47 replies to this topic

[cpsTN]

I ask this question because, years ago when 2 inch EPs became available, most 1.25 inch EPs were 50-60 degrees or so. Now, though, it is common to have 1.25 inch EPs with 82 degree fields of view and even larger. In light of this, are 2 inch EPs of limited use over what can be gotten with wide-field 1.25 inch EPs? Is this what prompted the production of 3 inch and larger EPs?

[TOMDEY]

Yes. The two and three-ichers are as useful as ever, in arenas where 1.25 simply can't compete. This graphic that I came up with shows that overtly >>>    Tom

 

~click on~ >>>

Attached Thumbnails

• 

[eyeoftexas]

At the minimum they are because they cannot make a 1.25" eyepiece with widefield (82, 100, etc) at focal lengths longer than about 26 or 28 mm (I forget the exact number, but it's around that FL).  For example, Panoptics are 1.25" up to 24mm, and 2" for longer.

 

And Tom's excellent graphic illustrates why!

Edited by eyeoftexas, 05 March 2021 - 08:42 AM.

[dark_night]

Don't know much about eyepiece design so can't provide a technical answer. But I have not seen 1.25" high focal length Eyepiece provide more than 45-50 degree.

[eyeoftexas]

Yes. The two and three-ichers are as useful as ever, in arenas where 1.25 simply can't compete. This graphic that I came up with shows that overtly >>>    Tom

>>>

Exactly graphic Tom.  Although a minor point, you forgot the TV 55 mm Ploessl.

[junomike]

Exactly graphic Tom.  Although a minor point, you forgot the TV 55 mm Ploessl.

TV 55 Pl is at the 2" Ceiling limit (46mm).  The rest of the TV Pl are at or below the 1.25" Ceiling limit.

[gnowellsct]

You might get the impression that wide fields can be made in 1.25" format because there a lot on offer typically at 24 mm and shorter. The 24 pan is 1.25 inch and 68 degrees. But to get longer focal length oculars for wide field views you need two inch oculars and a two inch focuser.

They offer some of the best views to be had.

Going the other way two inch format goes extinct around 25 mm, but the Nagler 17 T4 is an exception. Even it has dual barrel format and can be used in 1.25 inch focuser.

[eyeoftexas]

TV 55 Pl is at the 2" Ceiling limit (46mm).  The rest of the TV Pl are at or below the 1.25" Ceiling limit.

Yep, I knew that.    I was simply pointing out that it was missing.  Very helpful plot!  A picture indeed is equal to a thousand words.

[cpsTN]

Don't know much about eyepiece design so can't provide a technical answer. But I have not seen 1.25" high focal length Eyepiece provide more than 45-50 degree.

I depends upon what you mean by high focal length, but I generally know what you mean. 
 

[Sarkikos]

Among my eyepieces, here are the longest focal lengths with the widest AFOV for each format:

 

1.25":  13 Ethos 100 degrees

2":  25 ES 100 degrees

 

The 13 Ethos is a dual 1.25" / 2".

 

Are there any eyepieces for these formats with a longer focal length without a narrower AFOV?

 

Mike

Edited by Sarkikos, 05 March 2021 - 09:49 AM.

[Sarkikos]

TV 55 Pl is at the 2" Ceiling limit (46mm).  The rest of the TV Pl are at or below the 1.25" Ceiling limit.

AFAIK, there are 2" eyepieces with longer focal lengths than the TV Plossl, but they will have less than 50 degree AFOV.  Or will they?  I've owned some of these.  Owned.

 

Mike

Edited by Sarkikos, 05 March 2021 - 09:47 AM.

[SeattleScott]

Stellarvue Optimus 13.5mm. Meade MWA 15 would be a competitor, advertised as 100 AFOV, but probably more like 90.

ES 25mm 100

Scott

Edited by SeattleScott, 05 March 2021 - 09:46 AM.

[Sarkikos]

The longest focal length 1.25" eyepiece I've owned was a Rini Modified Plossl/RKE:  45mm, 36 degree AFOV.  Eye relief 36mm.

 

The longest focal length 2" eyepiece I've owned was a Russell Optics 68mm.  I measured the field stop with calipers as 40.5mm.  Eye relief 25mm.  The claimed AFOV was 50 degrees.

 

Mike

[Sarkikos]

Stellarvue Optimus 13.5mm. Meade MWA 15 would be a competitor, advertised as 100 AFOV, but probably more like 90.

ES 25mm 100

Scott

You're right.  I forgot about my ES 25mm 100.  I don't use it as much as the Ethos.

 

(Edited post #10.)

 

Mike

Edited by Sarkikos, 05 March 2021 - 09:51 AM.

[25585]

You might get the impression that wide fields can be made in 1.25" format because there a lot on offer typically at 24 mm and shorter. The 24 pan is 1.25 inch and 68 degrees. But to get longer focal length oculars for wide field views you need two inch oculars and a two inch focuser.

They offer some of the best views to be had.

Going the other way two inch format goes extinct around 25 mm, but the Nagler 17 T4 is an exception. Even it has dual barrel format and can be used in 1.25 inch focuser.

