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The smaller secondary mirrors are also made of low-expansion
borosilicate glass. Like the primaries, the secondaries are edged
and centered, then ground and polished. The secondary is a
convex mirror so during manufacture it is tested against a
concave precision reference matchplate to check both its radius
of curvature and figure. The secondary mirrors are also brought
to the QA Interferometry Lab where the radius and irregularity of
each mirror is verified through interferometric measurement to
assure that each one lies within specification.
When we designed the EdgeHD optical system, we strongly
favored spherical surfaces because a sphere can be tested by
optical interference to high accuracy in a matter of minutes. If we
had specified a hyperboloidal surface for the secondary mirror,
we would have been forced to use slower, less accurate testing
methods that might miss zonal errors. Furthermore, coma-free
SCT designs with hyperboloidal mirrors still suffer from field
curvature—an aberration that we specifically wished to avoid in
the EdgeHD design.
Finally, the sub-aperture corrector lenses are made using the
same manufacturing techniques used for high-performance
refractor objectives. The EdgeHD design specifies optical glass
from Schott AG. The 8-, 9.25-, and 11-inch use N-SK2 and K10
glasses, while the 14-inch uses N-SK2 and N-BALF2 glasses.
To ensure homogeneity, optical glass is made in relatively small
batches, extruded in boules. The raw glass is then diamondmilled
to the correct diameter, thickness, and radius. Each lens
blank is blocked, ground, and polished, then the radius and figure
are compared to matchplates to ensure they meet EdgeHD’s
tight tolerances.
Our assembly workstations resemble the optical benches used
to qualify corrector plates. The primary mirror and corrector plate
slip into kinematic support jigs, and we place the secondary
mirror in its holder. The sub-aperture corrector lenses meet
specifications so reliably that a master set is used in the assembly
workstation. Laser light from the focus position passes in
reverse through the optics, reflects from a master autocollimation
flat, then passes back through the optics. Tested in autocollimation,
the optician can see and correct surface errors considerably
smaller than a millionth of an inch.
If the combined optics set shows any slight residual under-or
over-correction, zones, astigmatism, upturned or downturned
edges, holes, or bulges, the optician marks the Foucault test
shadow transitions on the secondary mirror, then removes the
secondary mirror from the test fixture and translates these
markings into a paper pattern. The pattern is pressed against
a pitch polishing tool, and the optician applies corrective polishing
to the secondary mirror—as we show in Figure 11—until the
optical system as a whole displays a perfectly uniform illumination
(no unwanted zones or shadows) under the double-pass
Foucault test and smooth and straight fringes under the doublepass
Ronchi test. The in-focus Airy disk pattern is evaluated for
roundness, a single uniform diffraction ring, and freedom from
scattered light. In addition, the intra- and extra-focal diffraction
pattern must display the same structure and central obscuration
on both sides of focus, and it must appear round and uniform.
After we remove each set of optics from the autocollimator, we
send the components to our in-house coating chamber. Here,
the primary and secondary mirrors receive their high-reflectance
aluminum coatings, and the corrector lens is anti-reflectance
coated. Each set of optics is then installed into an optical tube
assembly (OTA).
Completed OTAs undergo the Visual Acceptance Test. In a
temperature-stabilized optical test tunnel, green laser light at
532nm wavelength is reflected from a precision paraboloidal
mirror to act as an artificial star. With a high-power ocular, a QA
Inspector views the artificial star critically.
To pass the Visual Acceptance Test, an optical tube assembly
must meetthe following requirements:
• The in-focus Airy disk must be round, free of scattered light
around the disk, and display only one bright ring.
• Inside and outside focus, the diffraction patterns must be
round, uniform, and appear similar on both sides of focus.
• Observed with a 150 line-pairs-per-inch Ronchi grating, the
bands must be straight, uniformly spaced, and high in contrast.
Because its optics have been tested and tuned in error-revealing
double-pass mode, and because each assembled OTA has been
tested again and qualified visually, the telescope’s images should
be flawless when observing and imaging the night sky.
