Off axis Newtonian Binoscope

I am interested in off axis reflectors.

Has anyone modified (or knows how to) a Newtonian to a non-obstructed instrument?

I have several 4.5 f8 Meades (parabs) to play with.

I would like to make a 4.5 binoscope.

Stu Lieberman said:

I am interested in off axis reflectors.

Has anyone modified (or knows how to) a Newtonian to a non-obstructed instrument?

I have several 4.5 f8 Meades (parabs) to play with.

I would like to make a 4.5 binoscope.

Stu,

If you simply use a standard parabolic mirror and tilt it so that the diagonal is off axis, huge aberrations are produced by the tilt. There are only a couple of ways to move the diagonal off-axis, and they are technically difficult and expensive.

The first is to make the primary mirror an off-axis section of a larger parabola...so for a 4" mirror you would need to atart with at least an 10" parabola. This is the approach that D&G uses with its off-axis scopes. They perform very well, but are as expensive as apo refractors of the same diameter.

The second way is to use a flat that is asperically ground into a sort of saddle shape. This is done in the Yolo design, and again is technically very difficult.

No free lunch.

Tim Iafolla

Well, a 4.5" f/8 is probably very close to spherical (it might even BE spherical despite claims otherwise), but even then you still cannot tilt them -- you would get the spherical abberation of a 9" f/4 (or worse). However, you can try the following.

Mask off half of each mirror (instead of obstructing the center). Maybe the absence of spider diffraction and large central obstruction will end up helping. You might also use smaller secondary mirrors and bring the scopes closer together, masking off a narrow edge of each mirror where the head obstructs. The trade-off might end up advantageous. (The standard "binewt" construction does not favor small primary mirrors. Bringing the foci together and avoiding head-blocking would require extra large secondaries, about 1.5" in your case.)

Hope this helps,

-- William (Saratoga, CA)