To: "Herb Kasler" 
Cc: atm{at}shore.net, owner-atm{at}shore.net
From: Hugues.Laroche{at}ses-astra.com
Reply-To: Hugues.Laroche{at}ses-astra.com



Hi Herb,

>I'm thinking maybe 32-36" would allow me
>For one thing I do NOT want to stand on top of a big ladder to  look
through it.
>Therefore it will have to be a cassegrain of some sort. I think  I would
>go for a Nasmyth focus directed out the altitude axis,

Looks good! I have myself a project with similar contraints running.

My direction is:
- make the primary parabolic and use the newton focus for a while
  (OK, with a big ladder)
- later, make a parabolic secondary (convex or concave) to make it
  a Mersenne-type scope, to be used as in Clyde Bone's models.
  (his 30" was featured in S&T 2 or 3 years ago)
- the newtonian secondary can serve as a Nasmyth tertiary.
- the final focal ratio is determined by the auxiliary optics put
  after the tertiary.
- the trade-off to keep the feet on the ground are more critical
   baffling (as a Cass-Nasmyth) and FOV issues. And, of course,
   3 reflections plus the auxiliary optics with everything this
   implies (collimation, wavefront deterioration).

I think of another design, optically similar, which I do not know if it would
work (but why not?):
- build the scope as a newtonian.
- very close to the focal plane, tilt the light path with a small flat
  (ellipical, 1 or 2" minor axis), towards the sky.
- a bit further, have once again a (Mersenne) *concave* secondary
  parabola which sends the light path back to the ground
  (parallel to the primary optical axis) where you again need
  an auxiliary optics.
- copmared to the existing version above, there is one more reflection
  (4 plus auxiliary optics), but here the
  baffling can be made excellent without obstructing the ligth
  path. Also, there is more weight&momentum put at the level
  of the secondary cage.
So the scope would look like a newtonian with a secondary OTA of smaller
diameter fixed on its side, carrying the focuser at a much lower position.

For sure those Mersenne designs must be carefully dimensioned so that some
full illumination diameter remains at the level of the system focal plane.

I hope this helps, and besides I would appreciate if some list members
challenge my proposal (which has surely been found already in the past, and
maybe rejected for a good reason).

Hugues Laroche
Luxembourg, Europe




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