4 replies to this topic

[chrysalis]

So I took apart the 20mm Orion Expanse EP. It does not have the optical configuration of any of the EPs I can find, including all of the ones on this webpage. It most closely resembles a variation of the Bertele EP.

From field lens toward eyelens:

1. Cemented element, flat side toward objective; lower element is very thick, upper element thinner at the edge; upper element is convex.
2. Spacer
3. Plano-convex singlet, convex side toward field lens.
4. Spacer
5. Positive lens that is shallow U-shaped in cross-section (not sure what this is called) with concave surface toward observer's eye.

Here is a pictorial representation (the distance between elements is the spacers):

Attached Thumbnails

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[NeoDinian]

Very similar to a Konig-II design.. But the field lens is quite possibly NOT flat. It would be concave. If even slightly...

[chrysalis]

Very similar to a Konig-II design.. But the field lens is quite possibly NOT flat. It would be concave. If even slightly...

No, it's very flat. Also, websites on Konig II's tend to speak of short ER, whereas this has generous ER. Maybe it's just a proprietary no-name design.

[Deep13]

Looks like a Kelner or RKE with an additional eye lense to correct for the wider AFOV. The Koenig has a doublet in the center with a single lens at the eye and below the cemented doublet. That bottom has to have some curve probably between it and it's cemented mate. Otherwise it has no purpose.

[Mark Harry]

That may be either a reverse Bertele, or a modified Konig. You'd have a reverse Kellner if you eliminated one of the single elements to the eye end, similar to an RKE. It really doesn't precisely match what I have here on a chart, so it could be proprietary.
The Bertele appears to have the surface nearest the field to be flat. (as in this case.)
Why would a flat surface on the field lens have no purpose? This would affect field curvature,from my way of thinking. There are a couple more designs with flats in this position, too.
Mark