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News & Updates - August 2000

Secondary Mirror Development

Among the many diverse SOFIA development activities taking place around the world, CSEM, or the Swiss Center for Electronics and Microtechnology, in Neuchatel, Switzerland, is building the telescope's sophisticated secondary mirror mechanism.

(Courtesy of DLR) In this schematic of the SOFIA telescope assembly, the primary mirror can be seen in gold and the secondary mirror toward the top in blue, while light waves are represented by moving lines. Click on image to see the animation.

The secondary mirror is a small but critical component within the full telescope assembly. Some would argue that it's the most complex part of the entire telescope. It holds special interest because it provides a lesson in how infrared astronomers deal with background, or interfering radiation, from sources emitting from throughout the universe, the galaxy and even from the aircraft and telescope itself.

Here's how it works: When astronomers use instruments such as cameras, spectrometers or bolometers to investigate celestial objects, their measurements inevitably include not only infrared emissions from the object under study but also infrared background, primarily from the atmosphere and from the telescope itself. Because telescopes emit infrared radiation, observing in the infrared is like observing in the optical in the daytime with a glowing florescent telescope.

Then how can a scientist isolate a real measurement of the object under investigation?

Simple. He or she can measure back and forth between the object-plus-background and the background alone. Later, it's just a matter of subtracting the background measurements from the object-plus-background numbers to derive true information about the object itself.

And how does one go about measuring back and forth in this way?

One arranges for the secondary mirror to "chop" first the object and then an empty part of the sky. When chopping, the secondary mirror rotates to and fro on a pivot point, much like your own head moves as you look left and right while your body remains still.

Chopping can be very fast, up to 40 movements per second, but it has the disadvantage of observing on two different parts of the primary mirror. This effect is compensated by "nodding" the whole telescope every 30 seconds to a third empty sky position. When the telescope assembly nods, both the primary and secondary mirrors move as a unit.

CSEM's prototype of the tilt-chopping mechanism for the secondary mirror was tested in 1999, with excellent results. The manufacturing, assembly and test of the final secondary mirror mechanism are planned for 2001. Meanwhile, the mirror itself for the secondary mirror is being developed and fabricated at Astrium SAS in Toulouse, France.

August 10, 2000