Meade RCX400 is NOT Ritchey-Chretien.

Nothing common between Meade RCX and true Ritchey-Chretien.
The only common is aplanatism - coma-free.

Calling these new scopes as Ritchey-Chretien is nothing, but
marketing hype to catch public attention. Peoples heard, that
Ritchey-Chretien are the best two mirror telescope systems.
But most of them don't know exactly what is what.

For CCD photography these new scopes will be not better, than
traditional SCT with focal reducer-comacorrector. Field is quite
enough, it is flat and coma-free.

New telescopes will have huge field curvature and will be
worser for visual observing - because of higher aspherics (less
smooth optics) and larger central obstruction.

If one will ask which scope I will use personally new RCX or
same size SCT, I will choose a SCT.


VD

Vader wrote:
Nothing common between Meade RCX and true Ritchey-Chretien.
The only common is aplanatism - coma-free.

Calling these new scopes as Ritchey-Chretien is nothing, but
marketing hype to catch public attention. Peoples heard, that
Ritchey-Chretien are the best two mirror telescope systems.
But most of them don't know exactly what is what.

For CCD photography these new scopes will be not better, than
traditional SCT with focal reducer-comacorrector. Field is quite
enough, it is flat and coma-free.

New telescopes will have huge field curvature and will be
worser for visual observing - because of higher aspherics (less
smooth optics) and larger central obstruction.

If one will ask which scope I will use personally new RCX or
same size SCT, I will choose a SCT.


VD


Does it have a hyperboloidal primary and secondary? If it does, then
why wouldn't it be a variation of a R-C? It's funny that they say the
front corrector will eliminate the contrast robbing diffraction of
spider vanes, yet it must have about a 40% linear obstruction! Even if
the design is sound, I don't have too much confidence in their
execution.

Jim Johnson

On 5 Jan 2005 15:23:54 -0800, wrote:


Vader wrote:
Nothing common between Meade RCX and true Ritchey-Chretien.
The only common is aplanatism - coma-free.

Calling these new scopes as Ritchey-Chretien is nothing, but
marketing hype to catch public attention. Peoples heard, that
Ritchey-Chretien are the best two mirror telescope systems.
But most of them don't know exactly what is what.

For CCD photography these new scopes will be not better, than
traditional SCT with focal reducer-comacorrector. Field is quite
enough, it is flat and coma-free.

New telescopes will have huge field curvature and will be
worser for visual observing - because of higher aspherics (less
smooth optics) and larger central obstruction.

If one will ask which scope I will use personally new RCX or
same size SCT, I will choose a SCT.


VD


Does it have a hyperboloidal primary and secondary? If it does, then
why wouldn't it be a variation of a R-C? It's funny that they say the
front corrector will eliminate the contrast robbing diffraction of
spider vanes, yet it must have about a 40% linear obstruction! Even if
the design is sound, I don't have too much confidence in their
execution.

Jim Johnson

40% isn't much large than the obstructions in Meade SCTs.
-Rich

wrote:

Does it have a hyperboloidal primary and secondary? If it does, then
why wouldn't it be a variation of a R-C? It's funny that they say the
front corrector will eliminate the contrast robbing diffraction of
spider vanes, yet it must have about a 40% linear obstruction! Even
if
the design is sound, I don't have too much confidence in their
execution.

Jim Johnson

Only in this case (both mirrors are hyperbolical) this system will be
a variation of a Ritchey-Chretien system.

However, this is a very hard task to accomplish two highely aspherized
mirrors along with flat optical window. This is a well known fact, that
it is more difficult to make flat, than average paraboloid. However,
here
are 2(!) flat surfaces.
Also, this system has very fast primary mirror - about F/2. This does
mean, that aspherics here is about 2.1x higher, than in traditional for
R-C systems F/3 primary. This also means, that aspherics on a secondary
mirror is also higher. The main problem is correct collimation of this
system, because it is much more sensitive to cillimation, that a system
with F/3 primary.
Meade says, that a primary mirror has no collimation. This does mean,
that a primary mirror is a sphere, because a sphere has infinite number
of optical axes and the only collimation require is secondary mirror
tilt. Corrective plate is not sensitive to collimation.

I bet, that with 99.99% probability this system consist aspherized
secondary and aspherized corrective plate.
Even in this case, this system will be WORSER visually, than
traditional
SCT, because it has aspherized secondary mirror. It is not easy tast at
all to make a smooth small convex hyperboloid or oblate spheroid. And
in any case it can't be made in large number and very smooth. Very
smooth aspherics requite a time, slow speed polishing with frequent
enough testing procedures.

2 Rich A. 40+% c.o. is really worser, than 33% or 35%. More and more
energy pulled out of Airy disk to diffraction rings and contrast
becomes lower and lower. According to your logic, 50% c.o. is not much
worser, than 40%. ;)

Conclution (with 99.99% probability). These RCX are not
Ritchey-Chretiens. Just marketing step.

