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!