Re: ATM another newbe question

[Dwight Elvey <elvey@hal.com>]

"john" <jp@premier1.net> wrote:
> 
> ive made a focal tester.
> my lightbulb is inbedded into a piece of 3/4 inch cherry block,
> above that i have a 3/4 hole too look through,
> my blade is screwed into the face of the block,
> (on the mirror side of the 3/4 block)
> is it too far from my eye too do me anygood,?

Hi John
 In general, you need to get your eye quite close to the
resolving knife edge or ronchi lines. Having an additional
3/4 inch makes it difficult.

> ,
> when i put a ronchi screen 25lines per inch in place of the blade,
> i see no lines,
> if i pull away from the bulb about 2-3 inches then i see lines,
> i looked at mells tester but couldent see enought to figure anything out,
> if i build another light platform ,
> can the light be further back,?

 In order to see multiple lines, the system needs to be slightly
out of focus. 25 lines per inch seems a little on the low side.
75 to 100 is more common but then I have little experience using
Ronchi gratings. You will never see more lines than the number
of line that are in front of your pupils opening. Too many lines
make it hard to interpret what is going on and too little make
it hard to see small effects on the surface.

> ,
> as long as the light and screen moves,togeather.
> or does the screen have to be at the light source for accurate
> measurements,.?

 The values measured have different meaning when you have the
source move with the knife edge than when the source is
stationary. When both move together, the numbers measured will
need to be multiplied by two to be the same as the fixed source.
This is because you are moving two things so it adjust twice
as fast.
 The fixed source introduces another error. This is generally
far less than your measurement errors when the source is at
about the same distance from the mirror as the knife edge.
The errors get larger when you move the two apart. Still,
with the right math, one can calculate the exact effect.

> ,
> for some reason im interpeting that you have the screen over the light
> source,
> and up into the viewing hole also,
> tell me im confused here also.

 I'm not sure what you are getting at here. Let me describe what
the Foucault tester is doing.
 When you image with a concave mirror, the image is flipped in
both axis relative to the source. This is important when considering
how a slit-less tester works. The tester makes a focused image
of the source slit at the resolving knife edge. The image is formed by
the light reflecting from the entire mirror. This is why we can
see the slight effects of changes in the surface of the mirror.
as we move the knife edge to cut off the light, we can see the entire
mirror go from light to dark all at once for a spherical mirror.
 In order for this to work well, we need a relatively uniform
light source. If you were to go to the mirror and move your
head sideways, you should see the same level of light across
the entire width of the mirror ( assuming a slit source. The
slit-less tester is a little more difficult to check ).
This can be achieved with a diffuser or by placing the light source
vary close to the slit ( or both ). If you use a small enough
slit or pin hole, the light will diffract to be relatively uniform
across the entire angle of the mirror. The problem here is that
this requires a brighter source and a vary small opening.
For the Foucault tester, all that is required is two edges
and a uniform light source, as has been shown in the slit-less
tester.
 Things like Cauder mask need to be place relatively close to
the mirror or they won't be focused to your eyes at the same
distance as the mirror is at.
 Imaging a cone extending from the edges of the mirror, converging
at the slit and then expanding to the light source on the other
side. We want the entire code to have uniform light level.
 Now, looking at how we place our eye relative to the resolving
knife edge. We need to imagine another cone of light, just
missing the knife edge at the point and then expanding to
your eye. If the eye is too far back or your pupil is too
small, you will not be able to see the light from the entire
mirror.
 Now, let us look at the effects of a Ronchi grating. If we
place the grating at exactly the focal point ( don't confuse
this with the term focal length ), it works just like a knife
edge sing the cone converges at only one line. As we move
away form focus, you allow different paths to the mirror to
pass by the grating, forming the bands. In order to use a grating,
one needs to use a point source, slit source or a grating
over the source as well. A slit-less source won't work since
it only has one side and the grating has two sides to each line.
 So, as an over view, we are focusing the source image at the
resolving knife edge. We place the knife edge close to our
eyes so that we can see where the light came form that formed
the focused image at the knife edge. This is the effect that
we see on the mirror. Remember, all the light rays go in straight
lines ( except those diffracted around edges ). Mostly, we are
looking at the straight line paths. If you can't form a straight
line from the source to the mirror and then to your eye, you can't
see the light.
Later
Dwight