Nikon’s original total reflection prism(?) (1 Viewer)

[Dr. K]

Nikon says it uses this original total reflection prism in the Monarch 82ed, and somewhere I read that this system is great, but while I do enjoy the scope I do not understand this prism. Is there somewhere that I can read more about it?

Thanks.

Nikon: Nikon MONARCH FIELDSCOPE 82ED-A WITH MEP-20-60 | Nikon

 

[henry link]

It's the usual 45º Schmidt roof prism found in most expensive angled scopes now. What makes Nikon's iteration "great" is simply that it's oversized, which offsets the eyepiece and places the roof prism edge just outside the objective lens' light cone, so even though it's a roof prism it doesn't need phase correction and is not subject to the roof edge defects that can show up in scopes with Schmidts that divide the light cone.

 

[Dr. K]

Is the oversized prism the reason for the large bugle near the eye piece? And, since the straight version has a porro prism, should that model actually brighter?

 

[henry link]

Is the oversized prism the reason for the large bugle near the eye piece? And, since the straight version has a porro prism, should that model actually brighter?

Yes, that explains the bulge. The Schmidt is a monoblock with the same four TIR reflections like a Porro and only two glass to air surfaces, so the transmission should be about equal to a Porro.

 

[Trinovid]

...even though it's a roof prism it doesn't need phase correction and is not subject to the roof edge defects that can show up in scopes with Schmidts that divide the light cone.

On the surface, this sounds like a very preferable way to build a roof, or is phase correction being done well enough that it's six of one, half a dozen of the other?
As someone who really understands all of the geometry, coatings, glass choices and prism systems available, how would you be likely to design a binocular (one if size is an issue, and 2 if it's not), or is it already being done?

 

[henry link]

Hi trinovid,

My understanding isn't quite that extensive, but remember a Schmidt is a 45º diversion prism. It's use is restricted to telescopes and binoculars with angled eyepieces, unless it's combined with another prism, as in the Schmidt-Pechan, but then its advantage of simplicity is lost. I don't have the knowledge to design a binocular, but the prism I would like to see used more often by those who do design binoculars is the Perger prism, just so we could see how well it really works.

Henry

 

[tenex]

What makes Nikon's iteration "great" is simply that it's oversized, which offsets the eyepiece and places the roof prism edge just outside the objective lens' light cone, so even though it's a roof prism it doesn't need phase correction and is not subject to the roof edge defects that can show up in scopes with Schmidts that divide the light cone.

Can you unpack this a bit? I can't envision how the roof edge can be outside the light cone, or see how that avoids phase interference.

 

[henry link]

Maybe the diagram below will help. Follow only the red rays through the prism as if they represented a pencil of light.

Entry is on the right side of the front face, the first reflection is off the right side of the back face down to the right face of the inverted roof at the bottom of the prism, then across the bottom to the left face of the roof, then up and forward to the left side of the front face, then exiting at a 45º angle through the left side of the rear face. The roof edge is located at the very bottom and falls outside the light pencil between the second and third reflections. The prism is large enough so that the light pencil represented by the red rays can travel though the prism by means of four TIR reflections without ever being split by a roof edge and reassembled as a mirror image of itself, so there is no destructive interference.