Re: ATM BK 7 Ideas

["Bob May" <bobmay@nethere.com>]

The use of the same melt is optimum as you don't need to change the basic
presciption to match the differing indices of refraction over the color
range.  I haven't done any real research on the subject but I'd suspect that
even if you used something only close (not even BK-7) you will still be
getting a color error well below what anything any other refractor can
really give you and you may never even see any color error.
Fof those who don't know the Schupmann design, it's probably the only color
free refractor design, although it does contain by necessity two reflectors
in the path, the first of is a plain spherical mirror while the second is a
Mangin mirror (second surface mirror) of another radius.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net