SAEP Eyepiece FOV Comparison in a 127mm Synta Mak

[Louis D]

I took a bunch of photos of the fields of view (FOVs) of many of my 12mm+ eyepieces in my Orion (Synta) 127mm Mak using a 2" visual back and 2" diagonal using an LG G5 phone's superwide angle camera.  This scope and camera combination really shows any spherical aberration of the exit pupil (SAEP).  I composited together a bunch of the most interesting images with the worst SAEP offenders on the left and the best on the right.  Rows are arranged by focal length, although I sometimes cheated and put interesting images on a nearby row to avoid the composite image getting too wide.

I hope y'all find this interesting.  It might help to explain why some people don't get on well with certain eyepieces.

Shadowing is nascent SAEP.  Rainbows indicate chromatic aberration of the exit pupil (CAEP).

I had difficulty suppressing eye lens reflections on some of the Rini eyepieces, so they look kind of funky as a result.  I included them because they are purely positive eyepiece designs that show lots of SAEP.

[Ricochet]

Some of those are quite extreme. Is some of the problem made worse by having the wrong spacing between the eyepiece and camera? If you were to see that visually many of those eyepieces would be completely unusable. 

[Louis D]

1 hour ago, Ricochet said:

Some of those are quite extreme. Is some of the problem made worse by having the wrong spacing between the eyepiece and camera? If you were to see that visually many of those eyepieces would be completely unusable. 

It's because of the slow f-ratio of the scope combined with the slow f-ratio of the camera.  My Galaxy S7 camera is faster and picks up less of the SAEP in comparison.  It mostly shows fleeting shadows instead of donuts.  In my AT72ED f/6 refractor, some of the worst offenders show SAEP with the slow camera and only the worst of the worst show it in the faster camera.

The analogous situation for the slow camera would be daytime, solar, or lunar observing where the eye's pupil constricts significantly, blocking more midfield rays as @Ruud shows quite clearly in this animation:

At night, with a fully dilated pupil, SAEP becomes much less problematic.

Camera-eyepiece spacing was at the exact point the full field stop came into view.  Obviously, pushing in further reveals blackouts, but not of the SAEP type.  Also, if you back off, you can avoid a lot of the SAEP as the two Meade MWA 26mm images show, but you lose some of the available field of view.  Both situations are shown in another of @Ruud's excellent animations:

Notice that it is edge rays, not midfield rays, that are truncated first in this situation.  Neither being too close nor too far would show the black midfield rings I photographed.

Some folks have complained about "finicky" exit pupils in the NT4s, Meade 4000 UWA 14mm, and the ES-92 12mm.  I believe it is undiagnosed SAEP that has been causing it.

Edited July 2, 2020 by Louis D

[Ricochet]

Thanks, Louis. The camera aperture being analogous to a very small pupil explains it well. I can't visualise why the Mak is modifying the exit pupil of the eyepiece though. 

[Louis D]

I can't figure that out, either.  The numbers from a digiscoping calculator indicate that until I get to an 8mm eyepiece, there is no decrease from the camera's f/2.4 native f-ratio for the 127 Mak and 3.5mm for the refractor.  And yet, eyepiece for eyepiece, I'm seeing more pronounced SAEP in the f/12 Mak than in the f/6 refractor.  However, the effective f-ratio remains f/2.4 for both.

[Don Pensack]

It's interesting.

It reveals that SAEP is common in ultrawides, a little less severe in wide angle eyepieces, and fairly minimal in narrower fields.

It also reveals where I, personally, see SAEP and where I don't.

I only notice it in 9 of those eyepieces, though it is present in the image in 33 of them.

Each of us has a greater or lesser sensitivity to SAEP, and it takes experimentation to find out your level of sensitivity.

I see a lot in the 14mm Meade Series 4000 UWA and 14mm Vixen SSW.

I see none in the APM 30mm UFF, though some is indicated in the pic.

One mystery to me is the TeleVue Nagler 22mm Type 4.  I notice some SAEP (but mild) when using the eyepiece without glasses, 

but none whatsoever when using the eyepiece with glasses, yet the image is horrendous.

One advantage to wearing glasses at the eyepiece (as I do with the 22mm) is you are not constantly moving your eye around.  Once you are there at the exit pupil, you stay there.

