Olomouc mirrors are used to observe cosmic rays around the world

How come that in Olomouc you started to manufacture mirrors for observatories studying cosmic rays?

The very first step, in the mid-1990s, was a project located in the village of Thémis on the last French slope of the Pyrenees. Here, the CAT (Cherenkov Array at Themis) observatory was built on the site of a former solar power station. The project required the development and supply of a mirror surface consisting of 90 spherical mirrors with a radius of 12 metres and a diameter of 50 centimetres. The problem was to find a partner who would be able to deliver the mirror segments in the required quality within a relatively short period of one year. As is often the case, coincidence helped; three classmates from high school met by chance. One of them was our colleague Jiří Keprt, the second was Jiří Vrána, who had emigrated and was working at the university in Paris, and the third was Ladislav Rob. The three friends agreed that it would not be bad if the Czech Republic also participated in this project and came to me asking if SLO could be a partner. It was a completely new thing for us, I didn't dare to promise something so complicated, but it was a challenge I accepted.

What was the beginning like?

From the scratch with minimal experience in the field we started to build an optical workshop that would be able to make such mirrors. I travelled the length and breadth of the country to find out that there was not a partner for this project who would be able to perform the deposition of the reflective layer on optical elements of the required dimensions in the required quality. So we undertook the complete production of the mirrors ourselves. I took advantage of the enthusiasm of people in various companies, to whom I explained what their cooperation was needed for and who were willing to help. In the course of time, a team of specialists in various technologies in optical manufacturing has been formed in our laboratory, which is able to respond flexibly to the needs of scientific projects with different requirements for mirror systems.

In those early days, before you learned to do everything by yourselves, who did you work with?

In Brno, they manufactured a mould from special cast iron for us. In Sázava, with local glassmakers we invented a technology for pressing mirror blanks from Simax. Simax is common glass you may know from the kitchen, but originally its production was introduced for use in submarine windows. Its great advantage is that it is very insensitive to changes in temperature. This means that when exposed to temperatures in the range of -30 to 50 °C, not only will it withstand such conditions, but its optical performance will not change significantly. In addition, a deposition apparatus had to be developed that could deposit a highly reflective layer on the mirrors, because the glass itself has only a small reflectivity and in these applications every single photon needs to be collected. This problem was solved with the assistance of specialists from Rožnov pod Radhoštěm. When our milling machine broke down, our colleagues from the former VOD in Turnov helped us a lot to complete the project.

And the project seems to have been a success...

A great success, the project was awarded a silver medal by the French Academy of Sciences. Right after that came a second project at the same location, called CELESTE. Which is an acronym for the Cherenkov Low Energy Sampling and Timing Experiment. The name implies that the project was a time sampling of the Cherenkov radiation waveform impact. The detector had very fast electronics by the standards of the time, and so it was possible to distinguish the time at which Cherenkov photons excited by cosmic rays hit the primary mirror. This was very important because the precise time resolution made it possible to determine the shape of the incident waveform and thus to obtain information about the primary signal.

How is the transport of such large and certainly very fragile mirrors to their destination actually performed? Whether to France, or perhaps to Argentina to the Pierre Auger Observatory, which we will get to later? One imagines government specials and security in gloves...

Quite straightforwardly – in crates. So imagine, for example, a glass baking dish you use for baking in the oven, it's the same glass as our mirrors, how do you treat it?

Probably not in the same way as a mirror. I put it in the dishwasher, scrub it with a brush...

As far as mechanical durability goes, those glass mirror substrates can take a lot. When we sent the first mirrors to France, they were bundled into one package, one mirror ten kilos with the box, so it weighed about sixty kilos in total. The parcel fell off the belt when it was unloaded from the plane, one mirror broke into pieces and the other one had the edge chipped off. I mean, if the mirrors are well packed, they don't necessarily get hurt even if they fall on the airport apron.  The transportation of mirrors to the Pierre Auger Observatory in Argentina was then carried by sea in a container. The transport company took everything from the FZU yard to the customs warehouse in Argentina. Then it was usually a difficult problem to get the shipment out of the customs warehouse, so it took some time for the mirrors to reach their destination in Malargüe. Malargüe is now already a town about 350 km south of Mendoza on the highway, basically on the northern border of Patagonia.

How is the construction of such an observatory perceived by the locals? It must disrupt their immediate surroundings quite significantly?

For example, for the aforementioned Malargüe it has meant a huge boom over the years. When we started to prepare the project, there was an asphalt central road running through Malargüe, four lanes, sometimes six lanes, with a statue of Christ on one side and the astronomical clock on the other side, but the other parallel roads were just gravel, and today a number of side roads have been asphalted. A lot of schools have been built, and not only primary schools, a lot of money has flowed into the town, and more and more educated people live there. Thanks to the project, the whole area has been making significant progress. The importance of the project for the city is recognised not only by the local municipality or the governor, but also by its citizens. Due to the rapid development of the area, however, the project naturally faces some pitfalls. The city grows, everyone is using lights, local businesses are lighting up large areas, the streets are lit up, and one of the telescopes has already had to be shut down because its camera, composed of photomultipliers, was glutted with light noise. So the presence of civilization obviously impacts optical detectors negatively. One of the reasons why remote areas are sought out projects of this type is precisely the absence of light smog.

But that's a problem when a project like this automatically means major development in the area...

