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off-axis refractor?

Started by yahganlang, 11/03/2005 06:32PM
Posted 11/03/2005 06:32PM Opening Post
You've all heard of off-axis Newtonians. My question is, can an off-axis refractor work? The idea I have (lacking suitable ray-tracing programs to check it) is that by suitably tilting the objectives of a refractor, one may then fold the scope at odd angles and thus reduce the number of flat reflective surfaces. Final diagonals would reparallelize the light for ep's. Kind of a refractive Schiefspiegler. This is for a large binoscope.

Would this be feasible? Or would the image generated simply be distorted out of all recognizability by the time it hit the eye? Also, assuming it would work for a simple biconvex lens, would added elements (achromats, apo, etc.) render it unworkable anyway?

Thanks.

Jess Tauber
Posted 11/03/2005 07:33PM | Edited 11/03/2005 07:35PM #1
jess tauber said:

You've all heard of off-axis Newtonians. My question is, can an off-axis refractor work? The idea I have (lacking suitable ray-tracing programs to check it) is that by suitably tilting the objectives of a refractor, one may then fold the scope at odd angles and thus reduce the number of flat reflective surfaces. Final diagonals would reparallelize the light for ep's. Kind of a refractive Schiefspiegler. This is for a large binoscope.

Would this be feasible? Or would the image generated simply be distorted out of all recognizability by the time it hit the eye? Also, assuming it would work for a simple biconvex lens, would added elements (achromats, apo, etc.) render it unworkable anyway?

Thanks.

Jess Tauber

Jess,

I have no idea why you are calling this an off-axis instrument. Folding a refractor does not result in using only an off-axis portion of the lens.

You can quite nicely fold a refractor in a variety of ways. The traditional approach is simply to use two flats and make a shorter tube, with the eyepiece at the bottom end as is traditional. Unitron sold a 3" version (I was never quite sure why anyone would bother with a 3"), and AstroPhysics brought a 6" f/12 folded this way to Astrofest one year.

You can also fold it as recommended by Henry Paul, so that it looks a bit like a Newtonian. A large flat at the bottom, up to a flat opposite the eyepiece, and then back across the light path using a small flat to the focuser.

Clear skies, Alan
Posted 11/03/2005 08:17PM #2
I guess I didn't make myself too clear. I KNOW about using flats and traditional folding strategies. What I want to do is not have to use TWO flats having one throw the light cone off at an unusual angle and the other reparallelizing or righting it using the same angle or its complement.

If the cone is bent say 27 degrees by the first mirror and then 35 by the second in a different direction (and 3-D) isn't the image going to get stretched more in one direction than another?

If this isn't so, and no astigmatism is introduced by this, then I beg pardon. If it is true, then couldn't tilting the objective in the tube compensate- i.e. by balancing the lenses and mirrors so that the error introduced by the former and latter cancel, and have the last leg of the light path (before the diagonal) come out at a cockeyed angle without astigmatism?

Jess Tauber