1.25" vs 2" eyepieces

[BoisBlancBoy]

If you were able to compare the two eyepieces, how much of a difference would you see?

Do most of the new scopes come standard with 1.25 or 2 capability?

[rowan46]

2" is mainly for wider or longer focal length eyepieces and most of the better scopes come with 2" focusser and 11/4" adaptor as standard

[AlexB67]

In theory you could see as much in any given 1.25 inch as you can in a 2 inch, however the limit will be greater for the two inch as to the max FOV that can achieved for a given scope. If that this is what you are asking than as an approximate guide ( when not limited by field stop ) you can work out the max possible FOV for you scope in each case.

{1.25 inch x ( 25.4 mm / inches ) x (180 / π )} / ( focal length scope / mm )

that would give it in degrees. For the 2 inch beast simply use

{2 inches x ( 25.4 mm /inches ) x (180 / π )} / ( focal length scope / mm )

[BoisBlancBoy]

Ok just making sure I understand you correctly he 2" will increase your FOV over any 1.25"?

[rowan46]

for sure that''s mostly true, but there can be a little overlap between the mid length super wides and and longer length not so wides. Somebody somewhere will have the equations but maths really isn't my forte

[AlexB67]

Ok just making sure I understand you correctly he 2" will increase your FOV over any 1.25"?

I only mentioned maximum achievable FOV in that post with the equations I gave, but usually yes, I think that 2 inches show more because the field stop can be made bigger ( wider in diameter ) and therefore are used to achieve wide views, that is the gist of it I think. No doubt the more knowledgeable will chime in on that.

[John]

I only mentioned maximum achievable FOV in that post with the equations I gave, but usually yes, I think that 2 inches show more because the field stop can be made bigger ( wider in diameter ) and therefore are used to achieve wide views, that is the gist of it I think. No doubt the more knowledgeable will chime in on that.

I think thats it in a nutshell

The physical diameter of the field stop defines the size of the apparent field of view. In practice the largest possible field stop diameter is limited by the internal diameter of the eyepiece barrel so 2" barrels allow bigger field stops than 1.25" ones.

Whether all that extra field of view is actually nice to look at is an entirely different matter of course !

[AlexB67]

I would not know wide angle from my elbow in practice not having had that luxury I am happy with my 60 degrees BSTs though and another one coming ( 25mm ) used at a reasonably good price and used twice. Coming in the next couple of days so lets hope it is what it says, never bought second hand so far except for the collimating sight tube tool I got yesterday at almost half price, and that looked as new

[RikM]

Ok just making sure I understand you correctly he 2" will increase your FOV over any 1.25"?

No, that is not quite true. You can get wider fields with 2" than with 1.25" eyepieces but it depends on the design. e.g. an Explore Scientific 18mm 82° 2" eyepiece gives a true field of 73.8 arcsec, while a Televue Panoptic 24mm 68° 1.25" eyepiece will show you 81.6 arcsec.

[Moonshane]

there's no real difference between the two in terms of what they will show you, it's just that there is potentially a wider field with 2". that said, in general terms cheaper wide field 2" eyepieces are not very good with faster scopes as they show a lot of distortion in the outer third or so of the field. personally, I'd sooner have a 50-60 degree apparent field that's all good and sharp than a wider field that's a bit gooey for the area wider than the 50-60 degree field in the wider 2". in other words, I'd sooner have a £60 used good quality 25mm plossl than a 25mm 2" wide field with distortion in the outer third of the field.

that said, if you can afford a good quality 2" eyepiece, they are pretty nice and I'd recommend you start saving.

[AlexB67]

there's no real difference between the two in terms of what they will show you, it's just that there is potentially a wider field with 2". that said, in general terms cheaper wide field 2" eyepieces are not very good with faster scopes as they show a lot of distortion in the outer third or so of the field. personally, I'd sooner have a 50-60 degree apparent field that's all good and sharp than a wider field that's a bit gooey for the area wider than the 50-60 degree field in the wider 2". in other words, I'd sooner have a £60 used good quality 25mm plossl than a 25mm 2" wide field with distortion in the outer third of the field.

that said, if you can afford a good quality 2" eyepiece, they are pretty nice and I'd recommend you start saving.

