The Hindle Sphere Test Bench
New for 2008 participants of the Mirror Workshop is a Hindle Sphere test setup for testing Cassegrain secondaries. The Cassegrain arrangement of reflecting telescope has long been sought after by users of telescopes. One difficulty with making a true classical Cassegrain is that the secondary mirror is a convex hyperboloid. Over the years there have been several compromise telescope designs developed to avoid this complex secondary the most notable being the Dall Kirkham, having a spherical secondary. This design, however, is significantly inferior optically to the true classical Cassegrain particularly having increased coma. With the Hindle sphere test, it is possible to test the convex hyperboloidal secondary to the precision required for perfect optical quality.

Test Concept
In the Hindle sphere test, light from a small light source at the system focus goes through a hole in the center of a larger spherical mirror then on to the secondary being tested. The light reflects off the secondary in a diverging cone to the larger spherical mirror which reflects the light back to the secondary being tested which then reflects the light back again through the hole in the spherical mirror to the knife edge or Ronchi screen for viewing. This arrangement is quite similar to the autocollimation test for a parabloid using a perforated flat. In the Hindle sphere test the flat is instead a spherical mirror whose radius of curvature is approximately equal to the focal length of the Cassegrain primary that will be used in the finished telescope. This test has all the advantages that the autocollimation test has for testing parabloidal primaries. The test is a double pass test and the test return is a null return.

The Test Bench
The difficulty for the amateur is that you need an auxiliary optic for the test, that auxiliary optic being the sphere. And you need a test setup that can place the secondary precisely within the light cone of the sphere and at precisely the correct distance from the center of curvature. It is generally more effort than justified for a single amateur making a single Cassegrain telescope. The Hindle Sphere test setup uses a 12.5 inch diameter mirror at slightly less than f/2. This diameter sphere in combination with the chosen diameter of the perforation provides a Hindle sphere capable of testing Cassegrain secondaries for any telescope having an f/4 primary that is equal to or between 12.5 and 16 inches in diameter.

Above is pictured the Hindle sphere test set up with the elements positioned for testing a secondary for a 16 inch f/17 telescope. The light source and Ronchi screen is on the end of a dowel sticking up from the white tester slides at the far left of the photo. The sphere is in a square box in a yoke in the middle of the picture. To the right is the stage with the secondary under test in place. To the far right is the primary focus.

Pictured to the left is a view from the primary focus end of the test setup showing the face of the Hindle sphere. At the distance of the camera, the sphere forms an image and it looks like a transparent glass, but you can see the central perforation.

To the right, this close up view of the stage that holds the secondary being tested shows that the center of the stage has provision to move the mirror laterally and vertically to position the secondary precisely within the light cone of the sphere. The tilt and pan features are controlled by threaded rods at the bottom of the view extending out of the picture border.

The picture below shows the control rods for the tilt and pan of the mirror being tested passing through the yoke of the Hindle sphere to a point where the rods are accessible to the operator sitting at the light source and Ronchi screen.

And pictured below you are looking through the Ronchi screen at the secondary through the perforation in the Hindle sphere. Visible is the Ronchi pattern. This secondary has not been corrected yet so what you see in the return is an overcorrected optic (remember double pass). Once hyperbolized the Ronchi lines will be straight, parallel, and even.

So if you've ever wanted to try your hand at making a Cassegrain, the Mirror Workshop has the Hindle sphere test bench needed to make it possible.