Dual-fit Eyepieces

[geministar]

I keep hunting for EPs with large AFOVs, but whenever I think I've found one, it's usually a dual-fit.  

I believe that using such an EP in a 1.25" barrel would actually reduce your effective AFOV from the figure stated.

Does anyone know if this is actually true?

Many thanks, I'd love to know.

[Stu]

I keep hunting for EPs with large AFOVs, but whenever I think I've found one, it's usually a dual-fit.

I believe that using such an EP in a 1.25" barrel would actually reduce your effective AFOV from the figure stated.

Does anyone know if this is actually true?

Many thanks, I'd love to know.

It depends upon the focal length of the eyepiece. Below a certain size, a 1.25" barrel is sufficient even for a 100 degree afov so it is no problem. The 13mm Ethos for instance is a dual barrel design, but the 17mm is 2" because the field needed is bigger than the 1.25" barrel.

As a rough guide, a 24mm 68 degree eyepiece or a 32mm 50 degree Plossl is about as big as you can get in the 1.25" design. Above that you need a 2" barrel.

[geministar]

What I should have said was, with dual-fit EPs like Hyperion, Ultima Duo, etc., do you get the stated AFOV if you use the EP in a 1.25" focuser?  

[rockystar]

Yes, you get the same FOV whichever fitting you use, as stated above at 68 degrees I think the longest possible focal length is 24, so if you go longer it will be 2" fitting, so if the shorter lengths also have a 2" fitting you don't need to swap out an adapter. So it seems to me that it is mainly a convenience, though I think some people prefer the 2" fitting for added security.

[Stu]

What I should have said was, with dual-fit EPs like Hyperion, Ultima Duo, etc., do you get the stated AFOV if you use the EP in a 1.25" focuser?

Yes, if it is a dual fit eyepiece then you will get the stated afov whether you use in 1.25" or 2" diagonals/focusers etc

[ollypenrice]

I'm thinking you must do because, at least in the only dual-fit I have, the 1.25 nosepiece remains in place inside the 2 inch barrel. If it were going to vignette you'd need to remove it but you don't do so in 2 inch format.

Olly

[bingevader]

So to answer your original statement...

I believe that using such an EP in a 1.25" barrel would actually reduce your effective AFOV from the figure stated.

the answer is, not so!

The piece of glass remains the same size and is housed within the 1.25" barrel.

The 2" fitting is for convenience/safety.

[John]

The only exception to the above is the Baader Hyperion Aspheric eyepieces which are 2" format but come with a screw in alternative 1.25" barrel. With the 1.25" barrel in place the apparent field of view of these eyepieces drops from 72 degrees to around 55 degrees.

The eyepieces with hybrid 1.25" - 2" barrels are 1.25" eyepieces with the 2" barrel section provided as an alternative fitting, usually because the eyepiece is large and heavy and may be more secure in a 2" drawtube / diagonal.

[geministar]

Brilliant - consistent and logical responses (as expected!) - so now I can widen my search for new eyepieces.

Thanks everyone.

Doug.

[DAVE AMENDALL]

Skywatcher do a SWA-70  8mm with dual fitting   70 degree F.O.V. but I do not know if you have to remove the 1.25" barrel to achieve this with with the 2" fit

 

[John]

48 minutes ago, DAVE AMENDALL said:

Skywatcher do a SWA-70  8mm with dual fitting   70 degree F.O.V. but I do not know if you have to remove the 1.25" barrel to achieve this with with the 2" fit

 

The 1.25" barrel is not removable - it contains optical elements that create the focal length and eye relief that the eyepiece has. The 2" skirt is not removable from these either - someone tried to remove one once using a lot of force and managed to ruin the eyepiece !

 

[Louis D]

On the other hand, if you use one of these 2" eyepiece to 1.25" drawtube adapters, you might see vignetting with some 2" eyepieces, if you can reach focus.

[Paul Winkler]

Whilst I have presumed that the AFOV would be unaffected by which choice (1.25 or 2") of barrel diameter, I cannot get my head around the fact that the light entering the eyepiece is coming through a 1.25" opening.  If you had two telescopes, one that had an aperture of 20", and one that had 12.5", the 20" one would have a larger clear aperture, and more light-gathering power.  Would this not also apply to eyepieces?  Particularly one where the 2" fitting is only a skirt around the 1.25" barrel?

[Louis D]

4 hours ago, Paul Winkler said:

Whilst I have presumed that the AFOV would be unaffected by which choice (1.25 or 2") of barrel diameter, I cannot get my head around the fact that the light entering the eyepiece is coming through a 1.25" opening.  If you had two telescopes, one that had an aperture of 20", and one that had 12.5", the 20" one would have a larger clear aperture, and more light-gathering power.  Would this not also apply to eyepieces?  Particularly one where the 2" fitting is only a skirt around the 1.25" barrel?

Think of it this way, the 20" just packs more photons into the same size image circle if the focal length is the same (faster optic).  If the 20" is the same f-ratio (longer focal length), the image will be larger but no brighter per unit area.  You're right, you would need a larger field lens on the eyepiece to grab the same TFOV on the sky.  There is no limit to this with refractors, but with catadioptrics, you run into cutoff from the baffle tube eventually.  On Newts, you have to enlarge your secondary to send that larger image to the side, decreasing contrast by increasing diffraction around it.

[michael.h.f.wilkinson]

A 1.25" barrel is sufficient as long as the field-stop diameter is smaller that about 27 mm. The field stop determines which part of the image plane of the telescope is visible to the observer. Field-stop diameter and focal length of the EP together determine the apparent FOV. In practice, for SWA EPs (68-70 deg FOV) your maximum focal length for a 27mm field stop is about 24-25mm. For UWA EPs (82mm) that figure is around 17mm.