I concur More from theory and reading rather than practice I should add but exactly for that reason I feel the BST has nice bang for buck performance in a scope such as mine at f/5. I'd go with a GSO/Vixen as well perhaps or some other in that price range, though I have not tried them, but there I feel lies a nice healthy balance for me in terms of what I can afford as well right now.

I wish restoring an older house was not so expensive right now, but once done comes the big stuff and a big DOB with some 2 inch whoppers for the lower mag

[Moonshane]

I have a 26mm Nagler and a 13mm Ethos and in all honesty, the only difference to my eyes is the difference in size and weight and the wider field when comparing with my televue plossls (which cost about £45-65 used); there is no discernible difference in quality in the field you can see.

[ollypenrice]

Oh, I like wide fields! I love them as an imager and have spent the kind of money that would (not joking!) have bought me a Porsche instead of my Panda to obtain them. In visual it is much cheaper to have a wide field with large aperture. It does seem to depend on how you observe. Some people direct their attention to the centre of the field. I think I'm scatterbrained and crave to roam aaround the edges.

I can only suggest that you raid the astrosocs and try some kit. If you are not a widefield junkie save yourself some cash and rejoice!

Olly

[John]

No, that is not quite true. You can get wider fields with 2" than with 1.25" eyepieces but it depends on the design. e.g. an Explore Scientific 18mm 82° 2" eyepiece gives a true field of 73.8 arcsec, while a Televue Panoptic 24mm 68° 1.25" eyepiece will show you 81.6 arcsec.

Yes, thats a good point. The wider field is not an automatic benefit of a 2" eyepiece. The larger format gives the designer the potential to deliver a wider field of view if they so wish. Many of them do seem to have taken advantage of this possibility though !

[Stu]

True enough.

I think I gave a similar answer elsewhere but it bears repeating.

As an example, in my 106mm frac, a 1.25" 24mm Panoptic gives x29 with 2.37 degree fov. A 2" 21mm Ethos gives slightly higher power at x33, but with a 3.04 degree fov. The benefits are a smaller exit pupil and darker sky background but significantly bigger fov.

The 21e is a big ep though, and there are times when the 24mm is just much more convenient, portable, and easier to maintain balance in the scope.

Stu

[AlexB67]

Yes, thats a good point. The wider field is not an automatic benefit of a 2" eyepiece. The larger format gives the designer the potential to deliver a wider field of view if they so wish. Many of them do seem to have taken advantage of this possibility though !

Exactly that, I didn't express the point so well to the OP, but in the end to put it another way, it is possible to have a wide field of view in a 1,25 inch compared to an 2 inch depending on design. It is just that the 2 inch gives more potential to give a wider angle given the field stop, and that can become a limiting factor in achieving that, hence I quoted the formulae to give an indication of that limitation. Of course, as already said, that is not accounting for other factors such as how good that quality over the entire view will turn out to be in any given scope. Anyway, that is how I sort of understand it to be.

[bingevader]

No, that is not quite true. You can get wider fields with 2" than with 1.25" eyepieces but it depends on the design. e.g. an Explore Scientific 18mm 82° 2" eyepiece gives a true field of 73.8 arcsec, while a Televue Panoptic 24mm 68° 1.25" eyepiece will show you 81.6 arcsec.

Yes, but that's because the 18mm ES is not a true 2" EP it's a 1.25" in 2" clothing, you're comparing apples with apples, not with pears!

Cheers

[John]

Yes, but that's because the 18mm ES is not a true 2" EP it's a 1.25" in 2" clothing, you're comparing apples with apples, not with pears!

Cheers

If the optics are mounted in a 2" format body surely it's a 2" eyepiece ?

Or is the definition based on the field stop diameter ?

[BoisBlancBoy]

Instead of starting a new thread I'll just ask it here cause I'm sure it's something simple.

