Re: ATM Diagonal mirror coatings

[Bill Marriott <btk@pacbell.net>]

Doug Bodie wrote:
> 
> I am going to purchase a diagonal for a 10" f5.5 scope I am building.  The
> choices are a 2.5" plate glass diag. with a standard coating(89%) or a Pyrex
> diag. with enhanced coatings(92%).  The pyrex mirror costs roughly 40% more
> than the plate glass mirror. Is it worth the extra money to buy the better
> diagonal?
> 
> Doug Bodie

In the realms of secondaries, better means flatter. Does the
manufacturer provide supposed flatness specs? Interferograms? Otherwise
your taking a chance. You'd like least 1/8th wave. As to material, as
long as it is flat, it shouldn't matter. On something as small as a
diagonal, they should reach equilibrium rather quickly.

One reporters opinion

Bill
--
Bill Marriott
http://www.starlightoptical.com

BTW, isn't a 2.5" a little big for a 10" f5.5? I believe a 2.1" (a
standard size) should work