Re: New MEADE scope: RCX400 RITCHEY-CHRETIEN

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 01/04/05 

Date: Tue, 04 Jan 2005 14:11:09 GMT

"Paul Winalski" <nobody@nowhere.net> wrote in message
news:bh7kt0tsndkqbuhdpthuvobmlqja39gnrn@4ax.com...
> So what's the quality of the figure on the optics for these scopes?
> R-C vs. Newtonian or Schmidt-Cassegrain configuration means squat by
> itself. And what percentage of the aperture is obscured by the
> secondary?
>
> R-C is clearly the choice for large-aperture astrophotography (which
> is why Hubble, the 200-inch Hale, etc. are in that configuration).
> What is R-C like vs. Newtonian or Schmidt-Cass/Maksutov for visual
> observing?
>
> This new 14" scope costs twice what I paid for a 14" Newtonian with
> 1/20-wave .99 Strehl optics. Yes, I've got coma, but I could throw
> in a couple of hundred bucks for a Paracorr and deal with that. What
> does R-C get me that is worth doubling the price of a scope?
The key thing that is not mentioned, is chromatic aberration. On a SCT, if
you ray trace in the centre of the field, using green light, you get a
nice small spot. However switch to white light, and the spot size is
already about three times the size of the Airy disk, from the CA,
introduced by the corrector. The spot diagram shown, in the Meade splurge,
is only for green light.
The suspicion is that this scope will produce results similar to the
central axis performance of an SCT, across a relatively wide field, but it
will not produce R-C performance, except when using narrowband filters.
For the money, it looks an interesting design, but like all scopes, it is
a compromise...
A Newtonian, is about the cheapest scope to make, introduces no CA, and
except for off axis coma, produces a good image. Visually it is hard to
beat, provided you do not want really wide fields. As you say, a Paracorr
brings this under control.
The Mak-cass, introduces much less chromatic aberration than the SCT (the
two faces of the corrector compensate for one another), which is one
reason it is seen as the 'better' planetary scope, especially when made in
slower versions, with a smaller central obstruction. My suspicion would be
that the CA on the new design, will still give the Mak-cass the edge here.
Now (of course), if the corrector is achromatic, it'd begin to be really
interesting. The problem of course is the extra cost. The same cost
question arises as to the overall quality of the optics. This will be a
matter of some testing, once the scopes actually appear....

Best Wishes

Quantcast