Re: ATM A large refractor experience

["John Sherman" <shermj@netzero.net>]

Hi David,

> Does anyone know more about these objectives, or
> perhaps indulge me with some sort of strategy
> to abate its chromatic aberration?

> with a longer focus eyepiece, what eyepiece would
> be better at coping with the field curvature
> of a fast f/3.3?


Try using the lens through color filters. If it produces a sharp image thru
red, yellow, and green then you probably only need to correct the chromatic
error. So you're halfway there. It is easy to produce chromatic aberration
in a refractive material. So use that to your advantage and solve your
problems by making a telephoto telescope. Make a negative focusing lens of
two or three elements down about a foot or two from the objective. Since it
is a negative lens the light now bends in the opposite direction as it did
when passing thru the objective. This allows you to make a lens with the
opposite chromatic error of the wavefront, so that you can end up with
almost no color at the eyepiece. This lens will also extend your effective
focal length to about f/5 or f/10, making all of your eyepieces much
happier. It is called a telephoto because the final focal length of the lens
system is longer than the length of the tube.

John