first time Foucault attempts kind of long

Hi guys

Since you've all been so helpful in the past, here's another question:

I have been attempting to use a homemade Foucault tester. It's been a
lot of work, because of the quite critical adjustability needed to get
stuff set up properly.

I use a little laser pen that also has a bright blue LED, as a light
source. You know, turn on the red laser and search the room behind me
for the blip reflected from the mirror. Then get it close to the knife
edge and switch to the blue LED.

I think I'm close. I was beginning to see some "patterns" this
evening, suggesting I have a pretty decent parabola rather than a
spherical surface, which suits me just fine!

The mirror image, when filled with light as it is at 2F, is not very
big, being an 8" with a focal length of approx 70".

It's hard to hold my head still though, and the image shimmers and
wobbles and the patterns change. Do I need a chin rest like the
optometrist uses!?

THe LED has a lens with a surface that looks sorta crystalline when
magnified. I will have to add a diffuser.

Is it correct that when the tester is set up right re the focal
length, without a diffuser, I should see a sharp image of the LED lens
surface visible behind the pinhole, at the knife edge? I mean will
seeing this image of the LED surface tell me the
LED=to-mirror=to=knife edge distances are good?

thanks, anybody and everybody! Texereau's book is good but does not
say much about the _experience_ of doing a Foucault test-.

James

It's hard to hold my head still though, and the image shimmers and
wobbles and the patterns change. Do I need a chin rest like the
optometrist uses!?

No, your eye will adapt to the change in position of the pinhole light
source.
The "wobbles" are most likely air currents (this is a very subtle
test).
Shut the doors and windows, let the air steady, then test (don't
forget to breathe, most important)
:-)) good luck, jc




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"John Carruthers" wrote in message ...
It's hard to hold my head still though, and the image shimmers and
wobbles and the patterns change. Do I need a chin rest like the
optometrist uses!?

No, your eye will adapt to the change in position of the pinhole light
source.
The "wobbles" are most likely air currents (this is a very subtle
test).
Shut the doors and windows, let the air steady, then test (don't
forget to breathe, most important)
:-)) good luck, jc


Or could it be his unusual light source?
A bulb behind two razor blades might be a bit too "iron age" these
days. But does offer a bright source that is not unduly affected by
head position. At least in the vertical plane. It also has the
advantage of making Ronchi testing a piece of cake.

Chris.B

On 11 Dec 2003 04:38:26 -0800, (Chris.B) wrote:



Or could it be his unusual light source?
A bulb behind two razor blades might be a bit too "iron age" these
days. But does offer a bright source that is not unduly affected by
head position. At least in the vertical plane. It also has the
advantage of making Ronchi testing a piece of cake.

Chris.B

Just FYI, I am using a pinhole in a strip of aluminum as a source. I
started out planning to make a slit, then decided to go with something
easier "for a while".

The pinhole was made with an 0.025" (0.64mm) drill. It's a bit biggish
I guess

The "iron age" solution may be what I need - will work on it.

Thanks-

James

brocpuffs wrote in message . ..

Just FYI, I am using a pinhole in a strip of aluminum as a source. I
started out planning to make a slit, then decided to go with something
easier "for a while".

The pinhole was made with an 0.025" (0.64mm) drill. It's a bit biggish
I guess

Thanks-

James

I'm only guessing but I'd say your hole is a bit on the large side.
Twirling a sharp sewing needle between the fingers into aluminium
(cooking?) foil lying on a hard surface is better. Or even into a
number of layers of foil and then selecting which best suits your
needs from trying them. Needles are tapered so it makes a series of
different sized holes in the layers for you to choose from. Use a
frosted low voltage torch bulb if you can for a more even light
source. Particularly if you go over to using a slit made from razor
blades.

Chris.B

James, my boy, do yourself a favor and make things easier (but less
intuitive, maybe). Take your led and file the top down flat until you have a
nice flat circle. Use an old credit card or library card for your knife
edge, *covering one half of the LED on the bottom vertical edge of the card
and extending up an inch or so above the top of the tester*. The idea is
that you use the credit card (or utility knife blade or whatever) as both
the slit and the knife edge. Then set the device at the radius of curvature
and adjust it until you can see the reflection of the light an inch or two
above the tester. Then slide the thing carefully across the mirror until the
shadow begins to become visible.

I know I'm not explaining this well, but it's late and I'm bushed.

I fooled around for a week trying to make a traditional tester until I ran
across how to make a "slitless" foucault tester. It works *so* much better
and *so* much easier. The light is *so* much brighter. It took me another
week or two afterwards to figure out why the blessed thing worked, but it
did. Like a charm. Look online for more instructions. Once you make one
you'll never go back to messing around with the PITA slits and pinholes.

'Luck,
Chris


"brocpuffs" wrote in message
...
On 11 Dec 2003 04:38:26 -0800, (Chris.B) wrote:



Or could it be his unusual light source?
A bulb behind two razor blades might be a bit too "iron age" these
days. But does offer a bright source that is not unduly affected by
head position. At least in the vertical plane. It also has the
advantage of making Ronchi testing a piece of cake.

Chris.B

Just FYI, I am using a pinhole in a strip of aluminum as a source. I
started out planning to make a slit, then decided to go with something
easier "for a while".

The pinhole was made with an 0.025" (0.64mm) drill. It's a bit biggish
I guess

The "iron age" solution may be what I need - will work on it.

Thanks-

James

(Chris.B) wrote in message . com...
Or could it be his unusual light source?
A bulb behind two razor blades might be a bit too "iron age" these
days. But does offer a bright source that is not unduly affected by
head position. At least in the vertical plane. It also has the
advantage of making Ronchi testing a piece of cake.

I have always used a tiny flat mirror right behind the slit
that angles a nearby 75w bulb's light into the slit.
Works like a charm.

D Chaffee

(Dan Chaffee) wrote in message . com...

I have always used a tiny flat mirror right behind the slit
that angles a nearby 75w bulb's light into the slit.
Works like a charm.

D Chaffee

Great idea to get way from the heat of a bulb close to the face. How
did you make or obtain this tiny flat mirror?

It seems the Foucault test is capable of being updated and modified
endlessly as in Chris' self obscuring(?) slit system in the post
above. I have never considered an LED as a light source until now.
Thanks for the idea. :-)

Chris.B