1 1/4" vs 2" eyepieces

[Paul Manuell]

My current telescope takes 1 1/4" eyepieces. I'm considering buying a different telescope, but the one I want takes 2" ones. I know I can continue using my current eyepieces with the new telescope if an adapter is used, but are there any disadvantages in doing this instead of using 2" eyepieces?

[Mr Spock]

The advantage of 2" is you can use eyepieces with a larger field stop, so low power, wide field. Normal focal length eyepieces are going to be 1.25" anyway, there's no point to them being 2".

[RobertI]

It’s common and normal to use 1.25” eyepieces in 2” focusers so don’t worry about it. 1.25” eyepieces are the most commonly used eyepieces, 2” eyepieces are much heavier and there are fewer to choose from. You’re probably aware but 1.25” eyepieces have a more limited maximum field of view than a 2”. For example a 24mm 68 degree or a 32mm 50 degree will give the maximum field of view possible with a 1.25” eyepiece. But this is usually enough for most people, and 1.25” will be much lighter and easier to manage. I have a 2” eyepiece and basically only use it for observing the Veil Complex, apart from that it’s all 1.25” for me. 

Edited September 3 by RobertI

[Don Pensack]

2 hours ago, Paul Manuell said:

My current telescope takes 1 1/4" eyepieces. I'm considering buying a different telescope, but the one I want takes 2" ones. I know I can continue using my current eyepieces with the new telescope if an adapter is used, but are there any disadvantages in doing this instead of using 2" eyepieces?

Inside every eyepiece, there is an iris known as the field stop that determines how large a field you see.

As the focal lengths get shorter, the field stop eventually shrinks to the size where it fits in a 1.25" barrel.  There is no reason to make the eyepiece 2" beyond that point.

Eyepieces will be 1.25" if the focal length is shorter than:

14mm in 100°

17mm in 82°

25mm in 68°

27mm in 62°

33mm in 50°

At those focal lengths or longer in each apparent field size, the eyepieces will be 2".

[Louis D]

Aside from what has been mentioned above, a scope with a 2" vs. 1.25" focuser will be heavier, so that is one disadvantage.  If it is a Newtonian, the secondary will need to be larger to better illuminate the larger possible field of view in 2" eyepieces.  This will slightly decrease contrast.  So, there are at least these two disadvantages I can think of off the top of my head if you never plan on getting 2" eyepieces.  However, I feel the advantages of using 2" eyepieces far outweigh these two disadvantages.

[Louis D]

12 hours ago, Don Pensack said:

As the focal lengths get shorter, the field stop eventually shrinks to the size where it fits in a 1.25" barrel.  There is no reason to make the eyepiece 2" beyond that point.

I wouldn't feel very confident stuffing my huge and heavy 12mm ES-92 into a 1.25" diagonal, especially if it was one of those cheap plastic bodied ones sold with so many low end scopes these days.  It should just about be possible to repackage the ES-92 since its field of view is slightly smaller than my 12.5mm APM Hi-FW which is dual 1.25" and 2".  Since I know of no plastic bodied 2" diagonals currently in production, I think that's a good enough reason make large, heavy eyepieces 2"-only.  It guarantees they'll be properly supported.

[Don Pensack]

4 hours ago, Louis D said:

I wouldn't feel very confident stuffing my huge and heavy 12mm ES-92 into a 1.25" diagonal, especially if it was one of those cheap plastic bodied ones sold with so many low end scopes these days.  It should just about be possible to repackage the ES-92 since its field of view is slightly smaller than my 12.5mm APM Hi-FW which is dual 1.25" and 2".  Since I know of no plastic bodied 2" diagonals currently in production, I think that's a good enough reason make large, heavy eyepieces 2"-only.  It guarantees they'll be properly supported.

Weight and size are pretty much the only reasons for making short focal length eyepieces in 2".

Examples: 9mm 120°, 12mm 92°.  Were it not for the weight and size, however, they could have been 1.25" eyepieces.

[hal9550]

On 03/09/2024 at 01:26, Don Pensack said:

Inside every eyepiece, there is an iris known as the field stop that determines how large a field you see.

As the focal lengths get shorter, the field stop eventually shrinks to the size where it fits in a 1.25" barrel.  There is no reason to make the eyepiece 2" beyond that point.

