"Tim Auton" wrote in message
...
"Bill Kelly" wrote:

I hope this discussion has not already been here, if so humor me. Here
in the US, from Orion, a newtonian telescope is being sold, a 91mm clear
aperture newtonion reflector. For about $900.00 right now. My question is,
are any of you familiar with it? Is anyone else making them? Perhaps
larger
ones? This seems to be a very good innovation. Can amnone comment on this?
The claim is that they are as good as apo's which would sell for much
more,
due to the lack of chromatic abberration, since the off-axis mirror points
the light off to the side, eliminating the need for the secondary mirror.
If
this is really so much better will we soon be seeing larger apertures made
this way?

Off-axis newtonians aren't new, but they are larger and more complex
(expensive) to make than regular newtonians so haven't really taken
off in large sizes. The mirror is (or should be) an off-axis section
of a paraboloid mirror. This is hard (expensive) to figure alone, it's
easier to cut multiple sections from a large mirror. Newtonians may
not have any chromatic aberration, but they do suffer from off-axis
coma. As the mirror in a clear-aperture newtonian is an off-axis
section coma is worse. You can minimise this with a larger focal ratio
(f/ratio), but for larger apertures you start needing very long tubes
and sections from huge mirrors.

I don't know how good the 91mm Orion is, but I bet they won't be
bringing out a 200mm. I doubt the 91mm newt is streets ahead of the
80mm or 100mm Orion ED refractors which sit either side on price.


Tim
--
This is not my helicopter.


Thank you, Tim. There had to be a reason why they haven't become extremely
popular fast. I suppose for those who aren't interested in a really large
light gathering capability they may be a good option, unless, as you
suspect, they aren't much better than a reasonably priced ED glass refractor
of similar size.
God bless,
Bill.