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Mirror dances
05/09/2003
The secondary mirror for the Gran Telescopio CANARIAS (GTC) will have a rhythm that will be the envy of all those who see it. It will capture light from the primary mirror to deflect it straight to the Cassegrain focus or the tertiary mirror (which in turn will deflect it to the Nasmyth and Folded Cassegrain foci) - but that is not all.
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Glossary
- Cassegrain focus
- Chopping
- Diffuse light
- Folded Cassegrain foci
- Infrared
- Nasmyth foci
- Primary mirror
- Secondary mirror
- Mechanical structure of a telescope
- Tertiary mirror
- Atmospheric turbulence
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It will also “dance” to whatever beat is set for it. This will enable it to accurately grasp the light deflected from the primary mirror, correcting any rapid disturbances and providing the instruments with a facility known as automatic chopping.
ACTION!
The secondary mirror assembly comprises a hexapod robot, a “chopper” and the secondary mirror itself.
The hexapod robot will couple the assembly to the telescope, align the secondary mirror with the primary mirror, compensate for flexing of the telescope structure and counteract thermal expansion.
To do this it will have six feet which, by extending or retracting, will give it six degrees of freedom - in other words it will be able to move in any direction. These are called slow drive movements.
The chopper will move rapidly. It is a drive system with two degrees of freedom that will be responsible for controlling the position of the mirror, counteracting vibration from gusts of wind and atmospheric turbulence due to local temperature variations. It will be able to move at a speed of 5hz - in other words it will oscillate 5 times per second. It will also be fitted with an system to compensate for inertia, required because it will have to avoid transferring energy to the hexapod and the rest of the telescope when it moves.
The chopper will allow differential measurements to be taken in the thermal infrared range. It is a procedure that requires the secondary mirror to change the direction it points in rapidly, capturing precise images of the observing object and the dark background. By contrasting these two images the diffused light in the background of the object can be eliminated, providing better quality observing material for the instrument.
Both the hexapod robot and the chopper were manufacured by the Barcelona company NTE designed by the Swiss outfit CSEM.
Natalia R. Zelman