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**The purpose of a forced air system
is to reduce the difference between the temperature of the mirror(s)
and the ambient air temperature. There are two distinct cases.
One is internal temperature gradients within the mirror. The
second is the bulk temperature of the mirror. Internal temperature
gradients can cause figure distortion in mirror materials that
are not zero-expansion, while the bulk (average) mirror temperature
variance compared to ambient temperature will produce performance
loss at the boundary layer; an incredibly sensitive area at the
optical surface, degrading
the performance of the system by surprisingly large amounts. |
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**Boundary
layer issues are a problem for all mirror materials. The use
of zero-expansion mirror materials (for solid or thick-featured
"lightweight" mirrors) does nothing to eliminate the
mirror's thermal mass. Zero-expansion mirror materials can
greatly reduce figure distortion from internal temperature gradients,
but this often comes at the expense of high costs and greater
mechanical figure distortion because the mirror mounts are often
made of inexpensive, high CTE aluminum. As temperature changes,
the zero-expansion mirror itself changes by parts per billion
but is being distorted mechanically because the aluminum mirror
mount is changing shape at greater than 400 times the rate of
the mirror, due to its high CTE. |
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**Within non-zero-expansion glass
types; Pyrex, Borofloat, E-6, etc., internal temperature gradients
distort the figure of the mirror. For a 16" solid mirror
that is 2" thick, a 1°C internal gradient will distort
the figure of the mirror by roughly 1/3rd wave (~183nm of distortion).
For plate glass mirrors this distortion is roughly 4x greater,
mostly driven by the fact that plate glass has a CTE that is
nearly 3x higher than borosilicate glass types; Pyrex, Borofloat
& E-6. |
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**The benefits
of Dream's unique Filtered Air System Technology (FAST™)
are numerous. It allows a much greater volume of air than other
designs. This is due to the enormous surface area of the high-performance
K&N filters used and because of the specific high-volume,
low-vibration fans used. The large surface area of the filter
media causes far less static pressure than placing small blocks
of filter media directly on a fan. The higher the static pressure,
the lower the volume air moved. |
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**The combination of Dream's carbon fiber, FAST™
and zeroDELTA™ engineered
lightweight mirrors produce an optical system that deals
entirely with the historic problems of both figure distortion
and the boundary layer. The entire instrument reaches and then follows ambient temperature
changes (delta) far faster than any other
system in the world, because Dream has dealt directly with the
known, age-old problems within opto-mechanical systems. Dream's
carbon fiber was designed to closely match the CTE of zeroDELTA™ lightweight mirrors so it maintains
performance of the mirrors over extremely large temperature changes,
while simultaneously producing an athermal telescope. |
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Why
so much effort into equalization? |
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Dream's use of FAST™ provides
numerous additional benefits that no other systems offer. We
are filling the optical instrument chamber with filtered, clean
air. This keepa the optics cleaner for a much longer length of
time. For remote imagers/science, we advise our clients to leave
the fans on 24/7/365. These fans are very light and efficient,
using less than 10 watts altogether. Leaving them on has additional
thermal benefits but it also eliminates the need for automated
front aperture covers for the instruments. |
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In high humidity environments
moving air does not allow dew formation. Moisture is the leading
cause of dust and other air borne materials adhering to surfaces.
Dream's FAST™ keeps the optics prestinely clean, the thermal
benefits are enormous and insects, especially spiders, do not
enter the tube. |
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Dream is the only company in the world utilizing FAST™. Dream is also the only company in the
world offering a standard optical instrument that utilize engineered
lightweight mirros for both the primary and secondary mirrors.
This combination produces unrivaled speed in equalization of
the optics & world-leading installed performance. |
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