Re: [ATM] 4" objective !

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

A single lens is going to have a whole lot of color error.
What happens is that glass has a different index of refraction
with the color of the light - look at how a prism works and
you'll understand.  With this difference in refraction, you end
up with a line along the axis of the telescope that is the focal
point of the individual colors just like the prismatic divergence
of the colors of the light.
Go back and look at the design work that early astronomers did to
get some colorfree images out of the primitive refractors that
they built before they found out that glass can have differeent
indices of refraction depending upon the materials used to make
that glass and how that diffraction worked.  Back then single
lens refractors often had focal ratios over 100 (and you think
that a F15 refractor is long?) just to reduce the color problem
to something usable.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
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