Without glasses, your eye is wandering a bit, and SAEP is made worse by wandering in and out relative to an eyepiece, or back and forth.

 

Too bad the TeleVue Apollo 11 is not included.  This is one ultrawide with just about zero SAEP.

 

 

[Louis D]

It's been a few years, and I've acquired a few more eyepieces, so I updated the SAEP/CAEP image with those new eyepiece views.  The order is a bit scrambled now to fit in the new ones without messing with the existing ordering too much.  Images were taken with the LG G5 for consistency sake despite having the much better LG G6 now.  That, and the two cameras respond slightly differently to CAEP/SAEP despite both being 2mm f/2.4 lenses.  Enjoy!

[Louis D]

Another two years, and a few more new eyepiece AFOV images.  I took the time to rearrange things a bit to make it better ordered.

[globular]

On 02/07/2020 at 22:00, Louis D said:

Camera-eyepiece spacing was at the exact point the full field stop came into view.

Using Ruud's vignetting animation, your method of placing the camera would appear to be here; which is as far away from the EP as possible where the green light is visible.

Clearly this is not the optimal placing for best viewing of the image, which would be here...

 

What your images are highlighting is which eyepieces have SAEP, and how severely, when you're a bit too far away from optimal placement.  

In practice, an observer would simply move a bit closer and have no problems.

The EPs where you show 'good' images might actually have bad SAEP if the observer is too close, but you will not have captured that. 

In practice, EPs with a large range of movement (between the 'close' and 'far away' extremes where the field stop is visible) and where there is no / little SAEP visible are the 'most comfortable' to use. But, again, I'm not sure your images allow this to be determined.

If you take another image, where you start with camera very close to the EP and you move the camera away until the field stop is visible, then you'll get a 'too close' view of SAEP. So here...

This might show some / no SAEP in different EPs than the existing set of images.

The number of millimetres between the 'too close' and 'too far away' positions will give a sense of the 'comfort' of the EP - the larger the distance the more comfortable an EP is likely to be. 

And an image taken half way between the two extreme positions should show no SAEP at all, or at least the minimum SAEP achievable in the EP - and if there is some then it's likely nearly all users will suffer it.

 

p.s. I'm not being critical Louis - I appreciate the time and effort you have put in to produce these quality images.  It's just a suggestion that might improve them further - and it might paint a picture of SAEP which is closer to peoples actual experience using these eyepieces.

[Mr Spock]

I use a 42mm LVW every time I observe. I've yet to see any blackouts.

The scope you are using creates an issue with this and other 2" eyepieces - the baffle tube is only 27mm. This doesn't allow full field illumination. I'd suggest using a refractor. Also use the right kind of diagonal if you are using one - some 2" diagonals have a restricted opening.

[Louis D]

4 hours ago, globular said:

Using Ruud's vignetting animation, your method of placing the camera would appear to be here; which is as far away from the EP as possible where the green light is visible.

Clearly this is not the optimal placing for best viewing of the image, which would be here...

 

What your images are highlighting is which eyepieces have SAEP, and how severely, when you're a bit too far away from optimal placement.  

In practice, an observer would simply move a bit closer and have no problems.

The EPs where you show 'good' images might actually have bad SAEP if the observer is too close, but you will not have captured that. 

In practice, EPs with a large range of movement (between the 'close' and 'far away' extremes where the field stop is visible) and where there is no / little SAEP visible are the 'most comfortable' to use. But, again, I'm not sure your images allow this to be determined.

If you take another image, where you start with camera very close to the EP and you move the camera away until the field stop is visible, then you'll get a 'too close' view of SAEP. So here...

This might show some / no SAEP in different EPs than the existing set of images.

The number of millimetres between the 'too close' and 'too far away' positions will give a sense of the 'comfort' of the EP - the larger the distance the more comfortable an EP is likely to be. 

And an image taken half way between the two extreme positions should show no SAEP at all, or at least the minimum SAEP achievable in the EP - and if there is some then it's likely nearly all users will suffer it.

 

p.s. I'm not being critical Louis - I appreciate the time and effort you have put in to produce these quality images.  It's just a suggestion that might improve them further - and it might paint a picture of SAEP which is closer to peoples actual experience using these eyepieces.

