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Making a ronchi grating

For those who are not sure, a ronchi grating is used to test the quality of a telescope's objective. In it's simplest and oldest form it's a finely spaced grille of wires mounted in a frame, held inside the focus of a telescope. When this arrangment is pointed towards a point light source like a star, the shadow of the wires appears across the brightly lit objective. In a perfect objective the lines should be striaght and true. Any distortions of the lines will be caused by errors in the objective. It's a simple test that doens't provide qualitative measurements of errors, but does show deviation from what should be ideal. In fact, it is a null test for telescope objectives, and shows errors like regular zones, lumpy surface (dog biscuit) and turned edge with clarity. 

Rather than buy or try and reproduce one by printing on mylar, I decided to try and make one the "old fashioned" way, with fine wire. The wire was taken from electrical cable, which was stripped of it's insulation and the indiviidual copper strands unbraided. After than was the problem of spacing them evenly and nice and straight. I decided to make a jig, using fine brass screws. The wire was wrapped around this as shown in the pictures. The substrate for the grating was a piece of heavy gauge brass with a hole in it.

ronchi1.JPG (30615 bytes) ronchi2.JPG (29526 bytes)

The next hardest part was actually stikcing the wires to the brass. Initally I was going to solder them, but I had problems getting the solder to flow evenly and with the wires buckling with the heat. In the end I used plain old superglue. Once dry, the substrate was snipped from the jig.

ronchi3.JPG (39344 bytes)

A simple eyepiece was made from 32mm brass tube.

ronchi4.JPG (28116 bytes)

This grating is very coarse in comparison to modern replicas that are printed or etched onto film - approx 30 lines per inch or so.... the classic method to achieve the fines possible spacing is towrap a double strand around the substrate. Once completed, unpeel one strand and you end up with wires spaced exactly one wire-diamater apart. This might be something I try for the next attempt.