2inch Eyepiece.

[Missedapproach (Ben)]

Hi all.
I have recently bought a new f10 refractor that has come with a 2inch Diagonal. I currently only have 1.25 eyepieces and would like to see what difference they make. Im not sure what the best size to get but just seeing whats out there. Any advice on the best size to get would be great.

[MortonH]

Since f/10 is quite forgiving on eyepiece design you might want to try something cheaper like the GSO/Bintel Superviews. The 2" models come in 30mm and 42mm.

[Missedapproach (Ben)]

Thanks for the advice

[janoskiss (Steve H)]

All else being equal, 2" eyepieces (EPs) don't make any difference. That is, if you replace your existing EPs with 2" versions of the same design, there will be no difference in what you see through them.

What a 2" barrel allows you to do is to use a broader range of eyepieces: ones with longer focal lenghts = lower magnifications, and/or with wider true fields of view. Basically, you can fit more sky into a 2" barrel than a 1.25" barrel.

Can you tell us more about your scope?

[floyd_2 (Dean)]

Have you considered the Baader Mk III Clickstop Zoom? I have one and absolutely love it. I noticed that there's one for sale in the classifieds right now.

[Wavytone]

Hi Ben,

The point of the larger 2" barrel eyepieces is that the true field of view is ultimately limited by the inside diameter of the eyepiece barrel. 2" covers 2.56X as much sky as 1.25" can.

Hold uo your eyepieces and look at the flied stop - the end that faces the telescope. The field stop is a black ring inside the barrel that defines the edge of field of view and quite simply it must be smaller than the inside diameter of the barrel.

Now... if you look at say a Vixen 42mm LVW, or a Televue 31mm Panoptic, or say an ES 40mm 68 degree eyepiece, they all have a field stop around 46mm which is about as wide as you will find in a 2" barrel. So these beasties will gove you a view of the widest patch of sky you can get in a 2" barrel.

But you can't get a 46mm field stop in a 1.25" barrel, its physically impossible.

There is also a trade-off between $$$ vs apparent field of view too, for example the ES 30mm 100 degree field of view will give the same ACTUAL field, with more magnification. The price is that it costs more, and distortion is also inevitable.

Now... with your f/10 scope, the longest useful focal length you could use is around 60mm (determined by the exit pupil) so you could easily opt for the lower cost eyepieces like the ES 68 degree series, there is really no point in spending megabucks for ultra wide Naglers, Ethos or similar.

[brian nordstrom (As avatar)]

Steve yes in all that is true , but my 27mm 2inch Panoptic is much better than both my 25 and 32mm TV plossls , as good as these both are the Pan is a nicer eyepiece to use in all my scopes that take 2 inch accessories .

But that being said ... my 31mm Celestron Axiom 82* is the bees knees when it comes to 2 inch eyepieces , ( 98% of the TV 31mm T5 telemangler for 1/2 the price ),
keep your eyes out for one of these 2nd hand even tho they don't come up very often , for good reasons .

Brian.

Originally Posted by janoskiss

All else being equal, 2" eyepieces (EPs) don't make any difference. That is, if you replace your existing EPs with 2" versions of the same design, there will be no difference in what you see through them.

What a 2" barrel allows you to do is to use a broader range of eyepieces: ones with longer focal lenghts = lower magnifications, and/or with wider true fields of view. Basically, you can fit more sky into a 2" barrel than a 1.25" barrel.

Can you tell us more about your scope?

[janoskiss (Steve H)]

Originally Posted by brian nordstrom

Steve yes in all that is true , but my 27mm 2inch Panoptic is much better than both my 25 and 32mm TV plossls

I qualified my statement by saying "all else being equal ... etc". For example, if you replace your premo 32mm 1.25" Plossl with another premo 32mm 2" Plossl it won't make any difference. (Although TV plossls are not a good example because they all vignette a little at any FL - I'm sure there is a good design compromise reason for that.)

I could quantify what I said earlier with formulas but the maths gets a little ugly and there is an easier way. Look at the true specs of the EP: the apparent field of view and the focal length. If these are the same for a 1.25" and a 2" EP, the barrel size won't make a difference.

But in the end it's still all just academic/nerdy stuff, because an EP typically will only have a 2" barrel if its design specs don't physically fit into a 1.25" barrel. You'll never find a 27mm 68-degree AFOV EP like a Pano with a 1.25" barrel and I've never come across a <=30mm 2" Plossl either. So, simply judge the EP for what it does and forget about the barrel size...

... except there is one reason why barrel size matters: convenience. Given your observing habits, how often will you have to swap out that 2"-to-1.25" adapter?

[dimithri86 (Dimithri)]

What kind of targets would one use a 40+ mm eyepiece for?

I have a 20mm on a 10inch dob, and so far all my targets (althought limited number) have fit in the view.

