What is the effective difference beween 1.25" and 2" diameter eyepieces?

[StutzJr]

So as per my other post I am a fresh starter having just got my first proper scope yesterday after appreciating many years of a simple fixed 20x50 refractor.
I'm primarily interested in planetary viewing and will eventually consider purchasing an additional planetary eyepiece to complement my existing 25 mm wide angle and 10 mm EP's once I've had a bit of practice with what I have.
Clearly there are as many choices with eyepieces as there are with scopes..
My unit's focuser assembly can take either 1.25" or 2" diameter eyepieces. Are there any reasons to choose one diameter barrel over the other *APART* from simple compatibility with a given focuser assembly?

[mental4astro (Alexander)]

G'day StutzJr,

The size of the barrel determines the maximum size of the AFOV achievable for a given eyepiece focal length.

The best way to describe this is an example.

Take 68deg eyepieces. The longest eyepiece focal length that can give a 68deg AFOV (apparent field of view) in 1.25" is 24mm. In the 2" format, the longest eyepiece focal length that will allow this is 42mm.

Yes, you can get longer focal lengths in 1.25", but they won't show more of the sky. The widest AFOV a 32mm can give in 1.25" is 52 deg, and it will show the exact same amount of sky, though you will see a smaller diameter image. A 40mm 1.25" will have an even smaller AFOV because of the smaller barrel.

The 2" eyepieces hence will allow you to get a wider true field of view, ie how much of the sky you can see for a given AFOV.

There are eyepieces that have a 100deg AFOV. In the 2" format, because the limiting factor is the barrel, the longest focal length here is around 21mm. Such an eyepiece will show the same amount of sky as a 42mm 68deg eyepiece, but with higher magnification.

Now, just to add more spice to this mix, there is a new range of 3" eyepiece being developed by Explore Scientific. While not new as a format, they haven't been very popular with amateurs until the recent development of 3" focusers for photography have made them a more viable proposition for amateurs.

[StutzJr]

Cheers mental/Alex,

Thanks for answering my question, makes a lot of sense now I think about it. Since I'm still on a beginner's planetary fixation, this may be a silly question - would it be fair to say that objects in a wider field of view for a given magnification would remain in view for a longer duration before needing to move the mount?

[naskies (Dave)]

Yep, that's exactly right. At higher magnifications (especially planetary viewing), having a wide field of view is very handy for finding the little objects

[wavelandscott (Scott)]

Originally Posted by StutzJr

Cheers mental/Alex,

Thanks for answering my question, makes a lot of sense now I think about it. Since I'm still on a beginner's planetary fixation, this may be a silly question - would it be fair to say that objects in a wider field of view for a given magnification would remain in view for a longer duration before needing to move the mount?

Yes...however, depending on eyepiece design (configuration and quality) sometimes the "edge" will not have the same quality of view as the center. No single eyepice design is perfect and different imperfections are a result and may require you to refocus as the object crosses the field of view.

Because we each have different preferences and likes with respect to our choice of use and view you can get can very different opinions expressed as to what eyepiece offers the "best" view. This of course generates some of the fun in this hobby as you explore the options to find what you like best and is often why the answer folks get to the question "what should I buy?" is varied.

I often suggest going to viewing nights and looking through other folk's gear to learn more about what other gear is like before spending your own money...