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Old 05-07-2019, 11:59 AM
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thegableguy (Chris)
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2" vs 1.25" eyepieces

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
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Old 05-07-2019, 12:15 PM
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Atmos (Colin)
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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.
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Old 05-07-2019, 02:11 PM
Renato1 (Renato)
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Quote:
Originally Posted by Atmos View Post
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.
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

Last edited by Renato1; 05-07-2019 at 02:21 PM.
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Old 05-07-2019, 02:37 PM
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Atmos (Colin)
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Originally Posted by Renato1 View Post
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
I stand corrected
I've been looking at ortho lens' too much
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Old 06-07-2019, 02:51 PM
Renato1 (Renato)
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I stand corrected
I've been looking at ortho lens' too much
You like sharp images!
Cheers,
Renato
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Old 07-07-2019, 07:21 AM
astro744
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Originally Posted by Atmos View Post
...

Moving to 2" should give better off-axis views because you're using less of the usable FOV.
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_p...n=Advice&id=79
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Old 13-07-2019, 04:34 AM
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Quote:
Originally Posted by thegableguy View Post
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?

Yes, that's true. But that makes all the difference in the world because along with the larger true field comes a larger apparent field.

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...?)
no, because exit pupil (brightness) is related to the eyepiece's focal length.

Are they sharp to the edges, or do they get blurry? Or maybe that depends on the quality of the ep in question?
It depends on the quality of the eyepiece. If you want to have a 30mm with wide field that is sharp to the edge, expect it will be large, heavy, and expensive.

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?
It does matter, but it won't be a problem in that telescope.

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.
If you get an ultrawide 30-31mm eyepiece, be prepared to enjoy it enough you will soom be changing all your eyepieces.

Cheers
~c
my answers are above, in BOLD.
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