New low noise CCD cameras for "Pi of the Sky" project

Transcription

1 New low noise cameras for "Pi of the Sky" project G. Kasprowicz d, H. Czyrkowski a, R. Dabrowski a, W. Dominik a, L. Mankiewicz c, K. Pozniak d, R. Romaniuk d, P. Sitek d, M. Sokolowski a, R. Sulej e, J. Uzycki f, G.Wrochna b a Institute of Experimental Physics, Warsaw University; b Soltan Institute for Nuclear Studies, Warsaw; c Center for Theoretical Physics PAS, Warsaw; d Institute of Electronic Systems, Warsaw University of Technology; e Institute of Radioelectronics, Warsaw University of Technology f Faculty of Physics, Warsaw University of Technology; ABSTRACT Modern research trends require observation of fainter and fainter astronomical objects on large areas of the sky. This implies usage of systems with high temporal and optical resolution with computer based data acquisition and processing. Therefore Charge Coupled Devices () became so popular. They offer quick picture conversion with much better quality than film based technologies. This work is theoretical and practical study of the based picture acquisition system. The system was optimized for Pi of The Sky project. But it can be adapted to another professional astronomical researches. The work includes issue of picture conversion, signal acquisition, data transfer and mechanical construction of the device. Keywords: GRB,, low noise cameras, FPGA, USB, Ethernet 1. THE NEW APPROACH TO FAST OPTICAL TRANSIENTS DETECTION In recent years astronomy evolved towards observations of dynamic objects varying in short (from the human point of view) time scales. Optical counterparts of gamma ray bursts (GRB) stand for a good example of such phenomena. These events occurs a few times a day and are placed in isotropic way in the sky. This implies simultaneous observation of large areas of the sky. The time of gamma emission varies between milliseconds and minutes, hence it is important to observe the optical counterparts with a detector having time resolution of the order of single seconds. The high optical and time resolution of detectors generates large amount of data. The cameras are ideal candidates as detectors. This data stream is then processed on-line in order to reduce the amount of data. The approach should increase the overall efficiency of the detector. The Pi of the Sky project [3] uses methodology described above. The project consists of 32 cameras pixels each, working in parallel, covering large part of the sky. Lenses with focal length of 85 mm give about 0.6 arcmin of resolution [1]. 2. THE CAMERAS There are many models of astronomical cameras produced and available on the market. But they have several disadvantages that seriously limit their usage in this kind of experiment. The first factor is their price, which would limit the possibility of project realization. Another reason is not sufficient reliability of internal mechanical shutter. They are rarely equipped with features like: lens heating, temperature and humidity sensors, remote lens focus adjustments. The ADC resolution usually does not exceed 12bits at 1MHz or higher readout frequency. These factors

2 forced us to develop new cameras, better suited to the requirements of Pi of the Sky project. The construction presented in this work is continuation of previous generation of USB cameras published in [5]. The features of the cameras are following: Remote control of all the functions: readout frequency, gain, temperature, mechanical shutter, lens position. Remote monitoring of atmospheric conditions. The temperature and humidity is measured inside sensor chamber and outside device. There is also monitored temperature of electronic board. The humidity of is a relevant factor, because sensor works at low temperature, which may cause water condensation. This can damage costly sensor. Possibility of remote firmware upgrade (upc program and FPGA configuration). Watchdog Timer which prevents electronics against unwanted conditions. Electrical specification: Sensor STA0820, resolution pixels, 15 15µm each Dual channel readout Readout time : 1s 1min Noise: < 12e - at 2 MHz readout frequency, < 10e - at 1MHz ADC: 16bit (AD9826) Interfaces: USB2.0, max transfer speed: 52MB/s, Gigabit Ethernet UDP protocol, 100MB/s max Mechanical specification: Active thermo-electrical cooling, 35 below ambient, equipped with local heat sink with fan. Optional lens heating which prevents against water condensation High durability (>10 7 cycles) mechanical shutter. Lens focus remote regulation Separate sensor chamber filled with a noble gas Immunity to the weather conditions present at the observatory site. 3. HARDWARE DESCRIPTION Camera electronics is based on FPGA technology (Fig. 1). sensor Analog frontend USB 2.0 interface Memory FPGA Gigabit Ethernet interface sensors M2 M1 Power supply drivers TE cooling Fig. 1 Camera electronics block schematic

3 This approach enables easy and seamless modification of device functionality and significantly reduces components count. In prototypes there Altera Cyclone devices were used, which Active Serial configuration scheme, resulting in further simplification of the circuit. Inside the FPGA the following functional blocks are implemented: SDRAM memory controller readout controller sensor and video processor control signals generator USB transmission controller Gigabit Ethernet PCI master/slave control circuit with RX/TX data buffers IP/UDP/NUDP protocol accelerator SHT11 temperature and humidity sensors readout block Control registers file The control functions are performed by Cypress FX2 microprocessor built into USB interface. It is enhanced version of popular 8051 and takes responsibility for the following functions: Interpretation of commands from host Shutter and focusing motor control Sending device status to host Setting up transfer, video processor, exposition and readout parameters FPGA configuration update - Active Serial FLASH programmer Readout of temperature sensors Ethernet interface PCI master/slave state machine control ARP,IP,UDP,ICMP,NUDP protocol stack implementation It s architecture with program RAM allows easy firmware update directly via USB or Ethernet. During normal operation it loads the program from external EEPROM. 4. THE ANALOG SIGNAL PATH The block diagram of analog signal path is shown in Fig.2. The assembled board with the sensor is presented in Fig.3. x20 /x7 drivers sensor x20 /x7 Dual CDS & MUX ADC & processing Supply Drivers FPGA amplifier ADC buffers Fig. 2 Schematic of analog processing circuit Fig. 3 Assembled analog processing board The signal generated during readout is initially processed by a preamplifier. It performs the following operations: Signal DC restoration

