D r e a m

Mechanical Benefits Of Dream's Engineered Lightweight Mirrors


 The previous page discussed the thermal benefits of Dream's engineered lightweight mirrors. There are numerous mechanical advantages of Dream's lightweight mirrors.

Solid Versus Ribbed Mirrors - mechanical
* Which one bends its mount/support less?
* Which one maintains tighter alignment tolerances?
* Which one lowers the mass of the whole instrument?
* Which one provides greater repeatability (mapping) for the telescope mount?
 * Which one will allow faster slews of the instrument?
 * Which one provides greater total performance?

Stiffness is always a driving factor in opto-mechanical systems. Remember, we are trying to achieve AND maintain the surface of the mirror, as it moves during real-world use, to a fraction of a wavelength of light.

If the mirrors can't stay optically aligned to each other at the required tolerances, then it doesn't matter how "perfect" their individual surfaces were on a static, horizontal test bench. It's not difficult to appreciate that a solid mirror that is 3-6x heavier than a Dream mirror is going to bend structures far more and be that much more difficult to achieve already tight optical alignment tolerances.
If alignment tools don't show errors in the system as the instrument/telescope moves, then more senitive equipment is needed. If alignment requires x tolerance, alignment tool(s) need to show a minimum of 1/2x errors.

As the mass of the optic goes up, so too does the self-weight deflection of the mirror itself. A 12" (6:1 aspect ratio) solid mirror is 4x lower in stiffness than a 6" solid mirror (6:1 aspect ratio). A 24" solid mirror is 16x lower in stiffness than the 6" solid mirror. It is therefore highly beneficial to have a low mass optic. The result is a finished mirror that has performance gains across the board.

Dream's engineered lightweight mirrors set up a domino affect for the rest of the opto-mechanical system. The mirror's supporting structure does not need to be as heavy, which in turn means the backplate does not need to be as heavy, reducing the weight of the main instrument structure as well. This reduces the mass carried by the mount and can lead to greater performance and/or the use of less expensive mountings; off-the-shelf products for example. The performance gains can come in the form of greater pointing, tracking and slewing capabilities.

Left are images of a 9.8 pound 16.55" (physical OD) f3 (-1 conic) Dream mirror during vertical interferometry. The left used a 5" diameter support behind the mirror, while the right used a 13" support. The difference in interferometry data sets, color graphics to the right, is imperical information of the stiffness of the Dream mirror. There was only a 2nm RMS (L/316) difference between the two support types.

The far left color graphic shows the state of the surface prior to the final two finishing runs, with the final data set on the far right. Below is the simplified data.
Before   After  

Goal

PV surface 0.419 PV surface 0.052

0.125

RMS 0.106 RMS 0.008

0.036

RMS Surface Roughness was 9Å. Radius spec: 2413mm, +/-2.4mm (+/-0.1%). Final radius achieved: 2413.218mm (off nominal by only 0.009%).


Dream goes through an iterative design process using Finite Element Method (FEM) and Finite Element Analysis (FEA) to show what the mirror will do. Initial designs are then modified until design goals are met.
There are two main cases that we are analyzing: polishing and gravity displacements. The former is an evaluation of how the mirror will perform during grinding & polishing. The latter is an evaluation of how the finished mirror will perform in the completed opto-mechanical system.
Dream does not scale a design (simply enlarging or shrinking the same basic rib design to make a larger or smaller version). We have found through experience since 2003 that such a simplified approach does not work. The performance of the mirror is complex and can be non-intuitive. There are no free lunches in optics.


Dream understands each of these important factors and benefits because Dream has been engineering and using these lightweight mirrors firsthand since 2003. This wealth of knowledge and experience is essential in creating optimized, precision opto-mechanical systems.

Dream not only creates the lightweight mirror blanks but processes them, designs & fabricates dedicated carbon fiber mirror mounts, as well as using the engineered mirrors in Dream's carbon fiber full instruments.

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