Thermal Benefits Of Dream's zeroDELTA Engineered Lightweight Mirrors


The first optical mirrors were solid metal, beginning around 1668 (349 years ago). The first glass mirrors were solid, beginning around 1851 (166 years ago). Early on optical tests and control of the optical surfaces did not allow for precision. Issues like thermal mass and the boundary layer were unknowns because the quality of the surface was by far the largest error in the system, making it impossible to see other errors and the potential benefits of alternatives. Over time this slowly changed. Almost 100 years ago G.W. Ritchey, the renowned optician, designer and inventor, recognized that solid mirrors came with a lot of performance-robbing baggage. This led him to produce his own lightweight mirrors.

No matter the substrate material, be it glass or glass-ceramic (zero-expansion), maintaining as small a delta between the optic and the ambient temperature is one of the keys to performance, as is maintaining optical alignment and optimal focus. Numerous independent studies have shown that mirror seeing degrades performance unless the mirrors are within +0.1°C to -0.2°C of ambient temperature. Visual cues of thermal issues disappear long before degradation stops. If extremely critical alignment, focus and maintaining the mirrors within this small tolerance of the ambient temperature are not met, then claims of quality are just that, claims. Dream's zeroDELTAlightweight mirrors provide the foundation for the utmost in performance.

Dream's zeroDELTA lightweight mirrors use the latest 3D CAD, FEM/FEA, CNC equipment and micro-processor controlled furnaces & annealing ovens to provide cutting edge mirrors that eliminate the performance losses associated with solid mirrors.

Thermal Time Constant definition: Think about a 500 gallon container filled with water. Drill a 1/4" hole in the bottom of the container and let the water drain out. The 500 gallons of water represents the thermal mass of a solid mirror. The 1/4" hole represents the efficiency at which the mirror will equalize to a static temperature. The Thermal Time Constant is the time it takes for the water to drain out; for the mirror to reach equilibrium.
Dream's zeroDELTA lightweight mirrors typically start off with only 100 gallons of water and the hole in the container is 5" in diameter. Lower mass, thinner profiles and greater surface area are all contributing to the Dream zeroDELTAmirror being able to not just reach a static temperature, but to also be highly nimble; able to rapidly react to dynamic temperature changes, creating the smallest performance loss possible.


A 1°C delta between an optic and the ambient temperature can produce a minimum of 0.3-0.5 arc-second of degradation.

There are many features of Dream's zeroDELTA lightweight mirrors that make their thermal time constant substantially lower than solid mirrors.

Solid mirrors have many negative side effects that are hurting the performance of the system. The less thought put into thermals related to the mirrors, the telescope, etc., the greater the chances that the solid mirror(s) are not reaching equilibrium at all. It may not be the site seeing that is limiting performance but the key component in the system; the mirror(s).

 

* lower mass
* thinner profiles
* greater surface area
 
 
 

When answering the questions below for the aluminum block and finned aluminum piece shown to the left, the answers are common sense. The answers are the same for glass and glass-ceramic mirrors as well.
* Which one has more surface area?
* Which one will equalize to ambient temperatures faster?
* Which one will stay closer to ambient temperatures, especially throughout it's bulk (internal)? 
* Which one provides greater total performance?
Glass and glass-ceramic mirrors have undesirable performance in their thermal properties; heat capacity & thermal conductivity. They hold onto their existing bulk temperature and they do not conduct away that temperature readily. Dream's zeroDELTAengineered lightweight mirrors resolve both of these problems by starting off with 3-6 times less material, 4-6 times more surface area, while using profiles that are 10-25x thinner than solid mirrors. This deals with the numerous thermal problems directly and is why G.W. Ritchey was experimenting with lightweight mirrors in the 1920's, roughly 100 years ago.

Internal temperature gradients within all mirrors will produce mirror-seeing; performance loss at the boundary layer. Within glass-based solid or thick-ribbed/thick-faced "lightweight" mirrors there is an added problem; figure distortion. This can be seen during optical testing as a constantly changing figure. Only when the mirror is much more uniform in overall temperature, within a fraction of a degree of ambient temperature, will both the figure and boundary layer thermals stabilize. Toward the end of figuring it is common practice for traditional opticians to leave a solid glass mirror on the test stand overnight to let it equalize to room temperature, then test the next morning. What happens the next day when more work is done to the surface and it needs tested again? Another day has to pass...

Figure distortion and mirror seeing don't require large temperature differences and/or larger diameter mirrors. It's wishful thinking to believe that solid mirrors, used in environements where the temperature changes, are performing at an optimal level, all of the time.

The thermal properties of the material, the geometry of the solid mirror substrate and often the incapsulated nature of the mirror mount and back portion of the instrument are all lengthening the time it takes for the solid mirror to equalize. The use of a zero-expansion material does not change this.

Read this white paper regarding thermals.

 

Thermals From A Solid Mirror
How does the optician know what the real figure is if it is constantly changing? How can the work be done so rapidly if the solid glass mirror was not allowed to fully equalize? You can have one or the other but not both. 1+1=2. It can't equal L/20... Figure distortion in solid glass mirrors is the main reason glass-ceramics were created. But they only address one of the two thermal issues; figure distortion. They do not address thermals at the boundary layer; the most sensitive location in the system for thermals to occur.

The larger the mirror, the larger the thermal problems, which is why it was stated that some mirrors won't equalize. Taking three days to equalize after a cold front moves in is a lot of performance, time and discoveries lost forever.

Dream's mirrors do not suffer the above thermal problems. The 396mm CA f1.376 mirror that Dream finished for an Adaptive Optics project equalized and showed steady test images within 30 seconds of being placed on the kinematic mirror mount for vertical testing of the mirror during finishing. The mirror could come off the polishing machine after 1-2 hours of work, be rinsed, dried, placed on the mirror mount for testing and show steady views/data, all within three minutes of coming off the machine.

This page discusses thin, solid mirrors.


Move on to the Mechanical benefits page

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