The single mirror 4m Davies-Cotton SST project
Our group is actively working on the design and construction of on of the prototypes for the Small Size Telescopes (SST). These telescopes are conceived to cover the highest energy range of CTA between a few TeV and 300 TeV. Our prototype uses the Davies-Cotton design for the optics and telescope structure, which is a standard and already proven design together with an innovative camera using Silicon-Photomultipliers (SiPM) for the photosensors reducing its cost while increasing the observation time of the telescope. Unlike photomultipliers SiPM can operate during high levels of moonlight which is a key factor in the high energy region where exposure is more important to improve the sensitivity than background reduction. Our SST-1M design has a focal length of 5.6 m, dish diameter of 4 meters, Field-of-View (FoV) of 9 deg. and pixel angular opening of 0.24 deg.
The prototype structure of this 4m Davies-Cotton telescope was installed in Krakow. The total weight of the current design is about 8.6 tons while the camera weighs in total 200 kg. The compactness of the SST-1M design is an advantage for transportation to the site while the solidity of the frame makes it suitable even for sites well above 2 km of altitude. Currently this design has the lowest cost among SSTs proposals.
The project is run by 7 Institutions for the University of Geneva and a sub-consortium of Polish institutions.
The mirror will be composed of 18 hexagonal facets with dimension flat-to-flat of 78 cm and will offer a total effective collection area of 6.47 m2 after removing shadowing due to mast and camera chassis, taking into account inclination of facets and considering the reflectance effect of the mirror coating with AlSiO2 which between 330 and 600 nm is 0.87 ± 0.01.
The camera is composed of two elements that required considerable R&D: the photo-detection plane (PDP) composed of custom made hexagonal SiPMs, produced by Hamamatsu in collaboration with DPNC, and a fully digital readout and trigger electronics, DigiCam. The PDP contains 1296 pixels that couple the sensors to an optimized optical light concentrator with a dichroic coating. The reflective coating was selected so that its maximum reflectivity is below 400 nm and for large incident angles. A filter on the entrance window of the camera will cut-off wavelengths (> 550 nm) where the night sky background (NSB) and albedo dominate.
The optical system forms the optical path for the Cherenkov light emitted by showers in the atmosphere which needs to be concentrated on the camerato be recorded.
The Davies-Cotton geometry provides the best imagining over a large field of view without introducing too much time dispersion in small sized telescopes which make an optimum solution for the SSTs. The primary mirror consists of 18 hexagonal facets of 780 cm flat-to-flat arranged in two concentric rings. Together they form a reflecting spherical dish of 4 meters diameter. Glass mirrors are the baseline technology for these facets. The reflectivity of these mirrors with the proper coating might achieve 90%. Another approach using Sheet Moulding Compound (SMC) composite mirrors is also foreseen. SMC is a low-cost, lightweight, widespread and semi-fabricated product used for compression
The mirror fixation is based on a 3-point support enabling secure fixing to the telescope structure. The final orientation of the mirrors with respect to the camera is done using the alignment system. The orientation of each mirror can be modified during observations using the actuators, if necessary.
The SST-1M camera uses a novel approach of having the photo-detection plane (PDP) and the digital readout electronic (DigiCam) as two physically separate entities.The camera will be equipped with 1296 pixels organized in 108 modules of 12 pixels each shaped into a hexagon which is about 1 m flat-to-flat and about 60 cm of thickness. The PDP follows the same geometry while the DigiCam is distributed in 3 mini-crates hosting its functional boards. Each mini-crate collects the data from a specific sector of the PDP.
The photosensors on the PDP will be coupled to a hollow optical concentrator in order to increase its collection area and the lateral shape of the concentrator was optimized to achieve the optimal efficiency. The angular entrance size of the light concentrator (angular pixel size) was selected based on the required angular resolution A value that is 4 times the Root Mean Square (RMS) of the telescope Point Spread Function (PSF) was selected in order to guarantee the inclusion of 95.4% of the photons from a sources in a single pixel (p = 0.24 deg.). A smaller angular pixel size will increase the number of pixels on the camera without any benefit in the sampling of the images while a larger pixel size will spoil the angular capabilities of the telescope. The light concentrators have a cut-off angle of 24 deg. guaranteeing that all the light reflected from the primary mirror will arrive the sensors.
The sensors are custom made hexagonal shape SiPM from Hamamatsu design in cooperation with DPNC. These are the largest devices of this type with an active area of 93.56 mm2 divided in 4 channels. The 50 micrometer cell size offer a 30% photo detection efficiency ( < 40% in latest versions of the sensor). They also have a cross-talk probability of 20% and an operational voltage of 70V (65V for the newer version).
The whole camera will be sealed with an entrance window for protection against water and dust. The window will be made of a hexagonal Borofloat 33 (3.3 mm thick) borosilicate glass from SCHOTT. It will be coated with a dichroic wavelength filter to reduce the amount of night sky background entering in the camera. Additionally and anti-reflective coating will be applied to increase the overall transmissivity of the entrance window.
Construction at UniGe
Our group constructed and assembled the camera parts early 2017. The camera has gone through the calibration and the commissioning phase with intensive tests on the temperature control, cooling process, pixel monitoring, trigger rates... And here is the camera ready for shipping to Poland!! It will be mounted on the telescope structure in Krakow where the entire ensemble will be tested. Exciting times ahead with the first images of the sky!