Re: [ATM] Bench star test

[Ken Hunter <atm_ken_hunter@yahoo.com>]

Would it not seem reasonable to try this test using a GRIN lens. Extremely small 
  diameter point source with wide fan-like output...
   
  Ken Hunter
   
  

Arjan te Marvelde <arjan.te.marvelde@hetnet.nl> wrote:
  I've experimented with a laser diode as point source, but found it almost
impossible to use, at least for KE and Ronchi measurements. The image is
swamped with diffraction effects due to the small size of the diode
aperture.

In the 5mW - 650nm device I used, the beam divergence is given as 32 and 8
degrees. The intensity profile is approximately gaussian, and therefore the
far field beam divergence is given by:
theta = lambda / (pi * waist)
where waist is the aperture diameter, theta in radians.
Hence in my case the aperture measures 0.37 x 1.48 micron.

When the diode is lasing, it usually has a set of distinct lines around the
fundamental wavelength. If it is non-lasing, the spectrum is less
determined, but still quite narrow. This, in combination with the close
approximation of a point source, caused the annoying diffraction effects.

What you would prefer for a star test on a parabolic mirror is a flat
wavefront, and hence you would ideally need a beam spreader of aproximately
the mirror size. If you also want a circular intensity distribution, you
would first need to project the beam on a circular pinhole of sufficiently
small size, again determined by the numerical aperture of the projected
beam.

Arjan

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