Why the huge variation in viewing glass size?

[steveex2003]

I realise this is a fundamentally noob question, but if one doesn't ask one doesn't learn.
I mostly use Morpheus EP's which all have a huge chunk of glass (25-30mm at a guess) to peer into. Something like a Meade MA plossl has a but a few millimetres diameter to peer into. Why would one choose this? Are these EP's less complex in design and that is their strength?
Having some plossls with smaller viewing apertures (but nowhere near as small as some designs I see) for my sins and ignorance I much prefer the big glass user experience. 
 

 

[andrew s]

It's down to two main factors.

The field of view of the eyepiece and the size of the exit pupil.

The bigger they are the bigger the eye lens needs to be.

Regards Andrew 

[Stu]

This diagram shows the huge variation in optical design. As apparent field of view and eye relief increase, so does the complexity of design and number of elements required to produce a well corrected image.

Orthoscopics are very sharp, but simple designs because of their limited eye relief and narrow fov. Something like an Ethos is likely no sharper on axis, but offers the huge field of view with still reasonable eye relief.

[John]

Having longer eye relief is a popular feature these days and that sometimes requires the use of large lenses.

Not all the wides and ultra wides have massive lenses though. The Panoptic 24mm and Nagler 31mm have more modest eye lenses.

The Explore Scientific 17mm 92 is the largest of the ones that I own:

 

 

[Ruud]

Above is how you can calculate the diameter of the eye lens needs to have given the apparent field of view (afov) and the eye relief.

Edited June 19, 2020 by Ruud

[Louis D]

6 hours ago, andrew s said:

It's down to two main factors.

The field of view of the eyepiece and the size of the exit pupil.

The bigger they are the bigger the eye lens needs to be.

Regards Andrew 

Make that these two main factors: apparent field of view (AFOV) and eye relief (ER).  As others have indicated above, the combination of AFOV and ER is generally what dictates the size of the eye lens.  An eyepiece designer can increase the AFOV while keeping the eye lens the same size by decreasing ER or increase ER and decrease AFOV.

Exit pupil doesn't really play into the equation at all as it is strictly determined by the eyepiece focal length divided by the focal ratio of the telescope.  As long as the lens system in the eyepiece is wide enough to pass the entire exit pupil without vignetting or truncation, the actual design has no effect on it.

[andrew s]

6 minutes ago, Louis D said:

Make that these two main factors: apparent field of view (AFOV) and eye relief (ER).  As others have indicated above, the combination of AFOV and ER is generally what dictates the size of the eye lens.  An eyepiece designer can increase the AFOV while keeping the eye lens the same size by decreasing ER or increase ER and decrease AFOV.

Exit pupil doesn't really play into the equation at all as it is strictly determined by the eyepiece focal length divided by the focal ratio of the telescope.  As long as the lens system in the eyepiece is wide enough to pass the entire exit pupil without vignetting or truncation, the actual design has no effect on it.

While I agree the eye relief should be added. If you exclude exit pupil size you can get vignetting if the eye lens is not large enough. You could just as well say if the lens is large enough field of view or eye relief have no effect.

Regards Andrew 

Edited June 19, 2020 by andrew s

[John]

2 minutes ago, andrew s said:

While I agree the eye relief should be added. If you exclude exit pupil size you can get vignetting if the eye lens is not large enough. 

Regards Andrew 

I'm trying to think of an example of an eyepiece / scope combination where the exit pupil might exceed the diameter of the eye lens

[andrew s]

13 minutes ago, John said:

I'm trying to think of an example of an eyepiece / scope combination where the exit pupil might exceed the diameter of the eye lens

You won't as that's not the issue. For on axis ray the exit pupil comes through the centre of the lens. For a ray at the extreme of the field of view it comes through the outer edge and if the eye lens is too small it vignettes it just as filters can do in imaging.

Regards Andrew 

Edited June 19, 2020 by andrew s

[Louis D]

3 minutes ago, andrew s said:

You won't as that's not the issue. For on axis ray the exit pupil comes through the centre of the lens. For a ray at the extreme of the field of view it comes through the outer edge and if the eye lens is too small it vignettes it just as filters can do in imaging.

Regards Andrew 

Okay, can you quote the geometric equation to ensure this doesn't happen as I've never seen it quoted anywhere?  You're saying that just because you've got a 4mm eye  lens, you may not be able to pass an entire 4mm exit pupil without vignetting which I'll grant you might be the case.  However, what is the required width to ensure that there is no vignetting for a given exit pupil?

[andrew s]

16 minutes ago, Louis D said:

Okay, can you quote the geometric equation to ensure this doesn't happen as I've never seen it quoted anywhere?  You're saying that just because you've got a 4mm eye  lens, you may not be able to pass an entire 4mm exit pupil without vignetting which I'll grant you might be the case.  However, what is the required width to ensure that there is no vignetting for a given exit pupil?

As you pointed out it would depend on the focal length of the telescope. As with other aspect of eyepieces  design you need to target a range of telescopes. For example,  focal ratio (how steep a rays must it accept without significant aberration) and telescope field curvature. 

If this were not the case we could all use the good old Huygen eyepiece that enabled our forbears to study the Universe with long focal length and ratio refractors.

