Here's a start on the Hindle Test Procedure The Hindle Sphere Test is a testing setup for convex optics which allows for the null testing of a convex surface at a real focus so that the tests like the Foucault Test and the Ronchi Test can be done.
The test requires a perforated sphere which is called a Hindle Sphere to do the test. The sphere required to be a good spherical surface of spherical shape and usually needs to be of a short focal length in order to be able to test the convex surface fully.
The first thing to do is to figure out what you are doing as related to how big the convex surface is that you are going to test and it's various focii. What I am interested in here is the testing for a hyperbolic surface for the cassegrain telescope. The secondary of a Cassegrain telescope is usually a hyperbolic surface of some pair of focii. The near focus is behind the surface (on the concave side) and the far focus is on the front side of the surface. You need to know both of these focii in order to be able to test the surface.
There are a number of formulas used for determing
what the surfaces of a Cassegrain telescope are like that will be needed
for the testing and let's review them now.
Tthe distance to the focus of the system which is given as:
The Ronchi or Foucault tester is located at the Cassegrain focus, which is the other focal point for the secondary. Light from the tester proceeds to the secondary, then to the Hindle sphere, where it reverses direction exactly, then back to the secondary, then back to Cass focus. The airspace between the Hindle sphere and the secondary vertex is the Hindle ROC minus the "p" (rear focus point of the secondary) distance. Now that we have all of the various numbers of the scope all figured out, the test is really pretty easy to setup (in theory anyway) and perform. First thing that we need is all of the hardware to hold all of the optics and testers. This means that we need:
Hindle sphere holder that can move the ROC up and
down and left and right.
An aiming device that will help aim the sphere to the right place - the Foucault tester can provide the necessary aiming capabilities.
It would also make things easier if the testing apparatus (Foucault Tester?) is attached to the back if the focal point is nicely close enough.
This will make that much of the system easier to handle.
You then need a holder for the secondary and that also needs to be able to tilt and rotate left and right.
The secondary holder should also have a good set of mountings so that you can have some kind of repeatibility (mark a top on the secondary) when you're testing it. Remember that you're going to be taking the secondary off, doing a couple of licks with the lap and then putting it back on the tester and being able to only move the secondary just a little bit will be a big help.
You should also have the secondary holder on a set of rails of some kind so that you can make short in and out movements. These can be a bit cruder than those of the other motions - the rails mostly will be just guiding the holder so that it doesn't get way out of the proper angle.
You will also need a good measuring stick to measure the distance from the Hindle sphere to the surface of the secondary. This measurement is very critical as it sets what the actual shape of the glass is going to be and we don't want any Hubble Telescope Mirrors made here if we can help it.
If you're ready to start testing, put the Hindle sphere
(you already know exactly what it's radius is) up on it's mount, place
the secondary so that the distance to it is the ROC of the Hindle sphere
minus the (virtual) near focus of the secondary is at the same place. After
the Hindle Sphere and Secondary is setup, the Foucault tester (where you
are going to actually test the secondary) goes at the position that is
the far focus point of the secondary (marked as p' in the formulas above)
from the surface of the secondary.
Get everything lined up so that you can see the Hindle sphere centered in the secondary thus making everything coaxial and test. Everything should be at the same height and when looking at the image coming back from the optics at the knifeedge, it too should be at the correct height.
As a last check, make sure that all of your measurements are correct and the Foucault tester should be exactly where you think that it should be.
If the Focuault tester isn't giving a nice spherical looking null, but rather it's inside or outside of Focus, you've probably measured something wrong or you just have the wrong shape to the Secondary and the latter is probably true.