Ronchi grating eyepiece

suitable for simple star testing

Making a Ronchi grating is fairly simple, albeit a bit fiddly.

    Materials:

• Two small pieces of thin metal, around 40mm x 20mm. 

• Fine fishing line.

• Empty 35mm film canister.

• Small self-tapping screw.

    Tools:

• Junior hacksaw.

• Drill, with 10mm bit.

• Glasspaper.

• Stanley knife.

• Superglue.

    Construction:

Cut out the two small pieces of metal, sand edges and fix them together in one corner with the small screw. Drill a 10mm hole through both pieces, about 20mm from one end. Now take a length of the fishing line (around 2m), and double it up. Fix one end of the double strand to the screw. Now for the tricky bit. Wind the double strand of line around the little metal sandwich, carefully pushing the line up with your thumbnail to keep the wrapping tight. Make sure the wrapping does not kink and forms a smooth covering, and keep going till the small hole is completely covered. Use a drop of glue to fix down ONE of the strands to the metal plate. Now take the free strand and carefully unwrap it. If all goes well this will leave the hole covered with a single strand of line with spaces of the same width as the line. Cut the free strand off and apply superglue to the line remaining all along, above and below the hole. Do this on both sides. When the glue has set cut the lines at the edges of the plates, remove the screw and separate them which should leave you with two Ronchi gratings.

To make the eyepiece, simply drill a 10mm hole in the centre of the bottom of the film canister (these things conveniently fit inside a 11/4" focuser) and glue one of the gratings over this.