John Klayer's Spatial Filter Plan

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A simple spatial filter design

by John Klayer

A few people asked to see my design, so here it is. It was published in the L.A.S.E.R. News Vol. X #4 Winter 1990-1991. It is very easy to use and not too difficult to build.


Parts list:

A Carriage: 1/4 by 1-3/4 by 2-1/2 inch plexiglas. A 1-1/4 inch hole is centered 7/8 inch from the top. The top and one side are drilled and tapped to receive D1 and D2. A slot is filed at 45 degrees to receive F. (See the drawing below for an alternate method.) A hole is centered, drilled and tapped 3/8 inch from the bottom to receive E. Two holes 3/8 inch from the bottom and 1/4 inch from each side are drilled thru the carriage and the base h to receive I1 and I2 - use a #6 drill. (These holes need to be drilled thru the carriage and base in one operation.) Around the 1-1/4 inch hole three holes are drilled and tapped 120 degrees apart for the 2-56 screws that secure the fender washer C.

B Magnet: A 1-1/8 inch ring magnet available from Radio Shack.

C Plate: a 1-5/8 by 5/16 inch fender washer with three holes drilled 120 degrees apart for the 2-56 by 1/4 screws that secure it to the carriage A.

D1, D2 Fine Positioning Screws: 0-80 by 1/2 machine screw, end rounded and fashioned into a thumbscrew by soldering onto the head the knurled disk part cut from a 10-32 knurled nut. The 10-32 knurled nut is cut by putting it on a 10-32 screw and turning it with a drill while applying a hacksaw blade. The soldering task can be simplified by using a wooden fixture to align and hold the knurled disk and the 0-80 screw: Bore a flat bottom hole 1/2 inch diameter by 1/8 inch deep into a block of wood then continue concentrically with a 1/16 inch hole. (I've thought about replacing the 0-80 screws with 2-56 screws.)

E Focusing Adjustment: Same as D1 and D2 but use a 3/4 inch screw.

F Spring: Closed and ground ends, length from 3/8 to 5/8 inch, about 1/8 inch diameter.

G Pinhole Holder and Coarse Adjuster: Any piece of ferrous metal shaped similar to the illustration. Glue the pinhole to the ring part. The ring and handle should be able to lay flat against the magnet - this is important, otherwise the ring would be drawn to the magnet center and there wouldn't be any coarse adjustment. The handle part should extend beyond the edge of the carriage. You may find an item like this at a hardware store called a "ring hanger".

H Base: 3/4 by 1-1/2 by 1-3/4 inch block of wood. Two holes drilled thru it 3/8 inch from the bottom and 1/4 inch from each side. These holes must align perfectly with the two in the carriage,A. To assure alignment, start with a sheet of plexiglas and a piece of wood about 2 inches wider, attach the plexiglas firmly to the wood with two wood screws and drill thru both, the saw to the 1-3/4 inch width. Make a notch 3/4 inch wide and 1/8 inch deep in the front to receive K. Centered on the top, drill a 3/4 inch flat bottomed hole partially thru and continue concentrically with a 5/16 inch hole the rest of the way to hold L.

I1,I2 Sliding Shabts: Three inch aluminum screw posts (manufactured by Charles Leonard, Inc.) available from office supply stores. The normal use is for binding books. These have two heads, one attached to the shaft and the other screws into the other end. Insert the posts thru the carriage and base so that the screw end is to the back. Epoxy the front 1/4 inch of the posts inside the carriage holes.

J1,J2 Focusing Springs: Any spring that will fit over the shaft with a lenght of about 1-1/2 inch and not too stiff.

K Lens Mount: 3/4 by 1/8 inch aluminum. 1-5/8 inch from the bottom drill a hole the diameter of the lens. Drill and countersink two holes 3/16 and 9/16 inch from the bottom for two wood screws to secure it to the base.

L Fastener: 1/4-20 by 3/4 inch T-nut secured in the hole in H by three small wood screws. This is to provide a means of mounting the assembly on a post.

Lens: You could use a simple lens. What I found works real well is a "Hastings Triplet" from Edmund Scientific. Forget the microscope objectives.

An alternate design for the carriage magnet spring:


Here I'm using a hole drilled at 45 degrees with a head of a small nail against the magnet, a small spring held in the hole by a short section of brass tube epoxied in the hole.

I used clear plexiglas for the first few I built then I found some black plexiglas. Use the black if you can find it. If you never tried drilling a 1-1/4 inch hole in plexiglas before, you're in for a real treat. I'm sure there are plenty other materials you can try besides plexiglas and wood. These materials worked for me. Good luck.