ABOUT THE TEST HEAD: The head will sit on top of the stage - it holds the light source, knife edge, (optional) Ronchi screen and (optional) setup aids. The large area surrounding the viewing hole provides a target surface for alignment - the returned beam will likely reflect off this surface and the mirror stand can be adjusted until the returned beam is in the viewing hole just above the light source. The knife edge is mounted in a standard 2"x 2" slide frame (shown at left), and a Ronchi screen can be mounted in one also (not shown). The sliding track allows a clear aperture for a knife edge of a Ronchi screen to be quickly positioned and used.

Note: The light source and knife edge should be just about at the same level as the center of the mirror under test. The dimensions used here for the test head were sized for our matching Mirror Test Stand. If you are using a different stand, you should adjust the size of the head appropriately.

CONSTRUCTION: Referring to the Head Base Plans, start by cutting the plywood base and the two triangular side braces. Drill four 1/8" pilot holes for the brace screws, and two 3/16" holes for the screws that will attach the head to the stage. Put these parts aside while we build the Head Face.

The Head Face is a built from a sheet of " plywood - refer to the Head Face Plans. It has a couple of tricky parts. The central square viewing opening should be tackled first. Mark it on the plywood, then drill four large holes (about " each) just inside each corner of the opening. You can then use a keyhole or jig saw to cut out the square.

The second tricky part is the pair of tracks used to hold 2" square slide frames (these in turn can hold knife edges and Ronchi screens). These are optional - see the box below for a simpler way to hold a knife edge. The tracks are are also made from " plywood. Make them short enough to fit between the side braces. Glue them with wood glue, and use short brads or staples to hold them in place while the glue dries. It is a good idea to have a slide frame handy to adjust clearances while doing this.

ALTERNATIVE KNIFEDGE ATTACHMENT: If you don't want to make tracks or use slide frames, you can just tape (use duct tape) or screw a knifedge over the viewing opening. Do this after you mount your light source, so that you can be sure the knifedge "cuts" (partially covers) it. A utility knife blade is longer than a standard razor blade and is better for this use. You can also tape or screw a Ronchi screen over the viewing hole - just get it flat.

KNIFDGE ASSEMBLY: Our knifedge assembly is made from a standard 2" x 2" plastic slide frame, sold in photography shops to mount 35mm slides in. Select the image at left to see several steps in making this assembly. A standard single-edge razor blade is just the right size to fit in this frame. But first, you must carefully remove the stiffener opposite the blade edge. Leave the cardboard guard on the blade and wear gloves, or mount the blade in a vise. Then use a tool to pry the stiffener open and then off the blade - we used a small pair of diagonal wire cutters. The bare blade is then placed in the frame and the frame snapped shut. Black tape is used to keep the blade from sliding in the frame, and to optically darken the flat part of the blade and the white frame.

RONCHI SCREEN: Glass Ronchi screens in 2" x 2" size can be bought and will slip right in the track assembly; see our ATM Suppliers list. A film type screen can be bought or made on a computer printer using overhead transparency film. Cut the film screen to size and mount into a slide frame also. Generally, screens between 80 to 150 lines per inch are used; 100 LPI is a good first choice. Select the Ronchi Screen image on the right for a Printable Ronchi Screen Image.

HEAD ASSEMBLY: With all four parts cut out and drilled, assembly is simple. The side braces are glued and screwed onto the base. Slide you knife edge and Ronchi screen into the tracks on the face - make sure the knife edge and Ronchi bands are vertical. The face is just screwed on, not glued, so it can be removed for access to the items in the track. Screw the face on before the glue for the side braces sets, to make sure everything lines up.

OPTIONAL SETUP AIDS: We present two setup aids: (1) a tape measure hook; and (2) a miniature flashlight holder. When working by yourself, these simple items can make setting up and aligning the tester much easier. Tape Measure Hook: Our Mirror Test Stand is shown with an optional tape measure attached. It is convenient to be able to hook the end of the tape onto the tester head, so your hands are free to move the mirror stand to the proper radius of curvature for testing. Our tape measure hook can be seen sticking out from on the left side of the photograph at the left. It is simply a countertop laminate sample (which comes complete with a pre-drilled hole) screwed to the back of the test head. The screw is tightened just enough to hold the laminate in position, yet it can rotate down and out of the way when desired. Flashlight Holder: The flashlight holder is shown at the left. The knifedge and/or Ronchi screen have been slid out of the way, the copper pipe holder has been rotated in front of the viewing hole, and a miniature flashlight has been inserted and aimed at the mirror. While the green LED is plenty bright enough for testing and aligning an aluminized mirror, it does not produce enough light to align an uncoated mirror. To use, you would aim the flashlight beam at the center of the mirror, and then adjust the mirror stand to return the beam directly back to the  flashlight. The roughly aligned reflection can be easily seen on the face of the head. Several club members have used a pocket laser pointer instead of a flashlight with good results also. The holder is built from a Copper Van Clamp, a short length of " Copper Pipe, and a " Copper Tee. Solder or glue the pipe to the tee, mount the Van Clamp just tight enough so it can be rotated out of the way, and clamp the pipe just tight enough in the clamp so it can be rotated (altitude adjustment for the light).