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Faucault and Ronchi Testing Equipment.

Over the years I have built three Faucault testers. The first was under my house and had a length of 6" x 1" dressed timber rail bolted between two of the house posts. On one end was fixed the light source and the knife edge. The mirror was held in a wooden slide that could be moved along the rail. I cannot remember what I used for alight. That was forty three years ago. A razor blade was used for the knife edge and was mounted on a compound slide made of aluminium.

When I acquired a workshop I built a test tunnel, twelve inches square and twelve feet long. I used the same compound slide with micrometers on both axes, but now had a straight filament bulb for the light source. This I still use. Today I do not know what type it is or who made it. Fortunately I have not blown it or broken it.

Then a few years ago when I went back to telescope making I built a new test set-up. My optical shop was a twelve by eight feet section of the house laundry. The test rail was a length of 50 x 25 mm. aluminium box section supported by brackets on the house wall. Shimmer is a bit of a problem from vibrations of the house wall. Cairns is built on water and the test rig is 100 feet from the street which is the third busiest. Traffic vibration is a problem. For a crucial test I get up about 3 O'clock of a morning when there is no traffic and the air is still. The opposite wall is brick and would have been better but its outside caught the evening sun full on and heated up. Heat currents would have been a problem.

I will not go into detail with these set-ups. I have now rebuilt the test rig and will do so now. The accompanying photos and drawings illustrate this. Fig 6.. At one end is the light source and knife edge while the mirror holder can slide along the full length of the rail. Maximum radius of curvature it can accommodate is 112".

The Minor Holder. This will take 8" and 6" mirrors. The mirrors are held in a ring with trunions that fit into a fork on the slide. Mirrors locate on the front face against nylon washers and are held by two nylon fittings. The rings are lifted out of the fork onto the bench to fit the mirrors. Once their initial adjustments have been made to return the slit image back to the knife egde, mirrors can be removed and replaced without any further adjustment. Control cables run the length of the rail to tilt the mirror up or down and left or right. Handwheels are within reach of the observing position to make the adjustments. When positioned the mirror holder can be locked in position. Fig. 1 and Fig 2 illustrate how the mirrors are held. Fig. 3 and Fig.4 are of the Mirror Holder while Fig. 5 shows how the mirror is tilted.

Light Source. This is a straight filament bulb with spring tension on the filament to keep it straight and would be considered the equivelent of a slit. It is plugged into a 12 Volt DC. power supply with electronic control. This is the power supply and speed control used on one model of faceting machine I manufacture. The light ranges from a dull red glow to a bright white light. As it is desirable to keep the seperation of the beam of light to and from the mirror to a minimum a small prism is used. This brings the knife edge into a position where the eye can be got in close without one's face fouling the light. Refer to the drawing. A slide carries a knife edge, a Ronchi grating and a wire. A 4X telescope behind the knife edge gives a closer examination of the shadows. The light and the knife edge move together. However I have found in taking zonal measurements that the light has to be considred as fixed. I suppose this is because of the length of the light filament.

Knife Edge Assembly. The whole assembly moves as a unit on linear ball tracks on the test rail. There is six inches of travel, which accomodates a fair variation in the radius of curvature without having to move the mirror. The slide can be moved back and forth by an arrangement likened to a rack and pinion. A 3/32" steel rod running in a grooved roller. The rod bears up against a ball bearing with a resilient bush and an eccentric mounting of the grooved roller allows it to be adjusted to give traction. For zonal readings a dial gauge can be brought into use. A longitudinal adjustable rod can be brought into contact with the dial gauge plunger wherever the slide happens to be. Fixed to the longtitudinal slide is a steel tape runing the length or the rail. The radius of curvature of the mirror can be read directly off it. The cross slide with the knife edge and Ronchi screen is moved by hand. I haven't devised an arrangement for micro adjustment yet that I am happy with. However I find I can slide this in accurately enough to observe the shadow movements.

 

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