[ZeroID (Brent)]

I have 30 & 32 mm 2 " EPs. Bought cheap for a test but they are both brilliant in the 10" f5 Newt for viusal as they just seem to give such a wide panoramic immersive view. Nice for scanning around with and no edge distortion that my old eyes can perceive.
Bought them mainly for the 2" barrels for camera adapters but they double up for my rare visual use. ( Told you my eyes were old )

[Camelopardalis (Dunk)]

Originally Posted by dimithri86

What kind of targets would one use a 40+ mm eyepiece for?

I have a 20mm on a 10inch dob, and so far all my targets (althought limited number) have fit in the view.

Depends on the scope, but regardless, a 2" EP at maximum field stop gives almost 3x the field area of a 1.25" EP at maximum field stop.

In a small refractor such eyepieces can give very wide and satisfying views of very big objects, such as the Magellanic clouds, while in a large Cat it might still struggle to get the Orion nebula in!

[Wavytone]

Originally Posted by Camelopardalis

Depends on the scope, but regardless, a 2" EP at maximum field stop gives almost 3x the field area of a 1.25" EP at maximum field stop.

In a small refractor such eyepieces can give very wide and satisfying views of very big objects, such as the Magellanic clouds, while in a large Cat it might still struggle to get the Orion nebula in!

Nope... my 180mm f/15 mak will fit it nicely in a 2" eyepiece (the Vixen LVW 42 or LV50mm). Tack sharp edge to edge.

[Camelopardalis (Dunk)]

Whatever...my imaging shows much more nebulosity than ~1 degrees worth

[ausastronomer (John Bambury)]

Originally Posted by brian nordstrom

Steve yes in all that is true , but my 27mm 2inch Panoptic is much better than both my 25 and 32mm TV plossls , as good as these both are the Pan is a nicer eyepiece to use in all my scopes that take 2 inch accessories .g

Brian.

Brian

That has 100% to do with internal eyepiece design and 0% to do with the larger barrel size.

Cheers
John B

[AEAJR (Ed)]

In the end it is a matter of what you want out of the eyepiece.

I have an Orion XT8i with 2" focuser.

I wanted a low power wide field eyepiece for those big DSOs like the Pleiades and the Andromeda Galaxy.

Specs on my scope say the minimum mag on my scope is 29X. I am told that if I go below that I am going to see the spyder.

For my 1200 mm FL scope I got a 38 mm 70 degree AFOV eyepiece. That gives me 31.5X and 2.2 degrees actual field of view. That is beautiful and no way I can get that in a 1.25" eyepiece, at leas not one at price I would be willing to pay. I also have a 25 mm 2" 70 degree. And I barlow them at 2X.
http://agenaastro.com/agena-38mm-sup...-eyepiece.html


So look at the specs on your scope and don't go lower than the minimum rated mag.

[AEAJR (Ed)]

So, what have you decided? Going 2"?

[brian nordstrom (As avatar)]

Brian

Originally Posted by ausastronomer

Brian

That has 100% to do with internal eyepiece design and 0% to do with the larger barrel size.

Cheers
John B

[Don Pensack]

Originally Posted by Wavytone

Hi Ben,

The point of the larger 2" barrel eyepieces is that the true field of view is ultimately limited by the inside diameter of the eyepiece barrel. 2" covers 2.56X as much sky as 1.25" can.

Hold uo your eyepieces and look at the flied stop - the end that faces the telescope. The field stop is a black ring inside the barrel that defines the edge of field of view and quite simply it must be smaller than the inside diameter of the barrel.

Now... if you look at say a Vixen 42mm LVW, or a Televue 31mm Panoptic, or say an ES 40mm 68 degree eyepiece, they all have a field stop around 46mm which is about as wide as you will find in a 2" barrel. So these beasties will gove you a view of the widest patch of sky you can get in a 2" barrel.

But you can't get a 46mm field stop in a 1.25" barrel, its physically impossible.

There is also a trade-off between $$$ vs apparent field of view too, for example the ES 30mm 100 degree field of view will give the same ACTUAL field, with more magnification. The price is that it costs more, and distortion is also inevitable.

Now... with your f/10 scope, the longest useful focal length you could use is around 60mm (determined by the exit pupil) so you could easily opt for the lower cost eyepieces like the ES 68 degree series, there is really no point in spending megabucks for ultra wide Naglers, Ethos or similar.

Just a clarification: the 30mm 100° ES eyepiece is a 3" eyepiece and has a field stop larger than 2", so if it were usable, it would produce a larger true field than any 2" eyepiece with a 46mm field stop.

[Don Pensack]

Originally Posted by ausastronomer

Brian

That has 100% to do with internal eyepiece design and 0% to do with the larger barrel size.

Cheers
John B

Though, the 27mm Panoptic has a field stop of 30.5mm, so it is larger than any 1.25" eyepiece. Hence, it would yield a larger true field.
Your comment regarding performance is true--that has little to do with barrel size and is related to design.
True field, however, can be related to barrel size, and, generally, 2" eyepieces do have larger field stops and, thus, larger true fields.