Cherenkov Telescope Array (CTA) / Small-Sized Telescope (SST-1M)

2008-2019

The scope of work carried out under management  of DAI members

The Institute of Nuclear Physics of the Polish Academy of Sciences is a member of the international consortium Cherenkov Telescope Array (CTA), under which an observatory is being built to study the universe in the field of the highest energy gamma rays. The CTA will consist of two observatories: one in the northern hemisphere, on the Spanish island of La Palma, which will consist of 20 to 30 telescopes spaced over an area of ​​1 km2, and the other in the southern hemisphere, in the Atacama desert in Chile, containing 70 to 100 telescopes covering an area of ​​10 km2. Due to its size and technological advancement, CTA will be 10 times more sensitive than currently used gamma observatories. Its measurements will cover a wide range of energy, from about 20 GeV to 300 TeV, and will also provide data with much higher angular resolution. The designed lifetime of the observatory is at least 30 years. The observatories will be built of three types of telescopes: large with a reflecting dish with a diameter of 24 m (LST), medium ones with a diameter of 12 m (MST) and small ones with a diameter of 4 m (SST-1M and SST-2M). These are the so-called Cherenkov telescopes, recording the bluish light glow of atmospheric particle showers, initiated in the Earth’s atmosphere by high-energy cosmic-ray and gamma-ray particles coming from space.

DAI has designed the structure of a small Davies-Cotton telescope with a single reflector (SST-1M), which was  manufactured  by PONAR in Żywiec. The structure has been developed in such a way as to obtain the highest efficiency possible, with relatively low production and operating costs. High optical performance was achieved due to a small shadowing of the mirrors by the mast supporting the camera causing the dead area. The exploitation costs were tried to be optimized through the use of drive and control systems present in other telescopes intended for CTA and appropriate balance of the structure, and its aerodynamic shape eliminating the influence of strong winds blowing from different directions. In 2014, the telescope manufactured and assembled at IFJ PAN was tested in terms of the requirements of CTA. The tests included, inter alia, determining the parameters of the structure modal model modal, geodetic measurements (tower deviation from the vertical axis, mast deflection, mast vibration measurement), measurements of electricity demand in various modes of operation and atmospheric conditions.

In 2015, the telescope prototype was equipped with mirrors (Joint Laboratory, Olomouc, Institute of Physics of the Czech Academy of Sciences) along with their attachment and adjustment systems (Space Research Center of the Polish Academy of Sciences in Warsaw).

In autumn 2017, a fully digital camera (University of Geneva-UJ-AGH) was installed on the telescope. Instead of classic vacuum photomultipliers, it uses semiconductor light sensors, which represent a new quality in this field and reduce the weight of the camera several times, as well as an innovative fully digital trigger and data acquisition system. By the end of 2018, both the telescope’s functional tests and sky observations were conducted.

In the years 2017-2019 a modified mechanical structure of the telescope was designed, which was produced by PONAR Żywiec and installed at IFJ PAN, a new control cabinet was designed and made, and the first prototype structure of the telescope was dismantled and packed into containers.

Project management at DAI

J. Michałowski, M.Stodulski (2008 – 2015), J. Świerblewski (2016 – 2019)

Project timeline

2011 – 2012
Design of the mechanical structure of the SST-1M telescope

2012 – 2013
Design and implementation of the telescope’s control cabinet
Development and launch of telescope software
The PONAR ŻYWIEC company installs the SST-1M telescope in the IFJ PAN

2014 – 2016
Telescope equipment and mechanics movement tests
Equipping the telescope dish with 18 glass spherical mirrors

2015 – 2018
Tests and calibration of the telescope’s optical system
Installation of the telescope semiconductor camera made at the University of Geneva
Tests of the complete telescope, first registration of bursts of gamma radiation

2017 – 2019
Design of the next, modified SST-1M telescope
Design and implementation at IFJ of a new control cabinet
Disassembly and packing the structure of the telescope into the transport container
The PONAR ŻYWIEC company installs the modified SST-1M telescope in the IFJ PAN

Cherenkov Telescope Array (CTA) / Mirrors for Medium-Sized Telescope (MST)

