Why the huge variation in viewing glass size?

[Don Pensack]

An eyepiece can be constructed with an eye lens larger than is necessary to field the entire light cone to the exit pupil.

The TeleVue Delites are a case in point.

The eyeguard has an opening smaller than the eyelens, yet does not vignette the field.  It does add a "light trap" when the eyeguard is raised, though.

 

One note about eye relief: If the eyelens is concave or convex, a pure calculation of the necessary lens diameter does not follow from the diameter of the visible eye lens.

The lens diameter merely defines the approximate eye relief and/or apparent field.  True eye relief is measured from the center of the top surface of the eye lens.

So if it is concave, the concavity would be added to the eye relief of the eyepiece (i.e. the eye lens diameter may not calculate the eye relief).

Likewise, if the eye lens is convex, the convexity would be subtracted from the eye relief derived from the eye lens diameter.

[Ruud]

1 hour ago, Louis D said:

I think what @andrew s is saying, what if the edge of the eye lens were masked with a circular aperture mask?  The telescope and eyepiece focal lengths would remain the same since none of the curves changed, and the exit pupil would remain the same, but some of it would have to be vignetted.  Any thoughts on the validity or speciousness of this argument @Ruud?

Hi Louis. It's a late answer, sorry for that: I was  mostly responding to John who was wondering if an exit pupil could be wider than the eye lens.

But yes, Andrew's argument makes a lot of sense. A circular mask right on top of the eye lens could entirely obscure peripheral beams, and partially obscure beams closer in, thus obscuring some and dimming other edge beams, narrowing the afov and vignetting the view.

A guide ring like the one that Don showed is intended for use near the exit pupil and is wide enough to allow all beams through. It's an aid for finding the exit pupil.

 

 

Edited June 19, 2020 by Ruud

[John]

1 minute ago, Ruud said:

....A guide ring like the one that Don showed is intended for use near the exit pupil and is wide enough to allow all beams through. It's an aid for finding the exit pupil.

To be fair to Don, it was me who introduced that into the thread, and I wish I hadn't because it's not really relevant.

 

[steveex2003]

Thank you all gents.
I feel I got real value for money out of that question.

[Stu]

7 hours ago, John said:

Wish I'd not mentioned them .......

Not at all John, I think it quite usefully adds to the info on the thread 👍👍

[Ruud]

I'm with Stu, the question about eye lens size was answered already.  The rest is a bonus. Much was clarified  and I got to see my first pupil guard.

 

[andrew s]

10 minutes ago, Ruud said:

I'm with Stu, the question about eye lens size was answered already.  The rest is a bonus. Much was clarified  and I got to see my first pupil guard.

 

Not seen your eyelids  before? 🤪

Regards Andrew 

[Ruud]

Oops, it's a pupil guide.

[Louis D]

I realized something based on this discussion.  On many of my eyepieces that lack a defined field stop and instead use the bottom of the insertion barrel as the field limit, it is possible to see slightly more field by peeking around the edge, so to speak, and looking into the eye lens from an angle at the very edge to see what is hiding behind the edge of the barrel on the opposite edge when viewing from the center of the eye lens.  Is this a truncated exit pupil or some other effect?