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Thermal Benefits Of Dream's Engineered Lightweight Mirrors

Solid glass mirror technology is 166 years old & solid metal mirror technology, the first optical mirrors ever used, is 349 years old. Close to 100 years ago G.W. Ritchey recognized that solid mirrors came with a lot of performance-robbing baggage. Dream's lightweight mirrors, through intelligent design and engineering (FEM/FEA) provides Dream's clients with a truly modern choice that addresses a centuries old problem. Read this white paper regarding mirror seeing.

Thermal Time Constant: Think about a 500 gallon tank 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 traditional 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 lightweight mirror starts 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 mirror being able to not just reach a static temperature far faster, but to also be highly nimble; able to rapidly react to dynamic temperature changes.

A 1°C delta between an optic and the ambient temperature can produce 0.3-0.5 arc-second of degradation. This is only one form of degradation. There are many.

There are many features of Dream's engineered, lightweight mirrors that make their thermal time constant (how fast they reach ambient temperature) substantially lower than solid mirrors. In Dream's telescopes this can eliminate mirror seeing below detection, as well as the negative side effects of mirrors that cannot equalize fast enough, or at all, in a given night.  

* lower mass
* thinner profiles
* greater surface area

Solid Versus Ribbed Mirrors - thermal
* 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? 
* Which one provides greater total performance?
Glass and glass-ceramic mirrors have undesirable performance in their thermal properties of heat capacity & thermal conductivity. They hold onto their existing bulk temperature and they do not conduct away temperature readily. Dream's engineered 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 it 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 a 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 outdoor environements, 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.

Read this white paper regarding thermals.

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.
No matter the substrate material, be it glass or glass-ceramic, maintaining as small a delta between the optic and the ambient temperature is one of the keys to performance, as is maintaining optical alignment. If these are not at the core of the instrument, then claims of quality are just that, claims. Dream's engineered lightweight mirrors can help your project meet or exceed it's goals.

This page discusses thin, solid mirrors.

Move on to the Mechanical benefits page

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