Thermal behavior
Once an optical surface has been ground and polished into a mirror substrate, it is
important for the figure not to change due to environmental exposure, temperature
changes, or release of internal stress. Well-annealed glass and glass-ceramic materials
usually do not exhibit such stress.

The mirror material must be insensitivity to thermal variations.
o Thermal changes occur early at night, when the mirror is trying to reach
equilibrium with the night air temperature, and through the night as the
temperature drops
o As long as the mirror material is homogeneous and isotropic, bulk
temperature changes will affect the focal length but not the figure
 More troublesome are temperature gradients between the back and
front surfaces of the mirror or gradients across the diameter, both
of which can affect focal length and figure
o Thermal effects will clearly be smaller as the coefficient of thermal
expansion of the material, , is lower
Typical variations of CTE with temperature for a variety of low-expansion materials.
Coefficient of thermal expansion as a function of temperature for common mirror blank
materials.

An alternate to a low CTE is to have lower temperature gradients
o Gradients are reduced if the thermal conductivity, , is high and if the
specific heat,
, and density,
, are low.That is to say, if the ratio
is high
o
o
This ratio characterizes how quickly a material will come into temperature
equilibrium is the thermal diffusivity
Low CTE and high thermal diffusivity, can be cast into a single figure of
merit for thermal behavior, which one will want to maximize for passive
optics:
o

With active optics, the requirement that mirror figure not be affected by
thermal change is much less stringent
 Thermal effects are slow and well within the bandpass of active
optics systems
 For ground-based telescopes, high thermal diffusivity is useful for
reducing mirror seeing
 Blurring of images due to air turbulence generated by a
difference in temperature between the mirror's optical
surface and the surrounding air
 Mirror seeing is less for materials with high thermal
diffusivity because they track ambient temperature
variations well.
 For active optics systems on the ground, the figure of merit which
one will want to maximize will then simiply the thermal
diffusivity:
The advantage of higher thermal diffusivity is significantly enhanced if the mirror
is equipped with a thermal control system to keep the mirror's bulk temperature
close to the predicted or actual night temperature.