[ATM] Ritchey-Chrétien

[Stanley Truitt]

Many enthusiasts make the mistake of assuming that any given Hindle  
Sphere must be
about the same diameter as the primary of the Cassegrain system being  
considered.
This is wasteful and overly simplistic; but follows the Figure 8  
diagram by John Hindle in
his chapter "The Compound Telescope" in the first volume of the ATM  
trilogy.

All that is required is that the test sphere be around 60% of the Cass  
primary mirror's
diameter, and that is easily determined by a careful scale layout of  
the test setup.
What is necessary, is that the center of curvature of the sphere, be  
placed coincident with the
position of the prime focus, behind the secondary's vertex; and the  
knife edge at the position
of the desired Cass focus on the other side of that vertex.  The size  
of the necessary perforation is
easily obtained from the scale layout, as is the radius of curvature  
at any given position of the
test sphere.

Blindly following Hindle's simplistic diagram has caused a great deal  
of unnecessary work
and expense for those not bothering to think this through. I will be  
glad to check the layout
dimensions for anyone considering such a test setup; and will be  
pleased to share an
Excel spreadsheet that, along with other Cass calculations, gives  
those handy conjugate
positions.

Stan Truitt



On Oct 19, 2009, at 11:15 AM, Bob May wrote:

> First off, a Hindle Sphere is just a fancy name for a spheerical
> mirror with a hole in the middle.  Its size for a telescope
> secondary will be about the same diameter as the primary and half
> the ROC of it.  The biggest difficulty in doing such is all fo
> the glass tht needs to be removed.  If you've got a full
> thickness piece of glass, make the Hindle Dpheree first and then
> make the secondary to the right shape.  After that, grind the
> primary mirror from the Hindle sphere and finish the scope.
> Final testing of the system can easily be done on Polaris with a
> Ronchi grating and you can correct any errors left after finding
> out which surface is the problem.   The grinding and polishing of
> the surfaces is not going to be difficult.  The biggest problem
> is actually testing the surfaces to at least a tenth wave and
> prefferrrrably better.  Do make suree that you haaa the
> mechanical parts built before testing on Polaris or things will
> be rickeety and you 'll be frustrated from not getting proper
> results.  Always having others to bounce your problems off of
> (like us!) will help a lot in doing the work.
> Bob May