Spectrum digitizer cards to measure diameters of distant stars

November 24, 2021 // By Jean-Pierre Joosting
Spectrum digitizer cards to measure diameters of distant stars
Digitizer cards enable ultra-clean signal handling and nanosecond data precision, delivering unprecedented sensitivity to the largest Cherenkov telescopes.

The Max Planck Institute has selected digitizer cards from Spectrum Instrumentation to precisely measure fluctuations of the light intensity on nanosecond time scales received by MAGIC (Major Atmospheric Gamma-Ray Imaging Cherenkov) telescopes on the Canary Island of La Palma.

The MAGIC telescopes on the Canary Island of La Palma were built to observe cosmic objects that emit high-energy gamma rays, i.e., supernovae or black holes. Astronomers also use the twin telescope to measure the diameter of stars to investigate the processes throughout their life cycle. This is a challenging task for earthbound telescopes, since the angular diameter of stars is extremely small: only a few milli-arc-seconds. That is about the size of a coin on top of the Eiffel Tower as seen from New York! Not even the largest telescopes in the world are able to measure them directly. Instead, the researchers record the light intensity of an object by combining the light from several telescopes at a distance of tens of meters – a technique called intensity interferometry. However, the signals are very weak so any spurious signals and crosstalk would swamp them. Having evaluated several makes of digitizer card, Spectrum Instrumentation M4i.4450-x8 digitizer cards were selected.

"We found that these cards not only had the lowest levels of spurious signals and crosstalk of all the PC cards that we tested," said David Fink from the Max Planck Institute for Physics, who is in charge of Electronic Development on the project, "but the performance of each card was also identical. The latter is so important as you are trying to compare the differences between the signals from each telescope. The technique is very sensitive to correlated signals and crosstalk between channels including anything picked up along the way from the optical sensors through to the computer that the digitizer cards are mounted in. To put it into perspective, these Spectrum cards enable us to precisely measure fluctuations of the light intensity on nanosecond time scales giving unprecedented sensitivity that is around ten times better than that achieved in the 1970s with the Narrabri interferometer.

MAGIC (Major Atmospheric Gamma-Ray Imaging Cherenkov) are the world's largest air-Cherenkov-telescopes each with a diameter of 17m.

Picture: 
The twin telescope MAGIC at an altitude of over 2200m on the island La Palma.

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