Re: [ATM] F5 Achromat Design Help

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

Jerry is right on the money for a first telescope.  Do a long
focal length doublet and you will minimize the color problems
(the color error is pretty much constant for a pair of glasses no
matter the focal length) and the longer length brings the color
error closer to the Airy disk at the focus.
One item he failed to mention when doing the lenses is the wedge
of the glass.  Wedge is when the edge of the glass isn't a
constant value but rather varies with one side thicker than the
other.  To explain, you can divide the lens up into three parts,
the front surface, the middle and the back surface.  If the
middle isn't a parallel plate then you will have a prism and that
will make the focus change laterally for the color and that would
mean that stars would be lines rather than dots in an image.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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