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 zeroDELTAlightweight mirrors.

Dream's zeroDELTAcompared to solid mirrors
* Which one bends its mount/support less?
* Which one maintains tighter alignment tolerances required on modern optical systems?
* 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. Dream's athermal telescopes do this, not just at one temperature, but over a very wide range of temperatures.

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. Intelligent buyers appreciate that a solid mirror that is 3-6x heavier than a Dream zeroDELTAengineered lightweight 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 traditional system as the instrument/telescope moves, then a more sensitive tool is needed. If alignment requires x tolerance, alignment tool(s) need to show a minimum of 1/2x scale 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 that uses the same 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 mechanical and thermal performance gains across the board.

Dream's zeroDELTAlightweight 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, especially if Dream's CF & CFSC was used, 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. Secondary mirrors can show just as much mechanical performance improvement as primary mirrors. A 3x magnifcation secondary mirror on a Cassegrain can create substantial performance loss when it weighs 3-6x more than a Dream zeroDELTAlightweight mirror. The gains can come in the form of greater pointing, tracking and slewing capabilities, as well better spot performance and far greater stability over varying telescope angles. This produces the lowest possible maintenance.

Q u a l i t y

S t a r t s

A t

T h e

S u b s t r a t e

Left are images of a 9.8 pound 16.55" (physical OD) f3 (-1 conic) Dream mirror during extreme resolution 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 illustrates the stiffness of the Dream zeroDELTAmirror. There was only a 2nm RMS (L/316) difference between the two supports.

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 right. Below is the quantified data.
Before   After  

Goal

PV surface 0.419 PV surface 0.052

0.125

RMS surface 0.106 RMS surface 0.008

0.036

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


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 Dream is 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, without doing new FEM/FEA). Real mechanical engineeering using modern analysis tools shows that such an overly simplified approach does not work. The performance of the mirror is complex and can be quite non-intuitive at times. Dream knows this from nearly 15 years of designing lightweight mirrors. There are no free lunches in optics.


Dream understands each of these important factors & benefits, because Dream has been engineering and using these lightweight mirrors firsthand since 2003. This wealth of knowledge and empirical 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 athermal carbon fiber instruments.

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