Depends on AFoV, eye relief in their design. 1.25" fit is not practical for heavy eyepieces, both stability and structure need a 2" stem as its "leg".

[Sarkikos]

Depends on AFoV, eye relief in their design. 1.25" fit is not practical for heavy eyepieces, both stability and structure need a 2" stem as its "leg".

That is true if you're putting the heavy 1.25" eyepiece in the end of a diagonal on a refractor or Cat.   It does make for a heavy, long and cumbersome cantilever. 

 

But not so much true on a Dob.   A 2" focuser with a 1.25" adapter is a pretty secure anchor.  Also, a number of the heavier eyepieces that are 1.25" capable are in a dual 1.25" / 2" format.

 

Mike

Edited by Sarkikos, 05 March 2021 - 12:49 PM.

[Sarkikos]

Are there any 3" eyepieces that would be worthwhile in any telescopes besides large observatory telescopes or maybe big slow refractors or Cats?

 

Mike

[cpsTN]

That is true if you're putting the heavy 1.25" eyepiece in the end of a diagonal on a refractor or Cat.   It does make for a heavy, long and cumbersome cantilever. 

 

But not so much true on a Dob.   A 2" focuser with a 1.25" adapter is a pretty secure anchor.  Also,  a number of the heavier eyepieces that are 1.25" capable are in a dual 1.25" / 2" format.

 

Mike

That brings to another point. If an eyepiece has both 1.25" and 2" skirts, then the 2" format isn't necessary.

 

[eyeoftexas]

Are there any 3" eyepieces that would be worthwhile in any telescopes besides large observatory telescopes or maybe big slow refractors or Cats?

 

Mike

I don't know of any commercial telescopes that come with 3" focusers to accept a 3" ocular.  But, since 3" oculars tend to be very low magnification (long focal length), the exit pupil will most likely be very large in any scope.

[Sarkikos]

That brings to another point. If an eyepiece has both 1.25" and 2" skirts, then the 2" format isn't necessary.

Not necessary, but welcome.  In a Dob focuser or a 2" diagonal, I'd rather have the eyepiece fully engaged in the 2" format.  It just makes for a more secure and less fussy accessory stack.

 

Mike

[Sarkikos]

My NP101is has a 3" entrance aperture.  I'm not sure if it could be adapted to use 3" eyepieces.  Maybe it would be like using 1.25" eyepieces in a 0.965" focuser with a step-down adapter? 

 

Mike

[Frenchy]

Meade MWA 15 would be a competitor, advertised as 100 AFOV, but probably more like 90.

Scott

Across the series, the MWA's are said to be 87-92 degress. 87 is still wide, but a little far off of being advertised as 100 deg I'd say 

[LDW47]

I ask this question because, years ago when 2 inch EPs became available, most 1.25 inch EPs were 50-60 degrees or so. Now, though, it is common to have 1.25 inch EPs with 82 degree fields of view and even larger. In light of this, are 2 inch EPs of limited use over what can be gotten with wide-field 1.25 inch EPs? Is this what prompted the production of 3 inch and larger EPs?

Does the fact that probably more than 90% of fellow astronomers have more than 1-2” ep answer your query, lol ? 

[PJBilotta]

I ask this question because, years ago when 2 inch EPs became available, most 1.25 inch EPs were 50-60 degrees or so. Now, though, it is common to have 1.25 inch EPs with 82 degree fields of view and even larger. In light of this, are 2 inch EPs of limited use over what can be gotten with wide-field 1.25 inch EPs? Is this what prompted the production of 3 inch and larger EPs?

A more nuanced answer might be yes and no - it all depends on your telescope.

 

If you are dealing with a Schmidt-Cassagrain, Mak, or other compound scope, your maximum FOV will always be limited by the baffle tube diameter. The maximum you can get out of an 8" SCT, for example, is virtually identical whether you use a 1.25" 24mm/68 degree with an f/6.3 reducer, or a 2" 40mm/70 degree like a Pan, Pentax or SWAN. Either way, you're going to get approximately the same maximum true FOV possible in an SCT. You can squeeze a tiny bit more out of the scope with a 2" 28mm/68 or 30mm/65, but just a bit more. Below 20mm limit, there are a plethora of exceptional 1.25-inchers (Pans, Naglers, Pentaxes, Morphs and other less expensive options) that provide exceptional views from 68-82 degrees and beyond. So, one could reasonably argue that there is little to be gained from 2" eyepieces in this circumstance - unless you just like using 2-inchers (which I do).

 

Any other kind of scope like a Newt that has no baffle tube and is only limited by the 2" or 3" barrel? Go BIG baby!!! There's no arguing that the big babies are the way to go for maximum field and comfort if you don't mind the size, weight, and price.

[Sarkikos]

 

Any other kind of scope like a Newt that has no baffle tube and is only limited by the 2" or 3" barrel? Go BIG baby!!! There's no arguing that the big babies are the way to go for maximum field and comfort if you don't mind the size, weight, and price.

But then you are limited by the entrance pupil of your own eye.  If the exit pupil at the eyepiece is 9mm but your iris only expands to 7mm, there is going to be some lost light.  Also, you might notice the shadow of the diagonal.

 

Mike