FLAT TESTER.

Over thirty years ago I decided to build a null tester for telescope mirrors. I obtained three eight inch pyrex blanks and ground and polished them front and back. One was perforated, as a monochromatic light source was required I purchased a sodium vapor street light and ballast. I don't remember its wattage, I built up a test apparatus.

However other things then occupied my attention and nothing further was done. But I undertook to make spherical mirrors for Erbert monochrometers. A convex test blank was supplied and the mirrors had to be within five fringes. The mirrors were tested by the Commonwealth Scientific and Industrial Research Organization ( CSIRO) using a scatterplate inferometer. Some of the mirrors tested astigmatic. To test for this fault is not easy and I had no experience with it. So I tested them by interference against the test plate. I found that I had to stand twenty feet back to avoid parallax errors. My eyesight was very good then.

A few years ago I decided to make optical flats as a hobby. They are probably the most difficult surface to produce, but I wanted something that would keep me occupied for along time. It takes no time to make a parabolic mirror. All I had kept of my original tester was the sodium vapor lamp and its ballast. So I built up a new tester. This is illustrated below. I was given a 10° condenser lens so I incorporated this into the design to spread the light evenly over a greater area. A ground glass diffused was added to help. So that the fringes could be viewed from directly above. a thin glass beam splitter was used. It was found that the light illuminated the background the fringes were viewed against and spoiled their definition. Note the black background set back so that the light from above cannot shine on it.

I did not have a great lot of room in the laundry and could only stand back about three feet so parallax was going to be a problem I overcame this by using a wire over the flats. Four reflections of the wire are seen. The wire and three glass surfaces. When all reflections are in line then the eye is directly above. A scale was incorporated to measure the fringes or fraction of fringes. To separate the two blanks under test 1 used strips of .001" nylon shim material.

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