4 and 2" Barrels

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#1 garyp1936

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Posted 02 September 2012 - 10:18 PM

What exactly is gained by using a 1 1/4--2" combo barrel, such as a Baader Hyperion, in a 2" diagonal versus using the same EP in a 1 1/4" diagonal? I assume one gets a wider field, but how is that calculated? If one does get a wider field then the TFOV formula---AFOV/Mag---can't be used in both cases. :question: Thanks.

#2 nicknacknock

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Posted 02 September 2012 - 10:31 PM

No benefit in this case. Wider field comes at around 27mm as I recall. Then you need to go 2" to increase the field depending on the eyepiece type.

The only benefit to the 2" adapter is that if you have other 2" eyepieces you don't have to use the 2" to 1.25" adapter on your diagonal.

#3 nicknacknock

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Posted 02 September 2012 - 10:37 PM

The formula holds, but when you want a bigger FOV than that rendered at the 27mm mark, you need to go 2" inch. Otherwise, basically the field stop steps in and you are simply not utilising the maximum FOV.

Take a Nagler 31mm for example, 82 degrees FOV on a 6" f5 refractor which has focal length of 900mm. The Nagler gives 29x magnification and TFOV of nearly 3 degrees. However, if the Nagler was 1.25' the field stop would be way below 3 degrees (bending the light through the EP elements still has to adhere to some laws of physics).

#4 garyp1936

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Posted 02 September 2012 - 11:18 PM

I was considering a Hyperion 36mm. Again, what advantage is there in using it in a 2" diagonal rather than a 1 1/4"? Specifically, given that it has a 72 degree AFOV, what is the TFOV when used in the 2"? In the 1 1/4"? Thanks.

#5 Jon Isaacs

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Posted 03 September 2012 - 05:02 AM

I was considering a Hyperion 36mm. Again, what advantage is there in using it in a 2" diagonal rather than a 1 1/4"? Specifically, given that it has a 72 degree AFOV, what is the TFOV when used in the 2"? In the 1 1/4"? Thanks.


The Tfov depends on your scope.

In the 2 inch mode the AFoV is 72 degrees, in the 1.25 inch mode, the AFoV is about 45 degrees.

Jon

#6 Scanning4Comets

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Posted 03 September 2012 - 06:02 AM

There is one benefit when you have a 1.25" and 2" barrel eyepiece. You can use both the 2" portion and 1.25" part of the barrels to get two different magnifications in a 2" barlow lens.

The further you are away from a barlow lens, the more magnification you get.

Cheers,

#7 Joe Ogiba

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Posted 03 September 2012 - 08:25 AM

You could use the 1.25" barrel in a 1.25" focuser like in binoviewers and spotting scopes then use the 2" barrel in scopes with 2" focuser. If all you have is a 1.25" focuser or 2" focuser then it has no advantage with the combo.

#8 star drop

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Posted 03 September 2012 - 09:32 AM

The manufacturer might gain a few more sales if the eyepiece has this capability.

#9 Jon Isaacs

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Posted 03 September 2012 - 09:59 AM

I was considering a Hyperion 36mm. Again, what advantage is there in using it in a 2" diagonal rather than a 1 1/4"? Specifically, given that it has a 72 degree AFOV, what is the TFOV when used in the 2"? In the 1 1/4"? Thanks.


The Tfov depends on your scope.

In the 2 inch mode the AFoV is 72 degrees, in the 1.25 inch mode, the AFoV is about 45 degrees.

Jon


Actually the unvignetted/fully illuminated field of view would be 45 degrees, you should be able to see more but with noticeable drop off in brightness.

Jon

#10 garyp1936

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Posted 03 September 2012 - 11:35 AM

Thanks. Live and learn!!! So all the 2"--1 1/4" eyepieces, such as the Hyperion and Orion's Stratus, actually have two different AFOV. The advertised 68 or 72 degree or whatever is ONLY if used in a 2" diagonal, if I understand you correctly. It would seem that it would be helpful if manufacturers would state the AFOV when using the 1 1/4" mode. Anyway, I now know something new; thanks again. Gary

#11 edwinh

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Posted 03 September 2012 - 11:40 AM

Not really - its only the hyperion aspherics really that do that... its really a 2" piece with a step-down. The fact that these exist make it confusing.

