>I'm confused. Reading some of the ads in Orion Mail Order Catalog, I get
>the impression that Coated prism diagonals for telescopes are "better" than
>First Surface diagonals.
>Is this true? If so - why. What should I buy? How much should I expect to
>spend?
Someone has fed you a line. Coated Prism Diagonals will transmit a higher percent
of the light involved. However when you go to shorter F ratio instruments or
wider field eyepieces (2" diagonals) the refraction effects of the Prism diagonal
become important and can really foul up your resolution. Go to a telescope shop,
have them put a 2" mirror diagonal on a televue genesis (F5) and use it at various powers. Then put a 2" prism diagonal on it and do the same. YUCH!
In otherwords, whether the better light transmission is worth it depends on the
instrument and eyepieces you are using.
Point 2: Mirror diagonals if properly coated can reflect 96% of the light compared to 98% for each surface of a coated prism diagonal. Then you have the fact there are two surfaces and some absorbtion in the diagonal itself. You can
easily end up with less light with the Prism diagonal. In fairness, properly
coated mirror diagonals are hard to find. I do have a properly coated secondary in my newtonian.
