Simple
designs, with little to no (real) mechanical engineering behind
them, using simplistic repeating rib patterns, with fixed height
& thickness ribs, often reduce design-related print through
by using a thicker face. The main reason this and other simplistic,
repeating patterns are so common is due to the fact that they
are so quick, easy and cheap to produce, as well as making a
traditional optician feel more at ease. Quick, easy and cheap
are three adjectives that are never associated with a high performance
product.
The face of a lightweight mirror
is one of the largest drivers for the mass of the mirror. It
is also one of the largest drivers for how quickly the mirror
equalizes to ambient temperature. For a typical astronomical
application, which is some upward-pointing angle for the primary
mirror, the added mass of the thicker face displaces (sags) more
during the gravity load case (real-world use of the mirror),
compared to a thinner-faced mirror. So although a thicker face
can reduce print through during polishing (all things being equal),
the mechanical performance of the optical surface in final use,
where it will spend 99.99% of its life, is made worse.
Traditional
shops rarely test mirrors vertically, or in the actual mirror
mount. Horizontally testing an optic puts the face (mostly) in
shear, which means the face itself will displace less than the
zenith-angle (vertical) case. Self-weight deflection that can
be seen during vertical testing disappears during horizontal
testing of a thick-faced "lightweight" mirror. When
horizontal testing is combined with the lower polishing displacements
of a thicker face, it gives the false impression that the optic
is better off with a thicker face. Dream is an engineering firm
that has never been bound by this type of narrow viewpoint. Dream
pursues the performance of the mirror in final use, not the performance
of the mirror on a test stand, in a temperature controlled room.
In the final instrument such
a basic design using a thick face will weigh more, force other
components to be heavier, will not be able to hold the figure
as well (glass, especially when plate glass is used) and it will
take longer to equalize. These are the same negative attributes
of solid glass mirror technology, which has changed little over
its more than 165 year history.
Dream's
internal polishing has proven
that print through, real and otherwise, can be caused by a dozen
or more factors related to the polishing machine and tool. Better
process control makes for a smoother surface on both lightweight
and solid mirrors. Our thinnest face mirror to date (3mm thick
for a 424mm physical OD mirror) has proven that Dream can provide
our clients with the best of both words; a thinner face and a
lack of print through. This provides a final mirror with unusually
high performance. As one outside vendor stated, the figures of
Dream's lightweight mirrors are just as stable as glass-ceramics
and they can finished to the same high
quality polish as well; L/20+ PV surface, L/125 RMS surface
and 2Å RMS surface roughness. This level of quality was
quantified using the second most powerful interferometer on the
market, while vertically testing the mirror. |