Re: [ATM] [ ] making Foucault tester

[Dale Eason <atmpob@yahoo.com>]

What I do is use the Bright White LED taken from those
flashlights the sell everywhere these days.  I got
mine at Target for about $19.  Then I cut the front
rounded part off of the LED.  That makes a nice
diffuse light source. 

Read Texerau again and you will see that he uses a
diffuse light source and a slit.  The diffuse light
source and slit meets all of the requirments for a
tester.  The slitless tester likt the Stellefane does
that as well.

In my opinion you and many here are overthiking this
and worring about things that do not matter.  I use
mine for both visual and CCD Foucault measurments
driven by a computer.  I also have an interferometer
and both tests systems agree very well.

I have also used a laser diode without its lens taken
from a $5 laser pointer to make an artifical star. 
For all pratical purposes it is a point light source
despite what Bob May says about it.  You can use it
with a CCD camera without the lens and can photograph
the diffraction star pattern produced by a mirror.  It
is mostly just an amusing exercise since there are
very few ways to analyze the data.  The one place were
that is useful is to look for astig in a sphere.  It
will show up as the usual ellipital ring pattern as
described in Texerau.



Dale Eason


--- Mitch_Bisnauth@Cknainc.com wrote:

> Just because this is all new to me I'm going to
> 1-line what I understand 
> from the emails regards laser diode utility in a
> Foucault tester - (and 
> not touch on the electronics part because I've
> broken more than I've 
> made...)
> 
> A regular LED is normally used but a laser diode can
> be used --- however 
> the laser emits with a gaussian intensity profile in
> 'both' traverse axial 
> directions with the divergence being larger in one
> direction...and that 
> intensity variation requires fairly precise aligment
> of the light source 
> to the mirror axis to prevent false
> readings/shadows.
> 
> Is the light intensity profile from the regular LED
> different? - maybe 
> flat/spherical w/o axial variation/bias?  - (I was
> about to polish off the 
> lens on the regular LED to form a point
> source..good/bad/indifferent 
> idea?)
> 
> I was under the impression that the light source had
> to be polar 
> perpendicular with the mirror plane anyway to a good
> level so that the 
> intensity is the same going to the 2 opposing zones
> in the C-mask --- my 
> intention was to use the collimator to get to better
> than the sweet spot 
> dia --- good/bad idea, not-needed?  (how critical is
> centering....?)
>  
> MitchB
> 



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