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The night does not move
16/11/2006
The secondary mirror's time has come. It too is entitled to an aluminium shower which will allow it, like a spy, to feed back what the primary mirror reflects to it. In this edition, however, we focus on movement: it's a frenzy of movement for the telescope's optical system, whether visible to the human eye or not. Movements like the active optics system adjusting each primary mirror segment, the tertiary mirror working to redirect light and, of course, the movements of the secondary mirror.
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Strangely, however, all this movement is designed to give us a fixed, clear image of the object we want to observe. Paradoxically, because the telescope does, the night does not move.
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The secondary mirror is serrated in form, adapted to the shape of the primary mirror. It is part of an equipment assembly which includes the hexapod and the 'chopper' system.
So, why does it need to move?
Firstly, so that it can accurately capture light arriving from the primary mirror using a movement called "optic system alignment". The hexapod, which provides 6 degrees of movement (to allow movement in every direction), allows the secondary mirror to be aligned with the primary mirror, compensating for flexing and thermal expansion in the telescope structure. This movement, like the effects that corrects, is very slow.
"Chopping" is a technique used to measure differentials in the thermal infrared spectrum: it means that the secondary mirror will make rapid pointing changes, taking two different images - one of the object being observed and one of the dark background. By contrasting these two images diffuse light can be eliminated from the object's background, providing the instrument with a better quality image to work on. This can be done 5 times per second, but it is used a 20% of the time (to go from one pointing to the other); the 80% remaining, the secondary mirror must stand still in one of those positions.
The third movement is called "rapid guiding," and it compensates for vibrations caused by gusts of wind and atmospheric turbulence due to local temperature variations. It can move at a speed of 70Hz, which means that it can oscillate 70 times per second.
For both chopping and rapid guiding, the assembly has a reaction mass system to counteract inertia from the mirror so that it is not transmitted to the hexapod or other parts of the telescope.
Natalia R. Zelman