**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. The former can
cause figure distortions, while the latter produces performance
losses at the boundary layer; an incredibly sensitive thermal
gradient layer at the optical surface, degrading
the performance of the system by surprisingly large amounts.
layer issues are a problem for all mirror materials. The use
of zero-expansion mirror materials (for solid mirrors) does nothing
to eliminate the mirror's thermal mass. It can greatly
reduce mirror (itself) figure distortion from internal temperature
gradients but this often comes at the expense of high costs and
greater mechanical figure distortion because the mirror mount's
most often used are 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.
**Within non-zero-expansion glass
types; Pyrex, Borofloat, E-6, etc., internal temperature gradients
distort the figure of the optic. 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 this distortion is even larger because the CTE
is nearly 3x higher than borosilicate glass types.
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 greater the loss to the air volume moved.
**The combination of Dream's carbon fiber, FAST
engineered lightweight mirrors produce a 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) 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
lightweight mirrors so it maintains performance of the mirrors
over extremely large temperature changes, while simultaneously
producing an athermal telescope.
does Dream put so much effort into equalization?