August 1, 2011: Different
tests for different defects - a 17.5" mirror
The results here are a bit different than for the previous 10" mirror.
Tests include 1) Foucault test images, 2) interferograms from a
spherical-wave interferometer, and 3)
artificial star test images.
This
mirror is a rather thin 17.5" with good, flat back. Its
figure is
close to a sphere (well at least closer to a sphere than a parabola).
Foucault
tests above show a reasonably smooth figure and decent edge, although
not
perfect in either regard. Some irregularity is visible in the
non-symmetric nature of the shadows, but this could be due to the thin
mirror itself. More tests are needed to see what's going on.
We could rotate the primary and see if the shadows change,
and
that would help rule out test stand sag.
This
is a very good time to pull out the interferometer and artifial star
test to get a better ideal of what's going on, so let's do that, shall
we?
Yikes!!!
The interferograms tell the tale. Note the triangular fringes
in
the image above at left,
cleary showing trefoil is present. It is
likely that this mirror was pinched or clamped during previous attempts
at polishing or figuring. Note the non-parallel fringes in
the
image on the right also betray this defect, but in a different manner.
Analysis of the image on the right yields the contour plot
below,
which clearly shows three high regions.
Finally,
an artificial star test shows an obviously triangular shape that is
impossible to miss. This
is a proprietary test that I use with a laser-illuminated pinhole
source, and which is very valuable for testing figure of revolution
after polish and after figuring.
This
test is also superb for testing the edge support of mirror cells while
the primary mirror is nearly spherical (just after finishing polish).
If offer a mirror cell edge support testing service for
mirror
that I am making, and often work with JPAstrocraft
on this. They make the cells, and we test and adjust the edge
support for best performance after careful testing in my shop.
We
have seen triangular images like this in large telescopes and have
fixed the problem.
John Pratte (JPAstrocraft) and I will be collaborating on and testing
40", 42", and 45" mirror cells in the future.
Clearly this 17.5" will need a lot of polish time to get it back to a
good figure of revolution. Then it can be figured.
Please
check back for future installements of "In
the Shop".
Mike
Lockwood
Lockwood Custom Optics
