Open-Backed Cellular Optics

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.

If you want to learn more about large scale cellular mirrors and see examples, please visit Steward Mirror Labs web site or LBT, Megellan I & II, ARC 3.5m, WIYN 3.5m, etc..





pricing, availability and specifications subject to change without notice