Case StudyMax Planck Institute uses Spectrum's digitizer cards tomeasure diameters of distant stars
Ultra-clean signal handling and nanosecond data precision leads to
unprecedented sensitivity of the world's largest Cherenkov telescopes
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 theangular diameter of stars is extremely small: only a few milli-arc-seconds. That is about thesize 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 distanceof 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 wereselected.
"We found that these cards not only had
the lowest levels of spurious signals andcrosstalk of all the PC cards that wetested," said David Fink from the Max
Planck Institute for Physics, who is incharge of Electronic Development on theproject, "but the performance of eachcard was also identical. The latter is so
important as you are trying to compare
the differences between the signals fromeach telescope. The technique is very
Figure 1: The twin telescope MAGIC at an altitude of oversensitive to correlated signals and2200m on the canary island La Palmacrosstalk between channels including
anything picked up along the way from the optical sensors through to the computer that thedigitizer cards are mounted in. To put it into perspective, these Spectrum cards enable us toprecisely measure fluctuations of the light intensity on nanosecond time scales giving
unprecedented sensitivity that is aroundten times better than that achieved in the1970s with the Narrabri interferometer.""The other important factor in choosingSpectrum digitizer cards is their excellent
reputation for reliability. The cards arelocated by the two telescopes that arehigh up in the mountains on the island ofLa Palma, one of Spain's Canary Islands,
so it is not a simple matter to swap in a
new card if there is an issue. Plus, there isthe cost of instrument downtime and lostobservation time. The fact that Spectrum
provides a five-year warranty shows their© Spectrum Instrumentation GmbH, Germany

Figure 2: MAGIC (Major Atmospheric Gamma-Ray Imaging
Cherenkov) are the world's largest air-Cherenkovtelescopes each with a diameter of 17m
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Case Studyfaith in the high quality and reliability of their cards which was supported when we checkedwith other users in the scientific community. Lastly, Spectrum assured us that they canrepair cards long after the five years have passed. That i very reassuring as long-term
experiments can often be confronted with a large effort to redevelop a system because theoriginal hardware at the heart of it is no longer available."
Because of the large quantities of data
being handled, the system uses
Spectrum's SCAPP software (Spectrum's
CUDA Access for Parallel Processing).
This is a way of sending all the collected
data from the digitizer not to the CPU of a
PC with 8 or 16 processor cores, but to an
NVIDA PC graphic card because the GPUgraphic processor has up to 5000 cores
giving much faster data processing. This
enables recordings to be run in highresolution with 500 Megasamples persecond.
The diameter of a distant star is measured
by digitizing the variations in the light
Figure 3: Spectrum M4i.4450-x8 2-channel digitizer
received from a star. The cross correlation with 500 MS/s speed
is then computed and averaged duringobservation to determine its variation as a function of the separation between telescopes.
The geometry changes as the star moves across the sky. Measuring a shape requires
observations along multiple axes.

Background
Imaging Atmospheric Cherenkov Telescopes (IACTs) have large mirrors and in the order of
one nanosecond time response to signals of a few photo-electrons produced by opticalphotons. This means that they are ideally suited for optical interferometry observations.
Thanks to their sensitivity to visible wavelengths and long baseline optical intensity
interferometry with IACTs, angular resolutions of tens to microarcseconds can be achieved.
This project has installed a simple optical setup on top of the cameras of the two 17m
diameter IACTs and observed coherent fluctuations in the photon intensity measured at thetwo telescopes for three different stars.
The link to the Institute's paper on this project that is called MAGIC (Major AtmosphericGamma-Ray Imaging Cherenkov) is at:https://arxiv.org/abs/1911.06029
Further reading on Interferometry is at:https://arxiv.org/abs/1204.3624© Spectrum Instrumentation GmbH, Germany

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