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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. |
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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. |
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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. |
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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. |
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* lower
mass |
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* thinner
profiles |
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* greater surface area |
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Solid Versus Ribbed Mirrors
- thermal |
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* Which one has more surface
area? |
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* Which one will equalize to
ambient temperatures faster? |
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* Which one will stay closer to ambient temperatures,
especially throughout it's bulk? |
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* Which one provides greater total performance? |
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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. |
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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... |
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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. |
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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. |
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Read this
white paper regarding
thermals. |
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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. |
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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. |
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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. |
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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. |
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This
page discusses thin, solid mirrors. |
