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On the previous page we
discussed the many benefits of Dream's FAST. We will go over
a few of the reasons why Dream has developed such a system to
deal with the thermal issues inside the Optical Support Structure
(OSS). |
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Knocking down (but not eliminating) the
boundary layer, that thermal gradient layer just above the optical
surface, is proven to reduce mirror seeing. Unfortunately this
merely acts as a Band-Aid, covering up the real problem. It is
the internal heat within the optic that is causing the issues
at the boundary layer. But internal temperature gradients within
the optic itself are also causing a separate problem in non-zero-expansion
glass types; Pyrex, Borofloat 33, etc. Internal temperature gradients
distort the figure of the optic. For a 16" optic that is
2" thick, a 1C internal gradient will distort the figure
of that optic by roughly 1/3rd wave (~183nm of distortion). This
is why knocking down the boundary layer is not enough. The figure
distortion will persist until the optic is approximately +0.1C
or less from ambient temperatures. |
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Even a thin monolithic mirror, which is substantially more difficult
to support without causing cell-induced astigmatism (yet another
form of figure-robbing), takes a substantial
amount of time to "equalize" when compared to Dream
Cellular's engineered lightweight mirrors. Dream Cellular's mirrors
have an independently tested CTE that is 23% lower than Pyrex,
Borofloat 33, etc. |
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