Angled spotting scope prism design (1 Viewer)

[MustangHX]

I've been curious about the prism used in different spotting scopes. The Swaro ATS series uses a single Schmidt prism, and the older AT80 uses a Porro prism (post#9), though I'm still not sure how to achieve a 45-degree angle with "standard" Porro prisms only:

Need Info about Swarovski ATM 65

The single Schmidt design in ATS would result in a larger physical length with a given focal length. The ATX is shorter and seems to use a Porro-like design in the not-well-described patent (post#3):

ATX ocular module design

Another unique design is used in the Meoptar S2, "a Porro prism divided into 3 parts", I tried to draw the optical path but have not succeed yet:

Meopta S2 Mysteries

My question is: what kind of prism do other manufactures use? Do KOWA 883 and Swaro ATX use different prism configurations?

PS: even for a single roof prism, there are two configurations, the first is the ATS with a Schmidt prism:

Another is the diagram from the Celestron website, but I doubt if it's correct, as the roof prism like that should only reverse the image horizontally.



PS2: This should be the more straightforward design: Porro+half penta prism:

The half penta prism does not erect the image:


This page from Edmunds has a list of prisms:
Introduction to Optical Prisms

 

[MustangHX]

Kowa 55A/55S
From post#8 we know that the larger Kowa scopes (77,88 etc) uses the single Schmidt erecting prism, and the smaller 55 series (553/4, 55a/s) uses "A Porro with a moving prism for focusing followed by a semi-pentaprism for angling", which should be similar to the 3rd figure.

 

[etudiant]

No optical expert, but perhaps a clue is the placement of the eye piece.
My Nikon scopes have it offset to one side, so presumably the porro prisms are side by side rather than vertical as shown.
Or is there another explanation?

 

[henry link]

There is another explanation. The offset prism in angled Nikon Fieldscopes is not a Porro. It's a normal Schmidt roof prism, but oversized so that the roof edge does not split the objective lens's light cone. There is consequently no need for phase correction or for a roof angle with any more accuracy than a Porro.

 

[kabsetz]

And just to clarify, the explanation Henry offers is actually how it is in that particular scope family.

 

[henry link]

I forgot, one angled Nikon Fieldsscope doesn't use an oversized Schmidt - the Fieldscope 50ED. It uses a Porro followed by a semi-pentaprism, a design now relegated to mostly inexpensive scopes. One of the reflecting surfaces in the semi-pentaprism requires mirror coating. The type of mirror coating is almost never specified.

 

[Hermann]

I forgot, one angled Nikon Fieldsscope doesn't use an oversized Schmidt - the Fieldscope 50ED. It uses a Porro followed by a semi-pentaprism, a design now relegated to mostly inexpensive scopes. One of the reflecting surfaces in the semi-pentaprism requires mirror coating. The type of mirror coating is almost never specified.

Which is the reason why the straight ED50 is ever so slightly better optically than the angled version. At least IMO.

Hermann