EDG Prism Coating (1 Viewer)

According to the Nikon literature (page 12) only the straight bodied scopes have “Dielectric high-reflective multilayer prism coating on the roof prism for the brightest views”.

Just curious why the angled bodied scopes do not require it.

Does this also apply to the Fieldscope 82 EDA?

http://www.nikon.com/products/sportoptics/lineup/dl/catalogue/NikonSportOptics2009-2010.pdf


Mike

Is it possible that the angled models are of the porro prism design?

That would explain the lack of coating on the prism.

The angled EDG scopes use a Schmidt prism with no reflection angle below the critical angle for total internal reflection, so no need for mirror coating. The straight versions use a Schmidt-Pechan prism which has one reflection below the critical angle so that one face has to be mirror coated.

The angled scopes that use Porros add a semi-pentaprism behind the Porro and that prism has one face that requires mirror coating.

The angled 60mm and 82mm Fieldscopes actually use a better implementation of the Schmidt prism. It's oversized and offset, which keeps the the roof edge from splitting the light cone, so there is no need for phase correction and the quality of the roof edge and accuracy of the roof angles are not so important. IMO it's the best prism design ever used in an angled scope. It's a shame such an excellent idea was dropped in the EDG scopes. It's also too bad the straight EDG scopes use Schmidt-Pechan prisms instead of Porro. All the old Fieldscopes really needed were a better zoom eyepiece and larger apertures. Alas, instead we have a complete redesign, inferior to the old ones in several ways.

Henry,
A little off topic, but could you explain why Amici prisms for astro scopes have such a poor reputation.
Is it a lack of phase coating on some of them, or is it the additional light path through glass, which was not accounted for in the original design?

John

John,

If you're asking about the 45 degree erecting prisms that are often labeled "Amici", those are really Schmidts, and as far as I can tell very poor quality with no phase correction. An angled Amici can be done, but it would need a long housing shaped like the 60 degree Televue mirror diagonal. The only true Amici prisms for astro scopes I've seen are 90 degrees like the Baader Amici.

Henry

45, 60 and 90 degree Amici prisms are possible. See the following site for excellent visual explanation of image inversion etc. of various prism setups.

http://www.tecplusplus.de/ManualLu/prisms.htm