The simplest way to appreciate the cellular technology
that Dream uses is to ask a question. We can use the 24"
primary as an example. What cools faster and then follows temperatures
more readily, a 130 lb mass of glass or 35 lbs of glass?
Keep in mind that the physical design of cellular
gives it numerous advantages over other designs. One is the ~25%
mass compared to monolithic. A second advantage is the enormous
surface area of the inside back of the cellular mirror. The third
advantage is that with open cellular, like Dream uses, the back
of the main polished face of the mirror can equalize just as
readily as the ribs. This is due to the open-backed design. Monolithic
or other blank designs that use a back (solid, closed off sheet
of glass) do not allow air to flow into the heart of the mirror.
They also isolate the face sheet from the back sheet. Because
of this they can easily see differences in temperature from the
front (face) to the back of the optic. Open-backed cellular does
not suffer this effect.
Another advantage to cellular is the greatly reduced
mass that needs to be rigidly supported. It is therefore easier
to maintain a highly rigid optical system. Dream's extensive
use of carbon fiber enhances the rigidity and low mass even further,
while having a low CTE (Coefficient of Thermal Expansion) that
very closely matches the CTE of the mirrors. Dream's structures
also have a lower thermal mass.
Until an optic reaches equalibrium, the focal plane
quality is dominated by mirror seeing and is thus mirror limited.
The general rule of thumb is that for every 1C of delta (ambient
temperature versus mirror temperature) there is a 0.3-0.5 arc
sec degradation of the image quality.
Temperature sensors that are +/-1C are therefore not
accurate enough for detailed analysis of a particular optic.
Placement of sensors is also a key variable. The use of open-backed
cellular allows us to rely less on such variable-prone systems.
The bottom line is both a higher throughput and higher
optical quality to the focal plane. Because cellular reaches
and then follows ambient temperatures so well, there is far less
time lost to mirror seeing and less time lost to slowed slews
due to heavy OTA (Optical Tube Assembly) masses that are at least
double what Dream's products attain. Stiffness is also a key
factor. There is much less self-weight-deflection in our products.
Click on the image below for a demonstration of what our advanced
composite parts can do. They are produced in-house and cooked
in one of Dream's two ovens.