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Thermal Time Constant: 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. As a comparative tool 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 zeroDELTA™
lightweight mirrors typically start off with only 85-125 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 industry-leading performance of the zeroDELTA™ mirror. They can not only reach a static temperature
far faster, but they are also highly nimble; able to rapidly
react to dynamic temperature changes, creating the smallest performance
losses possible. Download
this TTC calculator
to compare the zeroDELTA™ mirrors to either thicker, "lightweight"
mirrors or solid mirrors, of any material type. |
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A
1°C delta between an optic and the ambient temperature can
produce a minimum of 0.3-0.5 arc-second of degradation. |
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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). |
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*
lower
mass |
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thinner
profiles |
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* greater surface area |
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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. |
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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 (internal)? |
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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; heat
capacity & thermal conductivity. They hold onto their existing
bulk temperature and they do not conduct away that temperature
readily. Dream's zeroDELTA™ engineered lightweight mirrors resolve both
of these historic 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 problems
directly, which G.W. Ritchey recognized 100 years ago. |
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Internal temperature gradients
within all mirrors, including zero-expansion, will produce mirror-seeing; performance loss at the boundary
layer. Within non zero-expansion 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 the figure stabilize. Visual cues from the
boundary layer thermals can no longer be seen well before the
figure has stabilized. There are no free lunches in optics. Just
because visual cues can no longer be seen does not mean degradation
is gone. Out of sight, out of mind is not useful with an optical
surface that is trying to maintain the figure to a fraction of
a wavelength of light. 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. But what happens the next day when
more work is done to the surface and it needs tested less than
one hour later? 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 environements where the temperature
changes, are performing at an optimal level, all of the time.
Even inside a lab, temperatures are rarely controlled to +/-0.1°C,
due to the real-world difficulty of actually achieving this small
tolerance. |
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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. The
use of a zero-expansion material does not change
any of these facts because CTE has nothing to do with thermal
time constant; how quickly a mirror equalizes. Use the TTC
calculator to do your
own research and learn the materials to a deeper level. |
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 Thermals From A Solid Mirror |
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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.
Not to mention solid mirrors', of all material types,
issues with gravity and therefore mechanical performance
in the instrument. |
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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. 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 is the enabling and disruptive nature of Dream's
zeroDELTA™ mirror technology.
396mm CA f1.376
mirror was 16.6" physical OD, 2.5" edge height and
weighed 9.5 pounds. |
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"Your company does phenomenal work. There is a lot
of thought and heart that goes into your products. Dream's engineering
sets their lightweight mirrors apart from competitors. Your engineering
goes beyond the lightweight aspect. You focus on actual performance!" |
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- Ted Kamprath |
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39 years in professional optics,
using everything from million dollar test rooms to 144"
Continuous Polishers. |
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