Hardware Development
A major activity of the group is hardware development for future telescopes.
Active Mirror Control
To measure the faint flashes of Cherenkov light emitted by air-showers, a large mirror area is mandatory to get a measurable signal. The two MAGIC telescopes operated in stereo-mode have a mirror area of 230m2 each. To be able to point such large systems to any position in the sky within about 20 seconds, a light-weight structure is mandatory. But then the structure deforms under its own mass.
To correct for this, we developped the so-called Active Mirror Control. Each of the 234 mirror segments per telescope is automatically readjusted to correct for any deformation and keep best optical performance. Due to the success of this approach, the next generation Cherenkov Telescope Array (CTA) is going to use a similar system for its up to 50 medium and large size Telescopes.
SiPM Camera
It is not sufficient to concentrate the Cherenkov light with large mirrors, the cameras and readout electronics must be able to take a picture every nanosecond and have sensitivity to record single photons. This is usually done using photo-multiplier tubes. When the first generation of solid-state photosensors, nowadays called SiPM, became available, we pioneered their usage under the very harsh conditions intrincis to Cherenkov telescopes. The very first measurements of such flashes is documented in the PhD thesis of Ilja Britvitch.
After these successful tests, we formed the small FACT collaboration to build the first SiPM based camera and install it in a refurbished small telescope with 10m2 mirror area. Development and operation of the camera was topic of the PhD theses of Thomas Kraehenbuehl and Patrick Vogler. The intention was to operate this early prototype for about six months to investigate how to build future SiPM based cameras. Few hours after installation of the camera in October 2011, first events were recorded and since then, FACT is successfully taking data every night whenever observation conditions permit whithout any sensor related problem.
This unexpected success convinced CTA management that the future 70 small size telescopes shall use SiPM based cameras. The unprecedented reliability of FACT also allows to operate it fully robotic, while similar telescopes need several persons onsite during operation.
Telescope – Plenoscope
A larger mirror area of such telescopes allows to measure lower energetic and therefore fainter gamma-rays. But physical, optical and technical constraints limit the mirror area of such systems to about 400 m2, resulting in an energy threshold of few tens of GeV. In addition, the field of view is instrinsically limited to few degrees. In his PhD thesis, Sebastian Mueller develops the ingenious idea to build a Plenoscope instead of a Telescope, i.e. measuring the complete light-field.
This allows to overcome most restrictions and construct systems with a field of view of tens of degrees to monitor large fraction of the sky in parallel. It also allows to use much larger mirrors to measure gamma-rays with energies less than 10 GeV that so far could only be measured by more expensive and less sensitive satellites. But there is still significant research needed to optimize the design and individual components.