2" versus 1.25" eyepieces

[erick]

OK, I'm still new at this, so your comments are welcome to educate me. Since my first eyepieces were a 2" 26mm and 1.25" 15mm and 9 mm, I wrongly formed the view that low magnification eyepieces were more likely 2" barrels. However, I'm discovering that eyepieces as long as 40mm fl come in 1.25".
Are there things to consider here, or doesn't it really matter whether an eyepiece is 1.25" or 2" regardless of focal length (overlooking size of focuser, barlow, filters etc.)

Thanks
Eric :)

[janoskiss]

The 1.25" barrel limits the true field of view achievable. That means that for a 40mm 1.25" you are looking at a max of about 40 degrees for apparent field of view. You get the same TFOV with a 32mm Plossl with an AFOV of about 50 degrees. Better option for most scopes, including yours.

[bkm2304]

Also magnification is not affected bythe barrel diameter. That is to say an eyepiece of 22mm focal length and say, 40 deg AFOV is 83 x in a 16" F4.5 regardless of diameter of the eyepiece. I am not sure if this is true of exit pupil though - anyone know?:shrug:

[bkm2304]

Mind you, having said that I am not sure if you would actually go near a 2" with such a small AFOV at 22mm!

[MortonH]

A good 2" eyepiece can give amazing views, but a lot of the time they're not really necessary.

Maybe for people with big scopes they come into their own cos they will generally give a wider view.

Downsides are the weight and the cost (for the premium brands).

Morton

[ColHut]

The longer the scope with respect to any given field stop diameter (regardless of focal ratio) the smaller the true field of view (tfov). So to get larger fields of view in long scopes you need wider field stops and thus 2" or bigger. There is a related issue of exit pupil size but that is another matter. So an f8 4.5" reflector with 1.25" (field stop say 27-28mm) has a maximum TFOV of about 1.7 odd degrees. In an f5 12" reflector the same 1.25" barrel gives only about 1 degree. To get more a 2" (field stop about 48mm) allows about 1.8 degrees. This is regradless of the Aparent field of view. So putting that very large ultra wide angle in a long scope with only a 1.25" barrel means the field of view is simply cut off by the field stop.

cheers

[Louwai]

Eric,
I've got both 2" & 1.25" EP's. Next time we're on the field feel free to drag them all out & give different ones a go to compare.

Bryan

[erick]

Thanks all

I was thinking of a 1.25" 40mm eyepiece, but I've decided to stick with 2" eyepieces (and their wide field of view if so designed) for anything, say 24mm and longer.

Yes, Bryan, next opportunity (:sadeyes: looking at the sky, clouds/rain yet again) I'd be grateful to compare 1.25" and 2" eyepieces of the same fl.