I move the camera inward toward the eye lens looking for the sharp field stop boundary to pop into view.  If I continue inward, I will get blackouts with every eyepiece regardless of SAEP.  This is basically the same way I approach an eyepiece with my eye.  Move inward until the entire FOV pops into view, but no closer.  There really isn't much, if any, room for variance forward/back with either my eye or the camera.

What you suggest about pulling back from the exit pupil is what I ended up doing with the Meade MWA 26mm.  The "Full" view is right when the FS pops into view.  The SAEP blackouts with my eye or the camera are tremendous and prevent seeing the entire FOV at once.  Perhaps if my iris was fully dilated, it would be more usable.  Note the second image next to "Full" labelled "Easy".  That image was taken getting as close as possible before SAEP blackouts became too strong to see the entire visible FOV, which is less that the entire true FOV.  Notice that the shadow pattern is different.  It has a bullseye surrounded by a thin black circle.  I don't have a ray trace to explain what it is that I've imaged, but it is quite manageable to either photograph or to view with the eye.  That's why I called it "Easy".  You don't have to fight the SAEP blackouts, but you lose about 4 degrees of FOV and have a less sharp, false field stop.

BTW, every raytrace image you captured is for an eyepiece without SAEP.  All the rays converge in a point.  Your conclusions are based on the wrong raytrace diagrams.

Here's a screengrab from those same animations of what SAEP looks like:

The animation assumes quite a large iris opening.  Shrink down the iris opening, and the red rays will be cutoff even whenever the eye or camera is close enough to see the field stop, resulting in kidney beaning if the eye or camera is off-axis.  On-axis, it results in a black ring.  Notice that if the eye/camera pulls back somewhat, the red rays will be captured, but the green rays will not.  I see this as a false field stop when on-axis.  If at the same pull-back position I tip my eye or camera and move off-axis to view the green edge rays, the opposite red and even some orange on-axis rays get cutoff, resulting it loss of FOV.  It appears like I'm peaking under a black ledge to see the true field stop at the expense of the rest of FOV.

I can't explain why the slow Mak (f/12) shows SAEP so much better even when using the same camera (f/2.4).  With the faster Galaxy S7 wide angle camera, the shadowing is much less distinct, probably due to it being f/1.7 and thus having a larger iris opening.  Combine the faster camera with the faster ED refractor (f/6) that I use for my general AFOV images, and SAEP is much less pronounced.

Of course, if you are young and your eye is fully dilated, SAEP may not even be visible in the worst eyepieces for you because none of the red rays get cutoff.

Edited September 1 by Louis D

[Louis D]

3 hours ago, Mr Spock said:

I use a 42mm LVW every time I observe. I've yet to see any blackouts.

The scope you are using creates an issue with this and other 2" eyepieces - the baffle tube is only 27mm. This doesn't allow full field illumination. I'd suggest using a refractor. Also use the right kind of diagonal if you are using one - some 2" diagonals have a restricted opening.

I never claimed I saw SAEP in the 42mm LVW.  I also don't see it in the 42mm Pentax XW, 40mm ES-68, or even the 40mm Rini Erfle.  All four widest field eyepieces have a black dot surrounded by a widely separated black ring with an indistinct edge.  Notice that the Edmunds RKE 28mm and the two 26mm Plossls also have it to some extent as well.  Even my 29mm ES-92 (a 12mm ES-92 minus the Smyth group) has it.  Yet, none of these ever exhibit true SAEP which manifests itself as an unavoidable blackout shadow in my experience.  In photographs, it manifests itself as a clear center and edge with a black donut ring in between.

I honestly don't know what causes that particular shadow pattern from a raytrace perspective, but it's what I get with those eyepieces when the camera is at the position when the field stop just pops into view while moving the camera inward toward the eye lens.  Any closer, and the image gets more shadowy and more messy to photograph.  I don't see the shadows with my eye under the same daytime conditions.

