What are P-coatings?
by Dave Sworin

prism binocular I see only one light path, which does
not split; but perhaps, to avoid confusion, these idealized sketches only show one of the light paths. Another thought, is that hunters hardly care whether the light splits or not, while the typical amateur astronomer (who may understand more about optics) would never buy a roof prism knowing it splits the light path, so this detail is played down. 

   How could they ever get those light paths back together again precisely and still be 1/4 wave?  The roof prism is more rugged than the porro prism and this has lots of appeal for hunters who are roughing it, whereas astronomers are pretty fussy with their equipment and usually take great care of it. So, for astronomers, porro prisms are cheaper, better, and good enough.  Barsness claims that Japanese and American optical companies now use P-coating technology,
and competition is driving down the cost of roof prism binoculars.  I'm skeptical, but until I get a detailed optical explanation from a more technical source, this explanation seems to make the most sense.

   More than two years ago, I first heard about P-coatings.  I first noticed this terminology in a
binocular advertisement in the Orion Telescope and
Binocular Center catalogue that arrives in the mail
every few months.  The product was a high-end Zeiss
7x42 binocular, costing about $1000. First, I looked around in in the Orion catalogue because Orion has lots of sidebars explaining the telescope and binocular technology that they sell. This information is very very helpful to the amateur who is just starting.  It gives only conventional wisdom and is never too critical (which is expected), but it's a real service.

   Not finding an explanation, I next searched the astronomy magazines and literature, meaning the tons of astronomy books lying around my apartment.  Nothing was found.  I have since run across an explanation that makes sense; it does not agree with some other things I have found, but I think it's worth passing on what I learned.  So first I'll explain what I discovered, and then I'll reflect on the explanation.

   I was surprised to find an explanation in an unlikely source. In his book Optics For The Hunter,  John
Barsness reports on conversations and tours of the
Zeiss factory, which he got because he was the optics
editor for a hunting magazine.  He reports that in
roof prism binoculars, the light is split as it enters the roof prism and travels in two separate paths.  The field of view is therefore composed of two light paths.  Barsness gives a geometric explanation of the two components combining to form half of each field. I cannot tell whether he is technically serious or is trying to explain this concept to an audience that includes non-technical members.  Because the prisms cannot be made exactly the same, not just in their respective face angles, but in the exact  length of the light path, the light gets out of  phase.  He claims that Zeiss discovered that putting coatings on the prisms eliminates the phase shift, but he offers no detailed optical explanation. These are "phase coatings" or "P-coatings" for short.  In the summer 1999 Orion catalogue, the Swarovski SLC ad mentions this term. Zeiss ads evidently now simply note that Zeiss binoculars have special T-coatings, another unexplained term.

   What do I think?  If I look at the sketches of a roof

AstroCon '99 at Hopkins Junior High School

   The Astronomy Club at Mission San Jose High School plans to hold an AstroCon at Hopkins JHS in Fremont, on Sunday, November 7. The planetarium and lecture events will be free, and open to the public. Star gazing will follow in the evening. Club president Dave Anderson invites all TVS members to participate, and he particularly encourages members who own and use solar filters. For all the details, reach Dave at davidand@home.com or call him at 510/661-4249.

Planets (Continued from page 4)

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