A couple of days ago the European Southern Observatory began testing a crucial part of an upgrade for their Very Large Telescope based in the Atacama Desert of Chile – a deformable secondary mirror.
Speaking as an engineering student, I love the ingenuity that goes into the devices we use to study the deepest reaches of the universe and the clever technology behind modern optical telescopes is no exception.
Most large optical telescopes today (like the VLT telescope, which is actually a group of four separate telescopes working together – more on that later – use a system to control their mirrors called Adaptive Optics, normally in conjunction with a segmented primary mirror.
For about two hundred years now, most large telescopes have used a system where light enters the telescope and reflects off a large primary (light-collecting) mirror before bouncing it off a secondary (light-directing) mirror into either an observer’s eyepiece or photodetector. Larger diamater primary mirrors mean that more light is collected so fainter objects can be seen. Eventually these mirrors got too large and too heavy to be manufactured in one piece without distorting under their own weight, so the segmented mirror telescope was born – a large array of mirrors that act and have the geometry of a larger single mirror.
Adaptive Optics developed around the same time as a consequence of an effect particular to ground based telescopes – light travelling through the atmosphere is subject to distortion as a result of air currents at many different heights. Adaptive Optics is – a system that moves and deforms the many segments of the primary mirror around to cancel out the distortions from the atmosphere to produce a clearer image of objects in the night sky.
This secondary mirror delivered to the ESO at Garching, Germany, is actually an adaptive optics upgrade for the secondary mirror of one of the four telescopes that make up the Very Large Telescope. It is 1.12 meters across but only two millimeters thick and thus thin enough to bend slightly in response to 1170 electromagnetic actuators glued to the rear side of its shell support structure at up to one thousand times per second.
Putting an adaptive optics system on both the primary and secondary mirrors should allow much finer control and consequently, much clearer images to be taken of objects in the night sky. The mirror is the largest deformable mirror ever made for astronomical purposes and is set to be installed on the VLT in 2015.
Image Credit: European Southern Observatory.