2 inch vs 1.25 inch

[gooseholla]

Guys,

Help me to understand the difference between 2 inch vs 1.25inch, other than physical barrel size.

If I had, for example, a 32mm 1.25 and 32mm 2 inch, of same field of view, say, 65 or 70, what would the difference truly be? That is, is there any advantage to a 2inch over a 1.25inch in the same spec. Or, if one has a large 1.25 inch collection, is there any reason to add 2 inch eyepieces at the same sizes?

Thanks

John

[Ags]

It's simple - you cannot get a 1.25" EP with a field of view of over 52 degrees... :-) If you want a wide angle EP at that focal length it HAS to be 2".

If you go down to 24mm, for 82 degrees you have to go 2".

But if the field of view is the same and the focal length is the same, 2" offers NO advantages over 1.25".

[hemihaggis]

i knicked this ...lol

In simple layman terms there are a few differences other than the obvious 3/4" larger diameter in the barrel size. Typically 2" Eyepieces are used for lower magnification and or wider field of view. A 2" barrel will allow for a wider Apparent Field Of View through the Eyepiece that a 1.25" wouldn't normally allow. So if you had a 2" and 1.25" 32mm EP, the AFOV in the 2" would be greater, probably much greater.

There is another benefit to the 2" barrel that it also allows for larger lens elements to be used. In other words a larger eye lens so that even at higher magnifications / short focal lengths your not looking through a peep hole as you would in your standard 1.25" plossl. Even in a short focal length EP's you can be looking through an eye lens that is 1.25" in diameter. Another advantage to many of them is a more comfortable eye relief so you don't have to paste your eye to the EP.

And probably the most obviouse difference is the cost.

[RikM]

For the same focal length and same field-stop diameter, there is no difference between 1.25" and 2".

[Merlin66]

It all comes down to the physical size of the field stop in the eyepiece.

The "potential" is there in the 2" eyepiece to have a larger stop, but only really effective in longer focal length eyepieces....

[Markinaboat]

Sorry to reply to a 2yr old topic, but based on the above very useful information, what about with the Baader Hyperions? These have both a 1.25" & 2" fitting. 

[bambuko]

They really are 1.25" eyepieces with the ability to fit them straight into 2" opening.

[Markinaboat]

As I thought. So how come they have a  68 d FOV?

[RikM]

You can have a 68° aFOV in a 1.25" up to 24mm focal length. Above that, to maintain a 68° field without mega distortion, you have to go up to a true 2" fitting. I have eyepieces from 3.5mm up to 28mm, all with 68-70° fields, and only the 28mm is a 2".

[Markinaboat]

Thanks Rik

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