<24mm, 72 degree - its just convenience. Using the 24mm and down focal length hyperions in 2" or 1.25" mode makes no visual difference just a focus travel and mounting difference.

Likewise 12t4 nagler, 22panoptic, 8.8 meade uwa..... and others. a few ethoi...

#12 garyp1936

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Posted 03 September 2012 - 11:54 AM

But my understanding is that there is a significant difference in focal lengths longer than 30mm. Is this correct?

#13 csrlice12

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Posted 03 September 2012 - 12:52 PM

But my understanding is that there is a significant difference in focal lengths longer than 30mm. Is this correct?


Yes, because you have a larger field stop in a 2" vs a 1.25" eyepiece. Also, if it is a 1.25" eyepiece, with a 2" outer barrel (Think original 13mm Nagler), it is still just a 1.25" eyepiece and won't matter if you have a 1.25 or 2" focuser, you'll get the same view.

#14 edwinh

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Posted 03 September 2012 - 01:02 PM

Yes, for that class of AFOV eyepice, 30mm is too much for 1.25" you lose something.

But at that focal length a plossl type AFOV is just fine in 1.25":
32mm plossl, 35mm ultima.

For the 68-72deg class cut-off is about 24mm
For the 82deg type cut-off is about 17mm

Its really TFOV that is the limiter...

#15 faackanders2

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Posted 03 September 2012 - 04:17 PM

Benefit is it can be used in either 1.25" or 2" scopes.
2" can only be used in 2" scopes.

#16 Jon Isaacs

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Posted 03 September 2012 - 04:36 PM

Thanks. Live and learn!!! So all the 2"--1 1/4" eyepieces, such as the Hyperion and Orion's Stratus, actually have two different AFOV. The advertised 68 or 72 degree or whatever is ONLY if used in a 2" diagonal, if I understand you correctly. It would seem that it would be helpful if manufacturers would state the AFOV when using the 1 1/4" mode. Anyway, I now know something new; thanks again. Gary


Gary:

In general eyepieces that are both 2 inch and 1.25 inch are shorter focal length eyepieces with some of the optics in the 1.25 inch eyepiece barrel and this is not removed when using the 2 inch option. This means the field of view does not change when switching between the 1.25 inch and the 2 inch modes. I have several 1.25/2 inch eyepieces, the longest focal length is a 22mm Panoptic.

The 31mm and 36mm Hyperions are unusual in that you add an adapter to allow them to be used in a 1.25 inch focuser, there are probably other eyepieces setup this way but I do not know of any.

One thing to be aware of is that the field of view is determined by the diameter of the field stop. When you look through an eyepiece, the edge of the field is determined by the field stop, a ring of metal at the focal plane. With simple eyepiece like a Plossl, you can see the field stop when you look backwards down the open barrel. The larger the field stop, the larger the True Field of View.

For a 1.25 inch eyepiece, the inner diameter of the barrel is standardized at 28mm, the diameter of the filter, that is the maximum diameter possible. For a 2 inch eyepiece it's about 46mm.

One can estimate the approximate diameter of the field stop:

FS = Focal length eyepiece x AFoV (deg)/(57.3 deg/rad)

A 36mm eyepiece with a 72 degree AFoV requires a:

FS = 36mm x 72 deg/57.3 = 45mm,

So, the 36mm Hyperion has a field stop nearly the limit for a 2 inch eyepiece, it will be significantly restricted looking through a 28mm hole.

Jon

#17 garyp1936

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Posted 03 September 2012 - 05:17 PM

Thanks Jon. I'll be using the formula you provided. Gary

#18 Jon Isaacs

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Posted 03 September 2012 - 05:24 PM

Thanks Jon. I'll be using the formula you provided. Gary


Gary:

It's only an approximation but reasonably accurate. Here's a scope of others, the first one is reasonably exact:

- TFoV = 57.3 deg/rad x (Field stop eyepiece)/(Focal length telescope)

- TFoV = AFoV/Mag

Jon

#19 Gene7

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Posted 11 September 2012 - 05:16 PM

I am just not impressed with the 36mm eyepiece, OK, but not worth the extra money over the others. I digiscope or use A-focal. A 2 inch focuser is a lot more sturdy to hang a camera attached to an eyepiece on. You do not want any sag in the focuser. Gene

#20 Eddgie

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Posted 11 September 2012 - 07:10 PM

Thanks. Live and learn!!! So all the 2"--1 1/4" eyepieces, such as the Hyperion and Orion's Stratus, actually have two different AFOV. The advertised 68 or 72 degree or whatever is ONLY if used in a 2" diagonal, if I understand you correctly. It would seem that it would be helpful if manufacturers would state the AFOV when using the 1 1/4" mode. Anyway, I now know something new; thanks again. Gary


I don't know if someone has corrected your misunderstanding of this or not..

