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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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