Problems with old style testers: 1. If you use a pinhole, you don't get much light. 2. If you use a slit, it can become misaligned. 3. Dim light sources are hard to line up.
So, you sidestep these issues by making a slitless tester. Find a small prism and frost one of the sides. A small light bulb shines up from the bottom, strikes the diagonal side of the prism, and heads out toward the mirror. The light strikes the mirror, and is reimaged back at the radius of curvature reversed.
Now, imagine a knife edge taped to the mirror facing side of the prism. The image of the lit half of the prism face will be imaged back onto the unlit side. By shifting the assembly laterally, you can simulate a slit without having a slit, allowing a thin sliver of light to pass the knife edge.
When I finish making mine, I will try to make a better description of it available on my web page.
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Mark T. VandeWettering Telescope Information (and more)
Email: <markv@pixar.com> http://webspace.com/markv/
<markv@webspace.com> Clear Skies!