Example pictures of common eyepiece optical "aberrations"?

[Zermelo]

I should probably have asked this a couple of years ago, but anyhow.

Does anyone have a link to some pictures illustrating common aberrations of eyepieces? The kind of thing that could be used for a diagnosis.

I've found sites that show examples of some aberrations, such as astigmatism, coma, field curvature and rectilinear/angular distortions. Those are clear.
But what I'm after is examples showing some other issues* that are mentioned, but I can't find pictorially, such as: light scatter, ghosting, flare, glare, and internal reflections (which may be the cause of some of the other issues). Sometimes these are mentioned in the context of terrestrial photography, but I'm after example pictures of telescopic objects.

* some authorities don't seem to class these other effects as aberrations; either way, I see them as "undesirable features that might be reduced or eliminated by careful design or additional materials"

[Louis D]

You're right, I can't recall seeing poor light control documented pictorially anywhere for eyepieces.  I guess it's one of those "I know it when I see it" sorts of things.

Ghosting: Pretty obvious with planets.  If you move the planet around the field of view and a dimmer image, generally on the opposite side of the FOV, dances with it, that's ghosting.  Only on axis will the two images merge.  If the alignment isn't perfect, contrast is greatly diminished and you're better off moving the image off-axis.

Light scatter: An overall term applied to light not going where it's supposed to go.  It can be narrow angle or wider angle.

Glare: Generally seen as a glow or spiking around bright objects on-axis obscuring fine details or close, dim companions.  Professional telescopes resort to using an occluding disk of some sort along with precise baffles.  Amateur scopes and eyepieces rely on fine glass polish, smooth reflective surfaces, and smooth coatings to minimize light scatter.  I believe this form of stray light is referred to as narrow angle light scatter.

Flare: Generally associated with bright objects off axis or even outside the FOV introducing stray light into the FOV.  Again, moving a bright object in and out of the FOV and around the FOV will show light streamers pointing back to the source.  It can be handy for recentering a bright object once it strays outside the field of view.  Telescope and eyepiece design minimize this effect with light baffles, light traps, flocking, blackening, etc.

Internal reflections: Yes, these often lead to flare and can contribute to glare.  If your scope and/or eyepiece are poorly baffled and mechanical/optical bits are not properly blackened, you will see an overall glare or even flare in the worst cases in the FOV.

There is also stray light in systems such as catadioptrics where unfocused light from the front aperture bypasses the mirrors and goes straight into the eyepiece due to poor baffle tube design leading to glare and/or flare.

Sorry I can't help with any photographs.   When I take cell phone camera FOV images indoors, I have to have all lights turned off near the telescope, and I have to shield the top of the eyepiece from reflecting even dim stray light into the camera lens with my free hand, so I have dealt with photographic stray light extensively doing this.

Sounds like a project for you to document photographically. 😉

[Zermelo]

Thanks Louis. I like "poor light control" as a collective name for this class of annoyances.

 

34 minutes ago, Louis D said:

Sounds like a project for you to document photographically.

Yes, if I could I would post a "what's causing this?" image, but my attempts at smartphone-to-eyepiece photography so far have been disastrous 😳

[Louis D]

Adding a baffle ring at the bottom of an eyepiece with an inner diameter no smaller than the field lens can greatly reduce some forms of stray light.  The Vixen HR eyepieces did this:

They even extolled their careful attention to stray light control:

I've also read of folks disassembling their eyepieces to blacken lens edges and spacer rings as well as to add flocking to reduce stray light.