I stayed up until 1:20 one evening in my basement taking measurements using a Couder mask, and I kept getting different readings.
If I took a set of readings, and went upstairs and entered them into the Texereau BASIC program (Converted to C thank you) to get a wave rating, and then went downstairs and took another set of readings, they were very different (Man a marathon run-on sentence there).
I'll have to describe my setup so you can get an idea just how crappy it is.
I have a plywood board with a Sherline lathe bolted to it and a razor blade mounted in a machinists vice. I also have a plate with two other razor blades bolted to it, and a Halogen light bulb for a light source.
This is setting on two saw horses in my basement. My basement is COLD. This is Minnesota after all.
The lathe has a numbered wheel that I can use to position the knife.
But the numbers go the wrong way, and only go 0 to 49 so if I record the
following numbers: 26 47 12 15 what I need to do is remember where the
zero crossings were ^ ^
| |
+-----+---- Here
And I need to do this: Pretend the first reading is at zero. Then the second is at 50-47+26 = 0.029 Inches The third is at 47-12+29 = 0.064 Inches The fourth is at 50-15+12+64 = 0.111 Inches
Fun, and not very error-prone huh?
Well, I kept getting "Creeping" values. The center measurement kept increasing about 0.05 Inches between every set of readings. ALL NIGHT LONG!
So I decided that it was my basement that was cooling off, and taking the mirror along for the ride.
I decided that the basement just wouldn't work.
I moved everything upstairs into a room that I heat with an electric/oil heater and is kept at 70 Degrees.
A day later, I took more measurements, and they were much more stable, but I had the tester setup in the hall, and it was vibrating all over the place, so I couldn't get really good measurements.
Oh, I also took a dial micrometer which is a thing that looks like a speedometer with a straight shaft sticking out of one end, and if you push on this shaft, the dial measures displacements to 0.001 Inch. I mounted this on the lathe bed, pushing against the vice, and now I can directly read the distances. I can even set the dial to zero when I get the center zone reading. These things cost about $20.00 and the magnetic stand was $15.00 from the local Enco dealer. Highly recommended.
OK, now I have a question. I think that my knife edge and slits are quite poor. They are single edge razor blades, and look rough under a ten-power loupe. I am thinking of making some brass ones like Texereau suggests.
When I try to measure a zone, as I cut into the cone of light, things get really blurry, and hard to read.
Should the mask stay in focus as I cut into the beam?
My knife edge and slit are almost 10 Inches apart which I know is WAY to far apart, so I think I'll replace the slit, and use a bright LED for a light source, and get the slit and edge about 1.5 Inches apart.
Is this causing my blurring?
Anyway, I'll get things rebuilt this week, and I hope to have some more news later in the week.
-Alan Kilian