Posted 25 February 2004 - 12:48 PM
Additionally, how come cassegrain's are not more popular. You can only get a couple now on the open market. There was a recent article about a couple in one of the big magazines. Seems like they would be more popular in the big sizes due to portability. Then again I guess you NEED a good mount to hold one, with a newt you just need a dob mount. It would be interesting to see an 18in cass on an altazimuth mount, couldn't be much more complex than a truss tube dob.
Posted 25 February 2004 - 08:12 PM
Schmidt-Cassegrains are popular because they are easy to manufacture. The primary is a spherical mirror, which is one fo the easiest surfaces to make. The secondary is also pretty close to spherical. The corrector plate is made by applying pressure to deform the plate and grinding the surface flat. On the other hand a classic Cassegrain requires a very fast parabolic primary. The hyperbolic primary of the Ritchey-Chretien is even more difficult to figure and test.
The Dall-Kirkham is an interesting compromise - a variant of the Cassegrain but using an ellipsoid primary and a spherical secondary. The Takahashi Mewlon uses this design, as well as the recently announced 20" from Celestron.
Posted 25 February 2004 - 11:17 PM
And what is the correct spelling of a plural ellipse?
Posted 26 February 2004 - 04:49 AM
Elliptical is maybe the other word you are looking for. But I think mirrors can be spherical, parabolic, or hyperbolic, but not elliptical. I far as I know, parabolic and hyperbolic surfaces will not met if extended. Elliptical surfaces will. Spherical surfaces will too, but they are not as good at forming sharp images as the other two. The ellipse curves in the wrong way at the edges. Isn't a parabola created from the cross section of a cone?
You spend so much time trying to remember things at school, you would think some of it would stick.
Posted 26 February 2004 - 09:59 AM
But I think mirrors can be spherical, parabolic, or hyperbolic, but not elliptical.
No, they can be elliptical too. Elliptical mirrors come in two variaties prolate and oblate. In one case the foci of the ellipse lie along the optical axis, in the other they are on a line perpendicular to it. Don't ask me which is which, that I can never remember!
The DK uses the kind where the foci lie on the optical axis. The DK uses mirrors types that are easy for an amateur mirror maker to test. The spherical secondary can be null tested through its back at its center of curvature. The main mirror can be null tested by putting the source at one focus and the knife edge at the other.
I'm not sure it has been answered clearly but the Classical Cassegrain, the RC, and the DK are not catadioptric telescopes since they use no lenses. The new Celestron is a catadioptric DK becuase it uses corrector lenses in the baffle tube.
Posted 26 February 2004 - 10:17 AM
If I'm correct, Cassegrain, Newtonian, Coude refer to where the focal plane is located and how it gets there.
Posted 26 February 2004 - 05:43 PM
Posted 29 February 2004 - 02:39 PM
yup. And the hyperbola is the one where the crossing plane is parallel to the cone's axis.
. Isn't a parabola created from the cross section of a cone?