"Protagonist" wrote in message
...
Simon W. wrote:
I would still like to hear from those who have acquired a Meade LX200R
Ritchey-Chretien telescope. A few of them are now in amateur hands.
(I saw one yesterday in a local telescope store.)

I want to know whether the LX200R optics are truly apo-like, as some
speculate, or at least whether the LX200R optics are better than SCT
optics.

Thank you.

Simon W.
Uses mirrors, so no color if that's what you asking, APO-like.
No. Has a corrector, with different shapes on the two surfaces, so will
show a small amount of chromatic aberration. However in general not
noticeable.

It has a large central obstruction, (over 35%) like any other compound
telescopes, so star images will be bloated, from diffraction rings.
RC scopes advantages over standard SCT is flat field, no mirror flop,
because the stationary primary.
JS
RC, does not have a 'flat field'. The field curvature is dependant on the
focal length of the primary, and the secondary. Hence RC's with large
secondaries, and relatively long focal length primaries, have a _flatter_
field than a traditional SCT, but still have a very curved field. The
'point' about the RC, is that it has the largest _coma free_ field of most
scope designs. Hence though the stars get larger towards the edges of the
field because of the field curvature, they remain round. This also then
means you can use a field flattener, and have a flat field with coma free
images. The 'R', will have more field curvature than the RCX, because of
it's faster primary.
Think of the design as an 'SCT-GT', rather than as a RC.
Now the energy being moved from the centre of the Airy disk, will mean
that the star images are larger than an equivalent APO. However the amount
will be slightly less on the R, than the RCX (smaller secondary), and the
old 'rule of thumb', of aperture-obstruction, gives a good approximation
of how well an obstructed scope performs in the worst case part of the
mtf. The best case resolution, will be that of it's aperture, but for the
8" scope (say), a good comparison, would be with a 5" APO. The extra
aperture will allow you to see 'deeper' than this will manage.
Historically, the commercial SCT, has been a slightly 'bodged' version of
the design (with near spherical optics, rather than the ellipsoid
secondary that should be used), and the RCX, produces centre field images,
that are closer to the traditional SCT, but without the coma further out
in the field, that this design displays.

Best Wishes