Televue plossl patent - what we can learn on paper?

[russ.will]

The exit pupil aberration means that the eyepiece is more likely to kidney-bean under daylight conditions.

Can you put why that may be into layman's terms?

I ask, because I use my EPs for astro and white light solar and have noticed, that whilst I have no issues with kidney-bean in the dark, of course some do in daylight. I had assumed it had something to do with the fact that my pupil was obviously stopped down in daylight and that was interacting with the curvature of the field produced by the EP, or some such.

But how exactly, I'd like to know? I'd like to get a small set of EPs that are better suited to daylight, or rather solar, use and I seem to get weirdly inconsistent results across the various Meade/ES clones that I have.

Russell

[great_bear]

Can you put why that may be into layman's terms?

Sure - that following diagram is from www.telescope-optics.net, a free educational resource where a more technical description is provided at the bottom of the page.

Light exiting an eyepiece can be thought of as thick "beams" (rather than simply "rays") of light, projecting (via the eye's own lens) onto the various parts of the image formed on your eye's retina. The thickness of those beams is the exit-pupil width measurement, and in an ideal world, when leaving the eyepiece, all those beams all cross over in exactly the same place. The distance of that position in front of the top lens is the "eye-relief" measurement. "Spherical aberration of the exit pupil" (a very common issue) is where beams representing the outer edge of the image cross over further away, or closer to (as in the above picture) beams in the mid-field angular range.

The net result of this is that if you are too near or too far from the eyepiece, either the edge, or the mid-range of the image will lose illumination. When the mid-range loses illumination, an unstable kidney-bean-shaped shadow appears to swirl around the image. Finding the eye-position "sweet spot" to obtain stable illumination of the entire image, can be rather challenging.

The situation is worse in daylight simply because the substantially-reduced size of the iris makes perfect eye-position critical or even impossible if spherical aberration is severe.

[great_bear]

I borrowed a 16mm Meade 3000 and loved it. I bought a 24 or 25 mm hoping it would be the same only to be very disappointed.

The 16mm is indeed lovely - the main problem with (any) 25mm Plössl is the excessive eye-relief, which they fixed in (some) Meade 4000 models by setting the lenses deeper into the barrel to aid eye placement.

[YKSE]

Thanks for the detailed explanations Jeremy 

You can always learn something by asking questions, isn't?

[russ.will]

Indeed - Great explanation  and the link was a handy one to bookmark.

So, between that post and the link, am I correct in thinking that any very wide aFOV EP is going to be a daytime challenge? I must say that my ES100s are proving to easier to use for white light solar than I thought and indeed the S5000 6.7mm UWA more of a disappointment, but others who occasionally get to look through my scopes are less patient.

I may have to bag myself two or three Plossls to use with the ES Focal Extender in the ES 80 Apo and 'native' in the LX200. Mind you, with the surface detail the 8" scope is ponying up compared to the 80mm plus Lunt Wedge, the latter combination may be on borrowed time for solar anyway.

Russell

[great_bear]

I correct in thinking that any very wide aFOV EP is going to be a daytime challenge?

Not by necessity - it's all down to the design of each one. The Nagler type 1s suffered from it, and as such it was an issue that was addressed and reduced in later versions. Unfortunately though it's not a published specification for eyepieces, so it comes down to knowledge exchange and personal evaluation.