What is being referred to when talking about "exit pupil"?

[bingevader]

If the optics are mounted in a 2" format body surely it's a 2" eyepiece ?

Or is the definition based on the field stop diameter ?

The latter I would suggest, or I could just buy 2" adapters for all my 1.25" EPs and then call them 2" EPs!

Instead of starting a new thread I'll just ask it here cause I'm sure it's something simple.

What is being referred to when talking about "exit pupil"?

Exit pupil is the perceived aperture of light leaving/exiting the EP and so hitting your retina.

So the ideal (but not essential) would be an exit pupil that matched your fully dilated pupil.

Anything less (as in smaller focal length EPs) will only illuminate part of the retina, anything greater (as in larger FL EPs) and not all the light will enter the pupil and hit the retina.

But, before anyone starts, that's not to say it's necessarily a bad thing and doesn't mean that EPs at either end are unusable.

Cheers

[AlexB67]

The latter I would suggest, or I could just buy 2" adapters for all my 1.25" EPs and then call them 2" EPs!

Regarding the exit pupil, as a rule of thumb it is often said that the optimal eye pupil is somewhere in the 2 - 4 mm range. Neither having the exit pupil = to the fully dilated pupil is ideal, nor having it very small, but as you say, not that it cannot be used or that it is the end of the world.

Another crude rule < 0.5mm exit pupil ( high mag ) may make it prone to dimming a bit, and /or the eyepiece may not be so easy to use without getting some reflections caused by floaters in the eye, therefore it can make the eyepiece a bit less user friendly, I get that for example a bit more with a much narrower exit beam, but my missus suffers less from this at the same eyepiece, she's younger and has better eyes than me

[RikM]

The latter I would suggest, or I could just buy 2" adapters for all my 1.25" EPs and then call them 2" EPs!

This is sort of the point I wanted to highlight. EP Barrel diameter (1.25" or 2") doesn't necessarily dictate the field of view; the fieldstop the designer uses does that. But you can't determine the fieldstop diameter simply from the 1.25" or 2" barrel fitting descriptions. Certainly when I think about 1.25" or 2" descriptions I am thinking about the barrel size not fieldstop / field of view.

[John]

The latter I would suggest, or I could just buy 2" adapters for all my 1.25" EPs and then call them 2" EPs!

That would mean that the 20mm Type 5 Nagler is a 1.25" eyepiece really then as it's field stop would fit inside a 1.25" barrel but Tele Vue opted for a 2" barrel. Crazy

It's going to cause a lot of confusion for punters if vendors classify eyepieces by the field stop diameter !

[bingevader]

That would mean that the 20mm Type 5 Nagler is a 1.25" eyepiece really then as it's field stop would fit inside a 1.25" barrel but Tele Vue opted for a 2" barrel. Crazy

No, not really. I've assumed the issue with these monster EPs with lots of glass was more to do with stability, hence the 2" barrel. I don't think you'd like it dangling off the end of your 'scope in a 1.25" barrel?

It's going to cause a lot of confusion for punters if vendors classify eyepieces by the field stop diameter !

No, not really. We already talk more about focal length and AFOV/TFOV than we do about barrel size anyway. I don't know why they don't include the field stop diameter.

Cheers

[John]

No, not really. I've assumed the issue with these monster EPs with lots of glass was more to do with stability, hence the 2" barrel. I don't think you'd like it dangling off the end of your 'scope in a 1.25" barrel?

No, not really. We already talk more about focal length and AFOV/TFOV than we do about barrel size anyway. I don't know why they don't include the field stop diameter.

Cheers

You have me a bit baffled

The 18mm ES UWA has broadly the same optical design as the Naglers, has a 2" barrel and a 25.3mm field stop but you feel that should not be considered a true 2" eyepiece wheras the 20mm Nagler, which is actually physically smaller than the ES 18mm, weighs a bit more and has a slightly wider field stop, but still less than the inside diameter of a 2" barrel, is a 2" eyepiece ?

Perhaps we are going to have to agree to disagree on this. To me these are both 2" eyepieces