If I had an f/12 or slower refractor with a 2" focuser, I'd give it try, but I don't.  The only effect the 27mm baffle tube has is to cause outer field vignetting as is visible in the 29mm ES-92 image.  The image is actually wider in my refractors, but the Mak cuts it off.  If it was the 27mm baffle causing the issue, why does the Lacerta ED 40mm with roughly the same field stop and TFOV have no shadows of any type?  There is some vignetting center to edge due to the 27mm baffle, but no blackout shadows.  In use during the partial phases of the April 2024 total solar eclipse, the Lacerta was indeed the easiest to hold the view in of the widest field eyepieces, so I kept it in the focuser for the total phase.  It was only in comparison that I felt that the exit pupil was easier to hold in the Lacerta during solar observing.  I could have used any of the others quite happily, but chose to go with the easiest to view of the bunch.

I'm using a 2" GSO diagonal with a 46mm clear aperture, so no worries there.  Ideally, I should use a 3" diagonal with a 2" step-down adapter in a refractor with a 3" focuser to avoid all vignetting, but seeing as I have neither, that's not going to happen.

Edited September 1 by Louis D

[globular]

I've used enough of the EPs in your collection to know that these images do not represent my experiences when using them.  That's why I had a think about what might be going wrong - because clearly something is.

If these are the best images you can produce with the set-up you're using then the only conclusion I can draw is that your set-up is not representative enough to produce a meaningful analysis of what we can expect from these eyepieces when used visually.

It's a shame, because it could have been very useful.

[Louis D]

15 minutes ago, globular said:

I've used enough of the EPs in your collection to know that these images do not represent my experiences when using them.  That's why I had a think about what might be going wrong - because clearly something is.

If these are the best images you can produce with the set-up you're using then the only conclusion I can draw is that your set-up is not representative enough to produce a meaningful analysis of what we can expect from these eyepieces when used visually.

It's a shame, because it could have been very useful.

Which eyepieces?  Are you using them during the daytime?  If you only use them at night with your iris fully dilated, you probably won't have any issues.

In particular, the 12mm and 17mm NT4s are all but impossible to use during the daytime for solar observing in a slow scope.  They're fine at night in a faster scope, given enough time for one's iris to fully dilate.  Others have described them as having "fussy" exit pupils that make them tiring to use during the daytime.  That's what I find as well.  It wasn't until I photographed them with this setup that it dawned on me that they have such massive SAEP that nearly the entire field blacks out during daytime observing.  Even lunar observing with them is problematic if the phase is near full.

These images represent the worst case scenario which would be daytime observing in a slow scope.

They also capture why @John and others could never got on with the 12mm ES-92, yet the 17mm version was tolerable to them.  Just look a the relative difference in SAEP between the two to see why.  It's barely there in the 17mm.

However, in daytime use, I have no shadowing issues with either the 12mm ES-92 or the 12.5mm APM Hi-FW, both of which exhibit very similar SAEP photographically.  It is interesting to note that both have less than opaque shadows with fuzzy edges, indicating a somewhat different form of SAEP that is not quite as intrusive as the completely opaque and sharp edged version shown down the left edge.

Edited September 1 by Louis D

[globular]

10 minutes ago, Louis D said:

Which eyepieces?  Are you using them during the daytime?

ES92/12, ES92/17, N22T4, UFF30, UFF24, XW40

I have used them during the day - for tests and measurements - but basically I use them at night. Including full moon lunar with no dark adaptation.
I observe without glasses - my wife uses them with glasses.
Neither of us recognise your images as representing what we see.
The N22T4 and ES92/12 in particular.  I never get views like these and I'd class the N22 as very easy placement.  The ES92/12 is more picky about placement, but I've never seen any shadowing anything like your image.
 

3 minutes ago, Louis D said:

If you only use them at night with your iris fully dilated, you probably won't have any issues.

Isn't your camera equivalent to a fully dilated eye? Presumably it's sensor is larger than the exit pupil size? If not then isn't that a / the problem?

 

As I said before, I'm not knocking your efforts.  There is some value there.  I just think it's not representative of actual visual use.  Perhaps when you post them they should come with some caveats - or people with experience of some of the eyepieces might devalue their worth on EPs they haven't tried;  and people with no experience might be incorrectly put off trying some of the EPs even though they will have no issues whatsoever if they did.

[Mr Spock]

1 hour ago, Louis D said:

I never claimed I saw SAEP in the 42mm LVW. 

