What happened to the Classical Cassegrain?

All mainstream scopes now seem to have
found a place, but not the Classical Cassegrain.
There are the Taks, but they are Dall-Kirkhams.
Some outstanding shots seem to be taken by people
with expensive, large ones, but there are no
mainstream units out there.
-Rich

Unfortunately, for the visual observer, most of the commercially made RC's
are relatively fast (f8) which dictates a large CO. The D&G 20" CC is f16
with a 25% CO. A visual side by side of a 20" D&G CC with a 20" RC would
be interesting!

"John Savard" wrote in message
...
The Ritchey-Chretien.

Amateurs may grind Dall-Kirkham telescopes themselves because it is
easier to do so than to make a classical Cassegrain.

Originally, the Dall-Kirkham was popular because the ellipsoidal
secondary was difficult to test, but a method of dealing with that was
discovered. The thing is, though, that if you're in a position to make a
classical Cassegrain, you can also make a Ritchey-Chretien, which
corrects coma as well as spherical aberration - so there's no real
reason not to go all the way.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html

On Sat, 27 Nov 2004 20:15:59 GMT, lid
(John Savard) wrote:

The Ritchey-Chretien.

Amateurs may grind Dall-Kirkham telescopes themselves because it is
easier to do so than to make a classical Cassegrain.

Originally, the Dall-Kirkham was popular because the ellipsoidal
secondary was difficult to test, but a method of dealing with that was
discovered. The thing is, though, that if you're in a position to make a
classical Cassegrain, you can also make a Ritchey-Chretien, which
corrects coma as well as spherical aberration - so there's no real
reason not to go all the way.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html

But can an R-C be made with a reasonable sized secondary?
Seems like most of them from the past were setup for
imaging and had large secondary mirrors.
-Rich

But can an R-C be made with a reasonable sized secondary?
Seems like most of them from the past were setup for
imaging and had large secondary mirrors.
-Rich

I thought the large secondary was inharent in the design and indeed necessary
to achieve the correction. I also thought that main advantage was that it was
possible to deconvolve the image rather easily making the R-C ideal for
photography but not necessarily visual work..

jon

Back in the days of old (pre-PC controlled) I built a very nice 12.5-inch
f/30 Classical Cass with a 2-inch secondary. Images were great. Before
light pollution rendered my DSO's in my sky completely flooded out that
telescope presented nice DSO's as well as planetary images. I considered a
16-inch f/50 before building my long Newt. Would have been nice me thinks.
Good thing about long F/R Cass's is the depth of focus and nearly flat field
near the center.

Dusty
"Jon Isaacs" wrote in message
...
But can an R-C be made with a reasonable sized secondary?
Seems like most of them from the past were setup for
imaging and had large secondary mirrors.
-Rich

I thought the large secondary was inharent in the design and indeed
necessary
to achieve the correction. I also thought that main advantage was that it
was
possible to deconvolve the image rather easily making the R-C ideal for
photography but not necessarily visual work..

jon

Dusty wrote:
Back in the days of old (pre-PC controlled) I built a very nice 12.5-inch
f/30 Classical Cass with a 2-inch secondary. Images were great. Before
light pollution rendered my DSO's in my sky completely flooded out that
telescope presented nice DSO's as well as planetary images. I considered a
16-inch f/50 before building my long Newt. Would have been nice me thinks.
Good thing about long F/R Cass's is the depth of focus and nearly flat field
near the center.
I have an 8" f/15 CC. It weighs 22 lbs - OTA+focuser+finder. I'm
thinking of selling it because it's too heavy for my GP mount, and I
can't afford something big enough to carry it. It gives great views of
the Moon, Jupiter, Saturn. Once in a while, I will put it on a dob mount
and look at the Moon. Wish I could use it properly.

Eric.

lid (John Savard) wrote in message ...
The Ritchey-Chretien.

Amateurs may grind Dall-Kirkham telescopes themselves because it is
easier to do so than to make a classical Cassegrain.

Originally, the Dall-Kirkham was popular because the ellipsoidal
secondary was difficult to test, but a method of dealing with that was
discovered. The thing is, though, that if you're in a position to make a
classical Cassegrain, you can also make a Ritchey-Chretien, which
corrects coma as well as spherical aberration - so there's no real
reason not to go all the way.

The Dall-Kirkham has a spherical secondary and an ellipsoidal primary,
both of which are much easier to make (and test) zone free than the
hyperboloid/paraboloid set of the classical Cass or the more extreme
surfaces (both hyperboloidal) of the Ritchey-Chretien. Large
obscurations are not inherent in any of the Cass type designs. They
arise because of the need to achieve fast systems (small f/ratios) and
wide fields for photography. Any of the Cassegrain designs can be
made with an arbitrarily small obscuration just by going to a long
focal ratio like f/25 or f/30. A Dall-Kirkham of this type makes an
excellent planetary telescope and has a completely negligible coma
over the complete (but small) field of view.
Clif Ashcraft

very nice summary. Collimation sometimes proved a little tricky
for amateurs. At higher f-numbers integrity of optical surfaces becomes an issue if homemade. One of the best
large format cass makers was Edward Plamondon of the Lunar Planetary Lab. He made some real beaut's but they
are very rare.





Clif wrote:

lid (John Savard) wrote in message ...
The Ritchey-Chretien.

Amateurs may grind Dall-Kirkham telescopes themselves because it is
easier to do so than to make a classical Cassegrain.

Originally, the Dall-Kirkham was popular because the ellipsoidal
secondary was difficult to test, but a method of dealing with that was
discovered. The thing is, though, that if you're in a position to make a
classical Cassegrain, you can also make a Ritchey-Chretien, which
corrects coma as well as spherical aberration - so there's no real
reason not to go all the way.

The Dall-Kirkham has a spherical secondary and an ellipsoidal primary,
both of which are much easier to make (and test) zone free than the
hyperboloid/paraboloid set of the classical Cass or the more extreme
surfaces (both hyperboloidal) of the Ritchey-Chretien. Large
obscurations are not inherent in any of the Cass type designs. They
arise because of the need to achieve fast systems (small f/ratios) and
wide fields for photography. Any of the Cassegrain designs can be
made with an arbitrarily small obscuration just by going to a long
focal ratio like f/25 or f/30. A Dall-Kirkham of this type makes an
excellent planetary telescope and has a completely negligible coma
over the complete (but small) field of view.
Clif Ashcraft

RichA wrote in message . ..

But can an R-C be made with a reasonable sized secondary?
Seems like most of them from the past were setup for
imaging and had large secondary mirrors.
-Rich

Yes, an RC can be made with a small secondary, but by the
time you are operating slower than F/10 there is no particular
advantage to the RC design over a (slightly easier to execute)
Classical Cass. But the advantages an RC brings to the table
are best expressed in a wide field scope. If you are going to
do an RC why not make it wide? Conversely, if you are going
to make a long focal ratio Cass, why not make it CC or even
DK?

The RC has to have a slightly larger secondary in order to
obey the sine condition (coma free). There is a nice picture
illustrating the differences in "Reflecting Telescope Optics"
R. Wilson Book 1 in easy to visualize geometric form.