2" vs 1.25" eyepieces

[thegableguy]

Apologies for asking a question I'm sure has been asked dozens of times, but, well, I'm lazy...

Considering getting a 2" EP or two. Please correct my understanding if it's wrong, but is looking through a 30mm 2" eyepiece similar to looking through a 30mm 1.25" except with a larger field of view? ie same magnification, targets will look about the same size - you can just see more of the surrounding sky?

Is it a markedly nicer experience to look through a 2"? My little girls kinda struggle to line their eyes up in the right place.

Any difference in relative brightness between the two? (I would expect not, but maybe...?)

Are they sharp to the edges, or do they get blurry? Or maybe that depends on the quality of the EP in question?

Does it matter much what kind of telescope you're using a 2" EP with? ie would there be a lot of noticeable vignetting or any other issues with a basic 8" f/5 reflector?

Any other info would be great. I've just dusted off my telescopes after several years' neglect and am hoping a few nice new 2" EPs / Barlows will get me off my bum and looking & imaging through them again.

Cheers
~C

[Atmos]

I can answer a little, 30mm is 30mm whether it is a 1.25" or 2". A 30mm 1.25" plossl gives near to the maximum FOV that you can get from a 1.25" EP at maybe 42ºish but so does a TeleVue 24mm Panoptic with its 68º FOV. Moving to a 2" means that you may be able to get 80º FOV in a 30mm as opposed to the constrictive 42ºish.

Moving to 2" should give better off-axis views because you're using less of the usable FOV.

[Renato1]

Hi Colin,
42ºish FOV is more applicable to Kellner type eyepieces, Plossls are better than that.

44º FOV is applicable to 40mm 1.25" Plossl eyepieces, but most other 1.25" Plossls have FOV of 49º to 52º.
Regards,
Renato

[Atmos]

I stand corrected :)
I've been looking at ortho lens' too much :lol:

[Renato1]

You like sharp images!
Cheers,
Renato

[astro744]

Could you please explain this statement. Seems contradictory and I'm not really sure what you mean by it.

With regards to 2" or 1.25" if the field stop diameter and focal length is the same then the size of the barrel makes no difference. Dual skirts exist on some eyepieces for convenience. The eyepiece performs the same in either mode (except for position of the focuser as you either have to rack in or rack out depending on which mode you are using. e.g. 12mm Nagler.

The main advantage of 2" is that the field stop diameter can be up to 46mm and is not resctricted to 27mm as for a 1.25" barrel. This means larger true fields, (more actual sky covered).

Tele Vue have Field Stop (mm) column the following table for their eyepieces.

See http://www.televue.com/engine/TV3b_page.asp?id=214

Any eyepiece with the same field stop diameter gives the same true field for a given telescope and this is not dependent on other parameters.

True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3

See http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=79

[Don Pensack]

my answers are above, in BOLD.