Eyepieces will be 1.25" if the focal length is shorter than:

14mm in 100°

17mm in 82°

25mm in 68°

27mm in 62°

33mm in 50°

At those focal lengths or longer in each apparent field size, the eyepieces will be 2".

fascinating to see the limits laid out like that - curious about 3" EPs i have seen in one or two places. The 30mm 100 Degree Explore Scientific i have seen for example. Is there a corresponding limit for 2" - im trying think of focal lengths and i know there is a 60mm (i have a 56mm Plossl which has a narrower AFOV)

Genuinely just curious - i have never seen a telescope with a 3" Focuser!

[Don Pensack]

19 hours ago, hal9550 said:

fascinating to see the limits laid out like that - curious about 3" EPs i have seen in one or two places. The 30mm 100 Degree Explore Scientific i have seen for example. Is there a corresponding limit for 2" - im trying think of focal lengths and i know there is a 60mm (i have a 56mm Plossl which has a narrower AFOV)

Genuinely just curious - i have never seen a telescope with a 3" Focuser!

Well, 3" is 76.2mm.

The wall thickness of the lower barrel for a 3" eyepiece, with threading for 3" accessories is going to be at least 2mm, leaving 72.2mm as the maximum allowable field stop diameter, or maybe a little less to allow to a field stop iris.

The 30mm ES 3" has only a 52.2mm field stop, so nowhere near the limit of 3" eyepieces.  I think an 80mm focal length begins to approach the limit for field size in a 3".

The thing to remember is that if the field stop is at maximum, making the focal length longer cannot expand the field stop, so the apparent field just gets narrower.

That is why a 32mm 1.25" eyepiece can be 50°, while a 40mm 1.25" eyepiece is closer to 40°.  I have seen even longer focal length 1.25" eyepieces with field sizes of 25-28°.  They were not pleasant to use.

 

So 2" eyepieces can be made with any focal length you want if you are willing to accept the resultant apparent field.  So far, 30-31mm maxes out at 82°, 40-41mm maxes out at 68°, 55-56mm maxes out at 50°,

and longer focal lengths with narrower apparent fields are possible.  There are very few telescopes that can use eyepieces longer than 55-56mm, so the market for such eyepieces would be very small.  A 60mm 2" eyepiece would max out at 42°

An 80mm 2" eyepiece would max out at 32°.  Even Classical Cassegrain owners of f/16-f/20 scopes would balk at a 32° apparent field eyepiece.

 

The maximum in a 2" eyepiece seems to be a 46.5mm field stop, and for 1.25" eyepieces, currently in production, 28.0mm (though a few with 29.0mm field stops and vignetted edges have been made in the past).

And that limits the longest practical focal length in each size.

 

[Mr Spock]

The best I can get out of a 42mm LVW.

[Don Pensack]

20 minutes ago, Mr Spock said:

The best I can get out of a 42mm LVW.

Vixen claimed 46.6mm field stop in their literature.

I suspected what you measured.

[Mr Spock]

I don't care what people say, I go on actual physical results. I've measured this numerous times and it is repeatable - between 46.41 and 46.45 depending which part of the stop you measure.

[Second Time Around]

Televue's 67mm Plossl is very popular indeed for night vision.  It gives a 40 degree apparent FOV that is the maximum of nearly every night vision device.  Any longer focal length Plossl wouldn't give a wider true FOV as the field stop would be too big to fit into a 2 inch barrel.

In fact, such was the demand that Televue made an adapter especially for their 55mm Plossl to convert it into a 67mm.

[globular]

I have a 2" 56mm 50° Plossl that I use when my skies are very dark and I'm dark nebula hunting - it shows the same true field as my XW40 70° but with a larger 5.4mm exit pupil. 
It's not used often, but it's great to have available for when the conditions are right and the mood takes me.

[John]

I've owned a few 40 and 50mm 2 inch eyepieces but find that my skies, slightly light polluted as they are, are better viewed at higher magnifications so I much more often used 21/20mm 100 degree eyepieces and surrendered a little ultimate true field size for a darker background sky.

I owned a 55mm 1.25 inch eyepiece many years back - you can imagine the claustrophobic apparent field that one had ! (I didn't know much about field stop / barrel size limitations back then)

 

Edited September 6 by John