An eyepeice won't have a difference in AFOV if it is used in a 1.25" or 2" diagonal. The diagonal has nothing to do with the apparent field of the eyepeice. That is determined by the design of the eyepeice and nothing else.

If an eyepiece is equipped with a 1.25"/2" barrel, you can use it in any diagonal and it will work exactly the same.

It is only when the field scope of the eyepiece is bigger than about 28mm that it has to be used in a 2" diagonal, but in this case, it will not be equipped with a 1.25" barrel. It will only have a 2" barrel.

So if the eyepeice has both a 1.25" and 2" barrel, it will work exactly the same whether it is used in a 1.25" or 2" focuser or diagonal.

The 1.25"/2" design is strickly a marketing design. Some amateurs want to use all 2" eyepeices and this allows that. Others want to use only 1.25" eyepeices, and that works for them as well.

It has nothing to do with the way the eyepiece actually performs with respect to AFOV or any other attribute.

#21 ibase

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Posted 11 September 2012 - 09:02 PM

Here's an example of a double barrel EP, 2" on the outer ring, 1-1/4" on the inner ring:

Posted Image
Nagler 9mm T1

Whether using the 2" or the 1-1/4" sleeves in the scope, the views in this 9mm focal length EP are identical and the 2" won't give a wider field at all. 2-inchers are useful in longer focal length EP's, say in 30+ mm's where a 1-1/4" hole can restrict the field of view. I have the Aspheric 31mm which comes with a 1-1/4" adapter that is an option for use only with scopes that has no 2" diagonal and the view is vignetted or constricted in this case, as compared to a wider unrestricted view when using its native 2" mode.

Best,

#22 Richard Low

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Posted 11 September 2012 - 11:15 PM

Personally I prefer not to have the 2"/1.25" combo, if possible. Why? There is a possibility of the longer 1.25" end hitting glass when you use it in 2" mode in a prism diagonal or paracorr. Using it in 2" mode gives a more secured and stronger grip, as gripping on the short 1.25" portion doesnt seem so secure. Also, to use 2" filters, I have to add another 2"-1.25" adapter and sometimes plus 2" extension tube.

#23 Jon Isaacs

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Posted 12 September 2012 - 05:38 AM

Thanks. Live and learn!!! So all the 2"--1 1/4" eyepieces, such as the Hyperion and Orion's Stratus, actually have two different AFOV. The advertised 68 or 72 degree or whatever is ONLY if used in a 2" diagonal, if I understand you correctly. It would seem that it would be helpful if manufacturers would state the AFOV when using the 1 1/4" mode. Anyway, I now know something new; thanks again. Gary


I don't know if someone has corrected your misunderstanding of this or not..

An eyepeice won't have a difference in AFOV if it is used in a 1.25" or 2" diagonal. The diagonal has nothing to do with the apparent field of the eyepeice. That is determined by the design of the eyepeice and nothing else.

If an eyepiece is equipped with a 1.25"/2" barrel, you can use it in any diagonal and it will work exactly the same.

It is only when the field scope of the eyepiece is bigger than about 28mm that it has to be used in a 2" diagonal, but in this case, it will not be equipped with a 1.25" barrel. It will only have a 2" barrel.

So if the eyepeice has both a 1.25" and 2" barrel, it will work exactly the same whether it is used in a 1.25" or 2" focuser or diagonal.

The 1.25"/2" design is strickly a marketing design. Some amateurs want to use all 2" eyepeices and this allows that. Others want to use only 1.25" eyepeices, and that works for them as well.

It has nothing to do with the way the eyepiece actually performs with respect to AFOV or any other attribute.


This is true of most 1.25 inch/2 inch eyepieces but the post you responded to was part of a discussion of the 36mm Aspheric Hyperion which has a removable 1.25 inch barrel section. The AFoV is considerably wider in the 2 inch mode.

Jon

#24 sgorton99

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Posted 12 September 2012 - 06:39 AM

Sometimes, they come with both barrels.

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