Your image suggests otherwise. If they aren't accurate then perhaps you shouldn't be publishing them. Some of those blackouts look horrible. 
I've had both 22mm and 17mm T4 Naglers. They did have black outs but it was manageable and not evident if your eye was in the right position.

[Don Pensack]

The thing is, a lot of eyepieces have SAEP to some degree.

If it's egregious, like the original Naglers or the more recent Vixen SSWs, it will cause problems for most users.

If it's minimal, even if it's there most people won't notice.

It really isn't an issue with most modern eyepieces.

 

What I'd like to know is why and how some eyepieces have their fields seemingly at different distances from the eye.

I change eyepieces a lot when observing, and one eyepiece will seem to have its field a nice comfortable distance from the eye, while another seems to require me to use my "medicine bottle reading" eye focus because the image is really up close and personal.

I have verified that it isn't apparent field, because some 60° eyepieces yield a closer-to-the-eye focal plane than some 100° eyepieces.

I think the answer to this could be related to why some eyepieces seem comfortable and others don't.  I don't ever see this discussed.

 

I'd also like to know why some 100° eyepieces seem to hide their field stops under a lip, where you have to sort of "look around the corner" to see the field stop, while others seem to yield a 100° view that, while rocking of the head to look at the edge is still necessary,

there does not appear to be a lip or corner you have to look around to see the field stop.  Is it the shape of the interior?  A baffle added?  

 

Speaking of 100° eyepieces, why do some require a very steady head at the exit pupil, while others allow you to bob around in and out and side to side?

It's not the focal length or exit pupil since an 8mm and a 7mm seemed to be completely insensitive to head movement while a 13mm and 4.7mm on either side were a lot more sensitive to eye placement.

Not really hard to use, just a bit more sensitive to head placement.  I don't think it was SAEP because seeing the entire field without blackouts is possible on all of them.

[Louis D]

41 minutes ago, Don Pensack said:

I'd also like to know why some 100° eyepieces seem to hide their field stops under a lip, where you have to sort of "look around the corner" to see the field stop, while others seem to yield a 100° view that, while rocking of the head to look at the edge is still necessary,

there does not appear to be a lip or corner you have to look around to see the field stop.  Is it the shape of the interior?  A baffle added?  

Since none of the 100° eyepieces have enough eye relief for me to use with eyeglasses, I've never bought any.  Thus, I've never tried photographing them to try and unravel the mystery.

The "lip" or "shelf" you have to look under sounds indicative of SAEP as in the Meade MWA 26mm.  It is quite difficult to see that last 2° of field around the edges without tipping ones head to line up the eye's axis with those edge of field rays.

[Ratlet]

6 hours ago, Don Pensack said:

The thing is, a lot of eyepieces have SAEP to some degree.

If it's egregious, like the original Naglers or the more recent Vixen SSWs, it will cause problems for most users.

If it's minimal, even if it's there most people won't notice.

It really isn't an issue with most modern eyepieces.

 

What I'd like to know is why and how some eyepieces have their fields seemingly at different distances from the eye.

I change eyepieces a lot when observing, and one eyepiece will seem to have its field a nice comfortable distance from the eye, while another seems to require me to use my "medicine bottle reading" eye focus because the image is really up close and personal.

I have verified that it isn't apparent field, because some 60° eyepieces yield a closer-to-the-eye focal plane than some 100° eyepieces.

I think the answer to this could be related to why some eyepieces seem comfortable and others don't.  I don't ever see this discussed.

 

I'd also like to know why some 100° eyepieces seem to hide their field stops under a lip, where you have to sort of "look around the corner" to see the field stop, while others seem to yield a 100° view that, while rocking of the head to look at the edge is still necessary,

there does not appear to be a lip or corner you have to look around to see the field stop.  Is it the shape of the interior?  A baffle added?  

 

Speaking of 100° eyepieces, why do some require a very steady head at the exit pupil, while others allow you to bob around in and out and side to side?

It's not the focal length or exit pupil since an 8mm and a 7mm seemed to be completely insensitive to head movement while a 13mm and 4.7mm on either side were a lot more sensitive to eye placement.

Not really hard to use, just a bit more sensitive to head placement.  I don't think it was SAEP because seeing the entire field without blackouts is possible on all of them.

