To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 12:05 2003-03-26 -0800, Bob May wrote:

>I'll also note that I've gotten another test for the COC from William Maddux
>a few weeks ago and that test (called DAFT - he named it, not me!) seems to
>be quite interesting in both its concept and the implementation of the test.
>It uses diffraction spikes generated at the mirror end of the test to align
>the two returning beams from the left and right mask openings.  The thing
>that seems nice about the test is that you look at postions of the
>diffraction lines for whether you are in the right position rather than the
>brightness of the two areas.

Here's an ASCII drawing  of a dingbat I made for testing a
10"f2.6 RC cass primary. I called the test FPF (Focal Point Foucault),
and it sounds like what you guys are discussing.

      mirror    |_______________________|
      mask       -- - ------------- - --
                     \             /
                       \         /
                         \     /
                           \ /
                          +---+          ___________
      beamsplitter        |\  |.________| 5 mW laser|
                          |  \|         |___________|
                          +---+

                           +-+
      microscope           | |
                           | |
                           +-+


The beamsplitter & microscope are on an x-y stage I made originally for
the caustic test.  I tried a laser pointer on the stage, but it wasn't
bright enough, so I just push the laser until the beam hits the fiber,
which is perpendicular and spreads the beam out into a line across the
mask.  I measure the distance from mirror vertex to the point where the
returns from the two hole pairs merge (once with a tape measure, the rest
by increments measured by micrometer head).  I don't think the (classic
Gavriola) caustic test is accurate enough for RC casses, so I made this
rig.

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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