[billyharris72]

As above, the key thing is not the size of  the barrel, but the diameter of the field stop. Think of it like stopping down the lens of a camera. Even if the front (in this case the barrel) is larger, the field stop, which controls the amount of light entering the eyepiece (and is usually internal), will remain the same size.

A useful (approximate) formula here is: AFOV = 57.3 x d / f

Where d is the diameter of the field stop and f is the focal length of the eyepiece. When you consider a 1.25 inch barrel, the absolute maximum achievable field stop will be about 28-29mm. If we were to take a 32mm eyepiece with a 28.5mm field stop this would give a maximum AFOV of about 51 degrees, while a 25mm ep would have a ~68 degree maximum FOV. For a 15mm ep the maximum AFOV from a 1.25 inch barrel grows to a massive 109 degrees.

The upshot of all this is that the only optical benefit of 2" eyepieces is achieving wide apparent fields at longer focal lengths. Almost all of the dual fit EPs are fine with either fitting - the 2" nose is there for security and convenience.

Billy.

[Louis D]

On 8/31/2016 at 10:13, John said:

The 1.25" barrel is not removable - it contains optical elements that create the focal length and eye relief that the eyepiece has. The 2" skirt is not removable from these either - someone tried to remove one once using a lot of force and managed to ruin the eyepiece !

 

I have the Astro Tech AF70 version of the 13mm and 17mm eyepieces.  The 2" barrel does unscrew on them.  Only the 22mm is a true 2"-only eyepiece in that line.  I would think a capable machinist should be able to remove the SWA70 2" barrel on a lathe.  I'm just not sure if it would be necessary to remove the lenses first or if it could be with them covered over.

[John]

I'm not aware that 1.25" eyepieces actually loose / vignette light that has been gathered by the primary any more than 2" ones

I thought that the light cone from the primary would have narrowed to small point by the time it reaches the focal plane of the eyepiece where the field stop is placed.

I could be wrong on this though !

 

 

 

[Louis D]

On 9/28/2016 at 10:37, John said:

I'm not aware that 1.25" eyepieces actually loose / vignette light that has been gathered by the primary any more than 2" ones

I thought that the light cone from the primary would have narrowed to small point by the time it reaches the focal plane of the eyepiece where the field stop is placed.

I could be wrong on this though !

 

 

 

There is an image circle defined by the OTA design.  Baffles, baffle tubes, secondary size, etc. all conspire to minimize the maximum possible size of that image circle.  Eyepieces merely magnify the central part of that image circle to one degree or another.  The AFOV determines how much of the image circle is displayed at that particular magnification (the field stop).  A 9mm monocentric cuts off way more possible image than a 9mm ES-120, yet both show the same magnification for this reason.  What happens to the rest of the image?  It slams into the field stop beyond the imaging hole.  You could think of it as 100% vignetting beyond the field stop.  If there was no limit to the AFOV width, you could display the same image at 5mm as at 40mm, just way bigger.  To a rough first approximation, and disallowing for angular distortion, you'd need about a 560 degree AFOV to accomplish that.  Obviously, that's never going to happen.

It's pretty easy to convince yourself that the light cone doesn't narrow to a point by viewing the moon.  Simply center the moon in the FOV, then replace the eyepiece with a piece of tracing paper or tissue paper or even light weight printer paper.  Refocus the image until the moon is sharply focused on the paper as if it was a piece of film.  Is the moon a pinpoint?  No, it's an extended object with an extended image.  Moving the moon's image around within the focuser tube allows you to see the image circle dictated by the OTA and focuser combination.

Technically, each point in space (like a faint star) does render as a single point on the paper/film.  However, there are an infinite number of points in every FOV, and thus there is an extended image at best focus for every FOV.  Those ray trace diagrams showing the light cone narrowing to a point are showing one particular point in space, not the entire light cone.  They may draw several to try to convey the presence of a light cone (center and edges, usually).

[astronymonkey]

On 08/31/2016 at 16:13, John said:

The 1.25" barrel is not removable - it contains optical elements that create the focal length and eye relief that the eyepiece has. The 2" skirt is not removable from these either - someone tried to remove one once using a lot of force and managed to ruin the eyepiece !

 

Ah yes, I think I remember that thread too John. It was a xmas present I think where the poor chap resorted to some mole grips to remove the 2inch part and ended up with tears by bèdtime .

[Ruud]

Eyepieces with field stops that fit a 1.25" barrel are usually designed as 1.25" eyepieces. Occasionally, when the eyepiece is very heavy and/or has a broad top, a 2" sleeve is added to an 1.25" eyepiece so that the eyepiece can also be used in a 2" focuser. 

If an eyepiece's field stop will fit a 1.25" barrel can be seen in this diagram:

The blue line is for a field stop of 29mm and an Angular Magnification Distortion (AMD) of 0%. 29mm is the widest field stop that could possibly fit in a 1.25" barrel.

When an eyepiece has an apparent field of AFOV degrees and a focal length of EFL mm, it's field stop is calculated as follows:

           Field stop (mm) = EFL (mm) x AFOV (°) / 57.3

The blue line is for situations when Angular Magnification Distortion is absent. Sometimes AMD is present, and has a positive value of a few percent. In such a case the true field stop is as many percent smaller than the calculated field stop.

In the diagram you can see that a 15mm 110° eyepiece should fit in a 1.25" barrel, as it is below the blue line.

A 21mm 100° eyepiece will not fit a 1.25" barrel, as it is above the blue line.