VD

and a damned poor marketing ploy too. Negative publicity Meade does
not need.





Vader wrote:

wrote:

Does it have a hyperboloidal primary and secondary? If it does, then
why wouldn't it be a variation of a R-C? It's funny that they say the
front corrector will eliminate the contrast robbing diffraction of
spider vanes, yet it must have about a 40% linear obstruction! Even
if
the design is sound, I don't have too much confidence in their
execution.

Jim Johnson

Only in this case (both mirrors are hyperbolical) this system will be
a variation of a Ritchey-Chretien system.

However, this is a very hard task to accomplish two highely aspherized
mirrors along with flat optical window. This is a well known fact, that
it is more difficult to make flat, than average paraboloid. However,
here
are 2(!) flat surfaces.
Also, this system has very fast primary mirror - about F/2. This does
mean, that aspherics here is about 2.1x higher, than in traditional for
R-C systems F/3 primary. This also means, that aspherics on a secondary
mirror is also higher. The main problem is correct collimation of this
system, because it is much more sensitive to cillimation, that a system
with F/3 primary.
Meade says, that a primary mirror has no collimation. This does mean,
that a primary mirror is a sphere, because a sphere has infinite number
of optical axes and the only collimation require is secondary mirror
tilt. Corrective plate is not sensitive to collimation.

I bet, that with 99.99% probability this system consist aspherized
secondary and aspherized corrective plate.
Even in this case, this system will be WORSER visually, than
traditional
SCT, because it has aspherized secondary mirror. It is not easy tast at
all to make a smooth small convex hyperboloid or oblate spheroid. And
in any case it can't be made in large number and very smooth. Very
smooth aspherics requite a time, slow speed polishing with frequent
enough testing procedures.

2 Rich A. 40+% c.o. is really worser, than 33% or 35%. More and more
energy pulled out of Airy disk to diffraction rings and contrast
becomes lower and lower. According to your logic, 50% c.o. is not much
worser, than 40%. ;)

Conclution (with 99.99% probability). These RCX are not
Ritchey-Chretiens. Just marketing step.

VD

VD,

Thanks for the info. I know about the extreme sensitivity to
collimation of a true RC and the precision needed to execute it
successfully. I personally never had confidence in Meade's ability to
get that level of precision at such a low price. If the mirrors aren't
hyperboloids, then how can they get away with calling it a R-C? It
seems like it would be easier to use a subdiameter corrector near the
focus to improve the optics rather than a large front corrector.
JIm Johnson

It is possible to make a compact SCT corrected for spherical,
coma and astigmatism. For an f/8 system with f/2 primary, ~30%
minimum secondary and ~40% c.obstruction, it would require primary
conic of ~0.5 (oblate ellipsoid) and spherical or near-spherical
secondary. Corrector would be nearly twice stronger than in an
f/10, but the chromatism wouldn't change significantly, due to
lower secondary magnification.

But there is no basis whatsoever to call this design Ritchey-Chretien.
As Valery said, the RC has two hyperboloidal mirrors correcting for
spherical and coma. Any corrector that would significantly change this
configuration couldn't be called an RC. And two fast hyperbolical
mirrors with the rest of the package would definitely require higher
price, even for only decent optical quality. Still puzzled...

Vlad

On Thu, 06 Jan 2005 02:18:17 -0600, Brian wrote:

and a damned poor marketing ploy too. Negative publicity Meade does
not need.

Tell us which company does need it?
-Rich

But there is no basis whatsoever to call this design
Ritchey-Chretien.
As Valery said, the RC has two hyperboloidal mirrors correcting for
spherical and coma. Any corrector that would significantly change
this
configuration couldn't be called an RC. And two fast hyperbolical
mirrors with the rest of the package would definitely require higher
price, even for only decent optical quality. Still puzzled...

I'll bet that they use spherical primary and overcorrect for spherical
with the front corrector to exactly the amount required as if it was a
"true" Ritchey-Chretien (making the combo behave as hyperboloid), then
use hyperboloidal secondary for full aplanatism. But as Valery said,
there is no escape from field curvature as primary and secondary
mirrors' paraxial radii differ too much. Free from coma, yes, but with
field curvature (milder than in SCT as secondary magnification is
lower).
That impressive spot diagram would look very different if it was
focused on a flat focal plane ...

Bratislav

40% isn't much large than the obstructions in Meade SCTs.

If you were to graph CO size against the damage it does to the image, the
curve would be flat (next to zero damage) until you get up to 15-20%
obstruction (depending on who is doing the evaluating). From there it
increases slightly to about 25%, starts to angle up and somewhere between
30-35% goes very steep. There is a very big increase in this area, but it is
even worse for the 35-40% CO.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/

Are you interested in understanding optics?
Try http://groups.yahoo.com/group/ATM_Optics_Software/

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