By field a different distance do you mean (by analogy) if it were a TV how far away the TV would be from your eye?  I've never really thought about it but that is a very interesting concept if that is what you mean.  I could absolutely see why that would play a big part in the comfort of an eyepiece.

[Don Pensack]

2 hours ago, Ratlet said:

By field a different distance do you mean (by analogy) if it were a TV how far away the TV would be from your eye?  I've never really thought about it but that is a very interesting concept if that is what you mean.  I could absolutely see why that would play a big part in the comfort of an eyepiece.

Yes, that is exactly what I mean, in a way.  But since the apparent fields are the same, it would be an enlarging screen as it moved away to maintain the same viewing angle.  But the screen/field would obviously be at different distances from your eye.

[Ratlet]

2 hours ago, Don Pensack said:

Yes, that is exactly what I mean, in a way.  But since the apparent fields are the same, it would be an enlarging screen as it moved away to maintain the same viewing angle.  But the screen/field would obviously be at different distances from your eye.

Is the 'apparent distance' something that can be measured?  From your experience is it an eyepiece specific thing or does the scope impact on it?

I'm going to have to try it out with some of my eyepieces.  Generally I want to see how they all compare, some eyepieces have great views but are 'exhausting' to use. 

[Louis D]

If I had to hazard a guess, this apparent viewing distance is related to how steeply the exit pupil rays are converging.  If you have a 100 degree eyepiece with 5mm of eye relief versus a 100 degree eyepiece with 20mm of eye relief, clearly, the exit pupil rays have to be converging much more steeply in the former than in the latter.  As a result, the former is probably going to feel closer than the latter because our brain has become accustomed to perceiving a steeply converging image as being closer.  The example of trying to view a movie theater screen from 10 feet versus 40 feet away comes to mind.

In my own experience, whenever I have to remove my eyeglasses to take in the entire field of view of limited eye relief eyepieces, the view always feels more immersive and closer than when wearing my eyeglasses with long eye relief eyepieces of the same power and AFOV.  This perception happens irrespective of AFOV.

[Don Pensack]

2 hours ago, Louis D said:

If I had to hazard a guess, this apparent viewing distance is related to how steeply the exit pupil rays are converging.  If you have a 100 degree eyepiece with 5mm of eye relief versus a 100 degree eyepiece with 20mm of eye relief, clearly, the exit pupil rays have to be converging much more steeply in the former than in the latter.  As a result, the former is probably going to feel closer than the latter because our brain has become accustomed to perceiving a steeply converging image as being closer.  The example of trying to view a movie theater screen from 10 feet versus 40 feet away comes to mind.

In my own experience, whenever I have to remove my eyeglasses to take in the entire field of view of limited eye relief eyepieces, the view always feels more immersive and closer than when wearing my eyeglasses with long eye relief eyepieces of the same power and AFOV.  This perception happens irrespective of AFOV.

Uh, nope.  All 100° eyepieces have exactly the same angle of convergence or they wouldn't have 100° apparent fields.  The only difference is the length of the cone from the glass.  Shorter eye relief only means a smaller lens.

Your movie screen analogy has different apparent fields at different distances.

But I own 100° eyepieces that have 13mm and 15mm of eye relief and the image in the one with 13mm appears farther from the eye than the one with 15mm, so eye relief is not the reason.

It could be a purely psychological phenomenon, or a physical interaction with the eye, or a difference in distortion that determines the perception of the field.  I just don't know, but I see the differences all the time when switching eyepieces.

[Ratlet]

1 hour ago, Don Pensack said:

Uh, nope.  All 100° eyepieces have exactly the same angle of convergence or they wouldn't have 100° apparent fields.  The only difference is the length of the cone from the glass.  Shorter eye relief only means a smaller lens.

Your movie screen analogy has different apparent fields at different distances.

But I own 100° eyepieces that have 13mm and 15mm of eye relief and the image in the one with 13mm appears farther from the eye than the one with 15mm, so eye relief is not the reason.

It could be a purely psychological phenomenon, or a physical interaction with the eye, or a difference in distortion that determines the perception of the field.  I just don't know, but I see the differences all the time when switching eyepieces.

I've got a feeling that this will end up being one of those things that you don't really notice in a big way, but once you do see it you can stop noticing it.

[globular]

Don't look for it Ratlet... run away now.....