Engineered Lightweight Mirror Blanks

Understand The Benefits Of Dream Cellular Mirrors

D r e a m C e l l u l a r , L L C

"I can say that Dream's carefully designed blanks can be polished and figured to the same precision as monolithic blanks. In my mind, the "dreaded" print-through is a non-issue for a skilled optician." - Mike Lockwood: Lockwood Custom Optics

Print through is often sited as a major issue of "all" cellular/lightweight telescope mirrors. For a conventionally designed and produced lightweight telescope optic, this is generally true. Such simple designs can be analyzed to show that the easiest way a large, repeating and fixed rib pattern (say hexagons) can reduce print through from polishing is to thicken the face. However the added mass of the thicker face displaces (sags) more during the gravity load case it sees in the final instrument. So although it can reduce print through, the mechanical performance of the telescope mirror, where it will spend 99% of its life, is made worse. A thicker face will also slow the mirrors Thermal Time Constant (TTC). In the final instrument such a basic, "lightweight" telescope mirror design will weigh more, will not be able to hold the figure as well and it will take longer to cool down.

Opticians rarely test optics vertically, or in their actual mirror supports. Horizontally testing an optic puts the face (mostly) in shear, which means face itself will displace less than the zenith-angle gravity case. When this is combined with the lower polishing displacements of a thicker face, it gives the false appearance that the optic is far better than it is at other angles. If your instrument is only used horizontally, then higher mass and a slower TTC are the two main disadvantages. When the face is thick enough though internal temperature gradients in non-zero-expansion glass types will cause figure distortion of their own.

Traditional lightweight telescope mirror designs, for ease of fabrication (especially for fused lightweight optics), use the same repeating shape across the entire mirror. Repeating designs like this are highly inefficient and the optical support is usually an afterthought. This means those traditional lightweight optics are difficult and time-consuming to polish and their performance in the final telescope is greatly decreased. It's performance is decreased both because the face had to be made thicker (slower TTC and greater gravity displacements) and because it is far more difficult to properly support.

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Dream Cellular (DC) has tackled these issues head on. Instead of repeating the same, simple pattern that use fairly large, unsupported face details across the entire telescope mirror, DC designs from the mirror supports "up." This includes proper lataral support of the optic at the CG of the telescope mirror. This support up engineering method in and of itself is a huge design improvement over other lightweight telescope optics. The second main difference is that DC has not been afraid to make the designs far more complex in order to greatly increase performance. DC can reduce both print through during polishing, as well as greatly reduce gravity displacements the telescope mirror sees in final use. In basic terms the stiffness of DC's telescope mirrors is substantially higher for both polishing and gravity displacements. Although this added complexity adds to the front side of producing each mirror blank, when compared to the entire cycle, it represents less than a 5% increase in time. DC does not feel reducing this time out is worth the far greater reduction in performance one sees with vastly simpler designs.

An example of the added complexity in a DC lightweight telescope mirror is illustrated in our 16" design. It has five different rib heights, five different rib thicknesses and a tapered rib feature as well. Our engineering of the blanks does not occur in minutes or even hours but in much, much more detailed analysis and through many design/Finite Element Analysis (FEA) iterations. Running the proper amount of anaylsis takes time, even with the computers that DC utilizes. Dream has been designing lightweight telescope optics for over five years. But even with this specialized knowledge the designs still go through many iterations before the final design is reached.

Below is the polishing performance for Dream Celluar's 16" design. Polishing displacements are based on 0.25psi of polishing pressure. For this 16" optic that would be a 50.25 pound full diameter tool. If the polishing pressure is lower than 0.25psi, the displacements would be even lower than those listed below.

Polishing Displacement – 5.06nm (1/109th wave, at 550nm) RMS and 7.08nm (1/78th wave) P-V.

All of our telescope mirrors are designed for 0.25psi of polishing pressure. This was intentionally chosen so opticians could work our engineered telescope mirrors with conventional polishing machines and techniques. This optician uses conventional equipment and has had no difficulties or additional costs in working DC's engineered telescope mirrors.

All of our standard mirror designs have a plano back (flat). This too was a deliberate design choice. It was based on feedback from opticians, customers and years of empirical experience from supporting optics at Dream Telescopes. Deep conical-shaped telescope mirrors are notoriously difficult to support during polishing, without imparting astigmatism into the mirror. DC designs do NOT require specialized, expensive equipment to finish.

Look at DC's designs to see the mechanical differences compared to all other lightweight telescope mirrors. When the 3D design of the optic is optimized around an initial support architecture, then modified in thickness, height and sub rib locations using extensive FEA iterations (and cases), it gives DC's lightweight telescope mirror blanks incredible performance that is unrivaled in the 90 year history of cellular mirrors.

Contact: shane @ dreamcellularllc.com