|Silver coatings are available with additional charges:|
Semi-Enhanced Enhanced Aluminum Protected Silver
8" $90 $290 $365
10" $100 $290 $365
12.0" $110 $295 $385
12.5" $115 $315 $410
14.0" $125 $340 $435
16.0" $180 $360 $455
18.0" $220 $380 $515
20.0" $230 $400 $575
We do NOT recommend the Enhanced Aluminium and Protected Silver
coating for the primary mirrors.
The main problem is not the aberrations due to deformation, but the
layer of warm air in front of the primary mirror. This layer of warm air is
the main cause of the image distortion called "mirror seeing", which is
caused by the non-uniform index of diffraction in the cooler air over the
warm mirror surface. No mirror, regardless of the type of glass used,
will perform adequately until the mirror is close to the temperature of
the ambient air. This occurs when the temperature difference between
glass and air is less than one degree centigrade (°C), and best
performance is achieved when this difference is less than 0.2 °C.
Therefore, the goal is to bring the temperature of the mirror to within 0.2 °C
of the ambient air temperature as quickly as possible. This will greatly
reduce image distortion due to mirror seeing. This is why all large
professional mirrors, regardless of the type of glass used, employ
complicated cooling systems to cool the primary mirror. For example, the
Advanced Technology Solar Telescope (ATST) 4.24-meter primary mirror
uses a jet cooling system.
Why Lightweight Sandwich Mirrors?
Telescope mirrors are manufactured in a strictly controlled temperature
and humidity environment. Each mirror is carefully measured after it
has reached equilibrium in our laboratory. While cooling, gradients in
the mirror will cause deformation of the surface, and the aberrations
induced by these gradients will be proportional to the CTE of the
substrate. These gradient-induced aberrations die out as the mirror
cools and equilibrates.
Jupiter Images by Emanuele De Giorgio with a Hubble 16" F/5 Sandwich Mirror
Mars Images by Emanuele De Giorgio with a Hubble 16" F/5 Sandwich Mirror