It is just not as simple as it may seem. Eyepiece design if far harder than that of objectives.

Regards Andrew 

PS  Iwill provide the equation after dinner.

Edited June 19, 2020 by andrew s

[Don Pensack]

25 minutes ago, Louis D said:

Okay, can you quote the geometric equation to ensure this doesn't happen as I've never seen it quoted anywhere?  You're saying that just because you've got a 4mm eye  lens, you may not be able to pass an entire 4mm exit pupil without vignetting which I'll grant you might be the case.  However, what is the required width to ensure that there is no vignetting for a given exit pupil?

You won't run into that because the eyepiece that yields a 4mm exit pupil will not be a short focal length eyepiece.

The diameter of the eye lens is directly related to eye relief and apparent field.

If the eye relief is longer, the lens is larger.  If the eye relief is the same but the apparent field is larger, the eye lens must be larger.

if long eye relief is desired in an ultrawide eyepiece, the eye lens will be enormous.

Ruud gave the formulas in an earlier post to determine the diameter of the lens for a given apparent field or desired eye relief.

If you don't need long eye relief or wide fields, the lenses can be quite tiny.

[andrew s]

I missed @Ruud post where he gave the equation which includes eye relief, afov and exit pupil!

Given he has provided it I won't repeat it.

Regards Andrew 

 

[andrew s]

Just to round of my view. If you take the equation in @Ruud post then:

For an eyepiece only required to work on axis the desired exit pupil size defines the eye lens size alone.

As you increase the afov then both the afov and eye relief gradually become more and more important. If you don't want to vignette the edge of the afov you must still consider the design exit pupil diameter.

Regards Andrew 

Edited June 19, 2020 by andrew s

[Ags]

The Nagler 4.8mm is 82 degrees and has a tiny eye lens. That's simply because there is no eye relief - as your eyeball is right next to the eye lens it doesn't have to be big. The morpheus has something like 20 mm of eye relief so the lens has to be bigger.

Edited June 19, 2020 by Ags

[Ruud]

2 hours ago, John said:

I'm trying to think of an example of an eyepiece / scope combination where the exit pupil might exceed the diameter of the eye lens

It's impossible if you consider how the exit pupil is formed.

 

[John]

17 minutes ago, Ruud said:

It's impossible if you consider how the exit pupil is formed.

 

Yes, that's what I thought

[Louis D]

6 hours ago, Ruud said:

Above is how you can calculate the diameter of the eye lens needs to have given the apparent field of view (afov) and the eye relief.

Okay, I missed the point of the second diagram taking into account the exit pupil diameter.  Basically, just a one to one linear offset for EP diameter.

55 minutes ago, Ruud said:

It's impossible if you consider how the exit pupil is formed.

 

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?

[andrew s]

1 hour ago, Ruud said:

It's impossible if you consider how the exit pupil is formed.

 

 

3 minutes ago, Louis D said:

Okay, I missed the point of the second diagram taking into account the exit pupil diameter.  Basically, just a one to one linear offset for EP diameter.

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?

@Louis D  just look at the diagram . At the extremes the lens is just large enough to pass the whole light cone. If it were any smaller it would not as part of the beam would miss it.

I would have hoped you could see this for yourself. There is nothing spacious about this.

Regards Andrew 

[John]

 

13 minutes ago, Louis D said:

Okay, I missed the point of the second diagram taking into account the exit pupil diameter.  Basically, just a one to one linear offset for EP diameter.

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?

The pupil guides that Tele Vue supply with some of their eyepieces do mask quite an area around the edge of the eye lens but you can still see the full field and, from what I recall, with no vignetting

 

[Stu]

5 minutes ago, John said:

 

The pupil guides that Tele Vue supply with some of their eyepieces do mask quite an area around the edge of the eye lens but you can still see the full field and, from what I recall, with no vignetting

 

Isn’t that because they are positioned at the exit pupil? If they were closer to the eyepiece then they would mask the view.

[andrew s]

7 minutes ago, John said:

The pupil guides that Tele Vue supply with some of their eyepieces do mask quite an area around the edge of the eye lens but you can still see the full field and, from what I recall, with no vignetting

 

As the guide is placed close to the exit pupil it only needs to be the size of the exit pupil and not that of the eye lens to avoid vignetting. 

Regards Andrew 

Post crossed with @Stu

Edited June 19, 2020 by andrew s

[John]

Ah, yes that's true. I didn't used to use the things but I recall that the instructions said that they should be used with the top section of the eyepiece raised ie: not close to the eye lens. It's been a long time since I owned a TV eyepiece that came with them - T4 Naglers and Radians I think.

It is indeed to help find the exit pupil, hence it's name

I'll get me coat ......

 

 

 

[Don Pensack]

43 minutes ago, John said:

 

The pupil guides that Tele Vue supply with some of their eyepieces do mask quite an area around the edge of the eye lens but you can still see the full field and, from what I recall, with no vignetting

 

But the pupil guide had to be used at or near the exit pupil or it would vignette the image.

[John]

5 minutes ago, Don Pensack said:

But the pupil guide had to be used at or near the exit pupil or it would vignette the image.

Yep, I realize that now !

I hardly used the things when I had TV eyepieces that came with them and that was quite a few years back now.

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