Taking the Pierre Auger Observatory specifically; it occupies an area of 3,000 m2, which an area comparable to the Liberec or Karlovy Vary region, so there are still plenty of detectors working in absolute darkness. This project is a hybrid, it is based on two detection principles. Optical detection tracks the fluorescent radiation produced by nitrogen molecules in the atmosphere after colliding with a high-energy cosmic ray particle that has entered the atmosphere from space. The optical detector consists of four observatories on the hills around the plateau, each with six telescopes facing each other and looking statically into the atmosphere. The cameras of these detectors wait for a very short and weak optical signal, so they need as much darkness as possible. There are also ground-based radiation detectors dotted around the observatory. These surface detectors do not monitor the atmosphere, they are not typically optical instruments, they do not have mirrors. They are barrels of ten cubic meters of pure water placed in a triangular grid 1.5 km apart, 1 600 in total. The barrels of water contain photomultipliers that track flashes of Cherenkov radiation, which are produced when particles from cosmic rays move faster than light in the watery environment. This is a seeming contradiction in terms as we know that nothing is faster than light in a vacuum. Light in an optically denser environment than a vacuum, however, has a speed that is no longer equal to the speed of light in a vacuum, but its propagation is much slower. A high-energy particle moving in a watery environment produces braking radiation and photomultipliers detect the Cherenkov signal. In each barrel, there are photomultipliers monitoring the action inside. When a high-energy particle hits the barrel, a signal about the event is sent. After evaluating the signals from all ground detectors affected by the shower, the shower can be reconstructed. These detectors can operate normally in the daytime and are not affected by light.

You then take further care of the mirrors and maintain them. How are such mirrors cleaned?

With difficulty. The mirror system has to be dismantled, each mirror segment has to be taken down and taken outside the observatory, into another room. Water is then used to clean them, to rinse off the dust particles. If other contaminants are present, it is necessary to assess what type of contamination is involved. For example, we need to know if it is oily, because there are a lot of oiled parts on the detectors, and sometimes a bat flies in and leaves organic dirt, and there are also cleaning ladies who come in and are a source of other dirt, and all this sticks to the mirrors. It is necessary to identify the contaminants and select the right cleaning technology. Cleaning mirrors that will not significantly reduce their lifetime and reflectivity is a relatively sophisticated technological procedure using a variety of solvents.

So you also need to be accompanied by a chemist?

No, I do my own chemistry. I usually already know the origin of the contamination based on experience. When the mirror is clean, the functional area needs to be dried, and we use compressed nitrogen for that. Then the mirrors can be placed back on the support structure and the telescope set again.

What is the lifetime of the mirrors in the observatory?

Originally it was supposed to be 15 years, but the first mirrors have been there since 2004 and are still working. Their reflectivity must not change significantly, that would be a major problem for the detector. We monitor the reflectivity regularly, measuring the reflectivity parameters every six months to be able to correct the detector readings. But if at the beginning the reflectivity was 99%, today it is one, one and a half percent lower, which is still a great number.

Where is the development of telescopes at the Joint Optics Laboratory going now?

We are learning to make better detector calibrations, we are reducing measurement errors, simplifying the telescopes structurally, we want to make their deployment more variable and their use independent of external infrastructure, we are trying to radically reduce their cost. Together with colleagues from Japan and America, we have built five very compact detectors. We call these telescopes FAST, they are small – two metres in diameter, and very cheap. The detectors contain four photomultipliers, a segmented mirror, an optical filter that transmits only a certain part of the spectrum of optical radiation and are installed in separate containers. These containers are sealed, have automatically controlled shutters, their own power supply, their own energy, their own communication. So they can be built really anywhere. We operate three FAST telescopes in Utah, where they are installed in collaboration with the TA (Telescope Array) project, and two in the Los Leones area in collaboration with the Pierre Auger project. We are now collecting data from them and trying to prove that our idea is viable and that it will allow us to build a cosmic ray detection facility anywhere with incomparably less investment than is usual for such projects.

Applications of mirror surfaces can, however, also be found in experiments unrelated to cosmic ray research .....

You are right, we have developed the technology to make mirrors for detecting Cherenkov radiation at particle accelerators and in this area our mirrors are also popular components for building detectors at CERN for example, but that is a topic for another conversation.

 

An overview of optical signal detection projects using mirror systems supplied by the SLO Applied Optics Group over the last 30 years.

1. CAT – France
2. CELESTE – France
3. Mirror system of the Cherenkov radiation detector for the experiment DIRAC (DImeson Relativistic Atomic Complexes) at CERN.
4. Mirror systems for the optical detectors of the Pierre Auger project in Argentina.
5. Mirror systems for the optical detectors of the HEAT (High Elevation Auger Telescopes) project.
6. A set of prototype segments and the won tender for the supply of mirrors for  RICH detektor CBM (Compressed Baryonic Matter) in Darmstadt
7. Mirror system of the Cherenkov detector RICH -1 for LHCb (Large Hadron Collider blue) CERN.
8. Mirror prototypes for the balloon experiment of the JEM-EUSO-SPB2 (Joint Experiment Missions – Extreme Universe Space Observatory – SPB2) project
9. Mirror systems for the FAST (Fluorescence detector Array of Single-pixel Telescopes) type detectors
10. Mirrors for CTA – Cherenkov Telescope Array project

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