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TESTING METHODS FOR CONCAVE MIRRORS
Ronchi Ruling: we use this testing method during the initial glossing phase.
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An interferogram
obtained from a 410 mm (16") diameter and 1400 mm (55") focal
length spherical mirror, by Ronchi ruling testing.
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Ronchi ruling
testing image from a 520 mm (20½") diameter and 1000 mm (39¼") focal length spherical mirror.
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Ronchi ruling
extrafocal interferograms from a 610 mm (24") Ø and 2200 mm (86") focal length spherical mirror:
at starting phase of parabolization (on the left), and approaching to ideal parabolic surface (at right).
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Foucault knife edge Test and Couder mask: : it's a quantitative
method we used since 1995 to 2006. It gives a very precise value of
front wave correction on aspherical mirrors surfaces, but it needs an
accurate application, since it could lead to a subjective interpretation of measurements.
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Foucault test:
the knife edge is placed at the centre of curvature, of a 410 mm (16")
diameter and 1800 mm (71") focal length near-spherical mirror, before parabolization.
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Foucault
knife edge test with Couder mask on a 610 mm (24") diameter
and 2200 mm (86") focal length (f/3.6) parabolic mirror.
The high parabolic curvature of an f/3.6 mirror needs a
great number of slits on Couder mask, to allow measurements.
In this image the focus zone corresponds to the slits number 7,
because they appear with the same brightness. These values,
together with mask parameters and mirror data are computed by
a proper algorithm to obtain the optical quality measurement.
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Test Interferometrico:
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TESTING METHODS FOR FLAT OPTICAL SURFACES
Spherical mirror aided test:
this method is just on duty, cause its easily and quick use
The Reference surface is a 310mm diameter spherical mirror with a front wave accuracy of lambda/20.
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This Ronchi
ruling image is obtained placing a 250 mm diameter flat mirror
at a 45° angle from the Reference spherical one; it clearly
shows the surface development but does not give information about its flatness.
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. The
same displacement of the previous image is used with
an artificial star source. Comparing the intra and extra
focal images is useful for decide about the surface flatness;
both two images are similar and show the same elliptical figure
that forms placing the plane mirror at a 45° angle from the Reference spherical one
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Interferometric test: is done by a Fizeau interferometer for flat surfaces.
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Our Fizeau
interferometer for visual or photographic testing of up to 250 mm diameter plates. |
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This
is the interferogram of a Newton flat secondary mirror which minor
axis size is 165 mm, using a 532nm monochromatic laser source. |
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