2011-2019

The scope of work carried out at DAI

Construction of a composite mirror with an open structure

Since 2011, DAI engineers and technicians have been developing glass spherical mirror technology. The classic technology consisting in long-lasting grinding of the glass plate until the desired shape of the reflective surface is inefficient and very expensive. The CTA observatory telescopes will be built in total from several thousand large-size hexagonal optical mirrors, 120 cm for medium telescopes (MST) and as much as 150 cm for large telescopes (LST). These mirrors will have to be made,  in composite technology, in a short time, around 2-3 years. Such a number of mirrors can be produced in several specialist production plants simultaneously. The Institute of Nuclear Physics of the Polish Academy of Sciences is one of four entities that has been dealing with the technology of producing composite mirrors for eight years. DAI employees have developed mirrors for MST telescopes. They have the shape of a regular hexagon described on a circle with a diameter of 1200 mm. Each mirror consists of a front and back glass wall, which are connected by 350 thin-walled aluminum sleeves with a diameter of 40 mm and a length of 50mm. The side walls of the mirror, mechanically stabilizing its shape and preventing the penetration into the mirror of large dirt and insects, consist of thin perforated aluminum plates. The edges of the glass plates are protected by silicone rubber profiles mounted on adhesive. On the back wall of the mirrors three stainless steel discs (pads) used to attach the mirror to the telescope are glued with a special flexible adhesive. The back wall is also covered with white self-adhesive foil, protecting the mirror against excessive heating during the day in the parking position of the telescope, in which the mirrors face the back wall towards the sun.

The construction of the spherical mirror is based on the cold bending technique of a thin glass sheet. The sheet (glass) is placed on a metal convex matrix. In the case of mirrors for MST telescopes, the radius of curvature of the matrix is ​​32.14 m. The matrix is ​​equipped with a vacuum suction system, which causes even pressure of the glass pane to its surface. Such pressure causes the initially flat glass to take the spherical shape of a matrix. The matrix is ​​also equipped with a heating system because epoxy resin is used to laminate (glue) subsequent layers of the composite, requiring elevated temperature to cure. The production line for the mass production of mirrors consists of three stations. The first station has a matrix for producing the rear wall of the mirror, the second has a front wall, and the third is the matrix for final assembly. Each matrix has a different, specially selected radius of curvature. In 2014, DAI built a prototype production line with three mirrors per week. By the end of 2016, 100 mirrors had been built for the MST telescope prototype.

The front and rear wall of the mirror consists of two layers of soda-lime glass (standard window glass) 2 mm thick. To obtain the hail strength required by CTA, between the panes there is a wet laminated layer of fiberglass fabric. These three layers are glued together on the described matrices with vacuum clamp and after hardening receive a preliminary, concave shape. At the third station with the target radius of curvature, the final support structure of the mirror is glued. First, the front wall is sucked into the matrix using a vacuum. It can be pre-metallized to reflect light in the desired wavelength range. The rear wall is then glued to the rear wall with aluminum sleeves and perforated side plates previously glued in at a different stand. Then, for the next two days, the epoxy adhesive is heated and the structure stabilizes. After removing the mirror from the matrix, it goes to the optical bench for preliminary measurement of the curvature radius and spot size (PSF). After the measurement, the mirror is finished: pads, rubber edges and white protective foil are glued. If the front wall is not metallized, the supporting structure is sprayed with aluminum compounds and a silicon protective layer before finishing.

Project management at DAI

J. Michałowski

Project timeline

2011 – 2014
Development of first generation  of 1200 mm hexagonal mirror technology, weight 40 kg, thickness 80 mm, completed by installing four pieces on the MST prototype at DESY Zeuten.

2014 – 2016
Development of second generation of hexagonal mirror technology with a dimension of 1200 mm, weight 32 kg, thickness 60 mm, completed by launching the production line at DAI. 100 manufactured mirrors were sent to DESY Zeuten for another MST prototype and comprehensive testing.

2017-2019
In cooperation with DESY Zeuten and CEA Saclay, the development of technology of third generation of hexagonal mirrors measuring 1200 mm, weight 19 kg, thickness 40 mm, ended with the production of 12 mirrors sent to DESY Zeuten for comprehensive tests.  

More

www.obserwatorium-cta.pl

Publications

2009 – 2016
2017 – 2021