Interference tester - Version 1

I need to be able to do dependable interference testing for future flat making and testing of Cassegrain secondaries agains concave test plates.  Here's a picture of the test setup, and a closeup of the common compact fluorescent lights that I'm using right now.  A piece of ground glass (ground with 320 grit) is seen sitting underneath the lights.  Beneath that is a piece of tissue paper for further diffusion, and piece of 1/8" thick red plexiglass.  Beneath that is a piece of 1/4" thick two-way mirror (obtained from a local glass company) that lets the light through to the pieces under test and then reflects the image out to the viewer.


Here's a closeup of the test area, and a picture of some fringes for these two "almost flats". 

test areafringes1

The fringe contrast is adjusted to about what it looks like with my eye.  The three shiny pieces are aluminum foil to keep the surfaces separated.  Finally, here's a closeup of the fringes with the contrast adjusted.


These fringes should be curved more, but I didn't adjust the pieces to be close enough to show the deviation between the two surfaces.  Actually, the above pictures were of the defective 7" and 10" flats, which matched, but the 7" was figured convex and the 10" concave.

Since these photos were taken and after figuring two cassegrain secondary mirrors, the light source was changed to two F8T5BL fluorescent bulbs.  These are 8-watt (F8), 5/8" diameter (T5) black light bulbs (BL) without the normal blue or purple filter material on the bulb.  These bulbs have single, strong green emission line and don't take up much space.  They also don't give off too much heat, so they are ideal bulbs to enclose in the top of my tester.  A green theatrical gel filter from Lee Filters is used to isolate the green line and remove another weaker purple line.  Contrast with these bulbs and the green filter is better than with the red line of the compact fluorescents, and the wavelength is shorter, improving accuracy.

Some photos taken with the new light source are shown on my page about refiguring some flats.

I'm still looking for old low-pressure sodium-vapor lights for my ideal setup.

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