4 Amplification (switchable) Reset pulse suppression It s construction is based on AD829J amplifier and BF862 JFETs. The next stage of processing is done by AD9826 video signal processor. It consists of 16bit 15MS/s ADC and triple analog signal processing path. The functions are the following: DC restoration Programmable DC subtraction Amplification (factor 1-6 controllable in 64 steps) Correlated Double Sampling Analog to digital conversion Digital bus multiplexing (to reduce number of wires) Output data are then processed by the logic in the FPGA. In the same circuit the signals are generated that control charge transfer in the sensor. After level conversion they feed MAX627 drivers which are directly connected to the electrodes. 5. THE DIGITAL SIGNAL PATH Block diagram of digital signal path is presented in Fig. 4. The assembled board is shown in Fig. 5. The 8 bit data from video processor is fed into FPGA and recovered to it s original 16bit form. Next, it is written into memory, by SDRAM controller. One full picture takes over 8MB. The data is there stored waiting for USB or Ethernet transfer request. When such a request occurs, the data is read from the SDRAM and transferred into the USB/ETH interface. Here, the data is divided into 512B packets for USB (1024B for Ethernet) and sent to the host. CDD & VIDEO processor control SDRAM & USB controller FPGA PCI interface ADC Demux Gigabit Ethernet MAC+PHY SDRAM 8Mx16 USB2.0 microcontroller Fig. 4 Digital processing circuit Apart from digital signal path, the following additional circuits are mounted on the main board (Fig. 5) : Main power supply (analog path, digital logic, drivers) power regulators drivers power supply Motor driver for lens focus adjustment Shutter motor driver

5 TE cooling modules drivers Lens heating drivers SDRAM FPGA Gigabit MAC+PHY DC/DC 3.3V Ethernet trafo supply Connectors USB & Eth USB2.0 interface Motor driver Linear motor Motor supply Shutter Frame Supply drivers Fig. 5 Assembled main board Fig. 6 The shutter 6. MECHANICAL CONSTRUCTION The frame of the whole construction is specially fabricated core. It forms a gas chamber in the front side, and heat transmission to the heat-sink at the rear side. Inside gas chamber, just over analog board, the mechanical shutter is mounted. It was developed specially for this camera. The construction is based on linear motors used to position heads in HDDs (Fig. 6). They were brought through endurance tests, where withstood over 10 7 open-close cycles. There is under development new control circuit with capacitive sensor feedback which will improve shutter endurance eliminating impacts with bumpers. Fig. 7 Assembled camera during tests in laboratory

6 Two complete cameras were manufactured and are currently tested (Fig.6). The production of whole set had already been launched. The first test that were performed results in % lower readout noise in comparison with previous camera prototypes installed and tested in Las Campanas observatory. 7. TESTS & RESULTS Fig. 8 Projection of old cameras Fig. 9 Projection of new cameras

7 First test result in improvement of noise factor of analog signal path. In Fig. 8 and 9 can be seen projections (X projection values in each column, Y projection values in each line) for dark frame of old and new cameras. Thanks to improved DC level restoration at the input of LNA the gradients ware significantly reduced. There were added digital buffers at the output of ADC which improved noise immunity of whole analog block. Fig. 10 Histogram of dark frame Fig. 10 presents histogram of dark frame from new camera. RMS value is 25,44 ADU which corresponds to about 8e- of overall camera noise (Gain = 8). There was achieved further improvement in TE cooling, the current value of 35 below ambient further reduces dark current generation. 8. CONCLUSION The two-cameras prototype was successfully produced. The full electrical and functional tests were performed. In nearest future there will be made last tests needed to calculation the best conditions of cameras work. They are the prototypes of the final apparatus, which will consist of two sets of 16 cameras each. The results of tests allowed us to start the production of first set of 16 cameras. The Electronic and Mechanical parts are in final phase of production. Final tests of 16 camera-system are planned to be performed in Autumn 2006 The previous version of detector, after successful tests in Poland, was installed in Las Campanas, Chile in a special dome used by ASAS experiment. During over one year of work, each camera captured over images of the sky. Several optical flashes have been observed. No confirmed GRB afterglows were detected, although 2 of them were in the field of view. Limits on optical counterparts of these GRB have been derived and published [1]. In addition, observations of variable stars, meteors and other phenomena were conducted. ACKNOWLEDGEMENTS This work was financed by Polish Ministry of Science in as a research project. The authors acknowledge the support of B.Paczynski, G.Pojmanski and Las Campanas Observatory team. REFERENCES 1. M. Biskup et al., Study of rapidly varying astrophysical objects with the "Pi of the Sky" apparatus, these proceedings 2. NASA web pages -

8 3. A.Burd et al., Pi of the Sky - all-sky, real-time search for fast optical transients, New Astronomy, Volume 10, Issue 5 (April 2005), pages Grzegorz Wrochna, Cosmic perspectives of particle physics, Proc. SPIE Vol. 5125, p , Oct A. Burd et al., Low noise cameras for wide field astronomy, Proc. SPIE Vol. 6159, pp , 2005.