Researchers at Sandia Laboratory and the University of Kentucky are developing enabling technologies that may be the future of space telescopes and surveillance satellites.
Whereas the Hubble and the NASA Next Generation Space Telescope (NGST) use the traditional polished glass mirror approach, the new design employs a ‘smart’ material that changes shape when struck by electrons fired by a computer-controlled electron gun. Thus the mirror could take the shape of a lightweight thin film that could be folded up and carried on a small booster rocket and opened to its full diameter in orbit.
The electron gun would then be used to correct the shape of the film mirror to its desired form to within 10 millionths of an inch, which is the required accuracy for optical-quality imaging applications. The material is also incredibly light, weighing less than one kilogram per square metre of mirror area compared to 15 kilograms per square metre for NGST and 250 kilograms per square metre for the Hubble.
Such technology would make possible really large space mirrors- as much as 20 to 30 metres square- to allow the collection of light from the dimmest and smallest of sources. With conventional materials this is precluded because of the expense and the size limitations of launch vehicles. The new technology would allow extremely large mirrors to be launched from small boosters, saving millions of dollars per launch.
The flexible nature of the piezoelectric mirror material means that it will be misshapen once it is launched in space, necessitating reshaping with the electron gun. Laser optical sensors measure the shape of the mirror surface, and this information is fed into the control algorithm programmed into the computer connected to the gun. The algorithm determines the excitation profile necessary to change the mirror surface to its desired shape via electron gun excitation.
Since the initial mirror shape will be very far from the desired shape, the mirror figure sensing method must have both large dynamic range and high resolution.The gun fires electrons into different areas of the mirror to make the surface change its shape in either a more convex or concave direction. The new shape remains fixed for a period of several hours to days. Then the beam is reactivated to add or remove the charge to make small corrections to the mirror surface shape.
According to principal investigator Tammy Henson, a further advantage of the mirror is the speed with which it can be fabricated and deployed: ‘a matter of months- as compared to many years with the Hubble telescope.’