## 2 Hardware of Transient Luminous Event (TLE) Observation System
### Hardware Component  
The hardware of the TLE observation system is composed of six main components: an SoC chip, a multi-anode photon-multiplier tube (MaPMT), a high voltage supply for the MaPMT, an analog-to-digital converter, a CMOS image sensor, and two synchronous dynamic random-access memories (SDRAMs) as video buffer. The SoC chip, Cyclone II SoC by Intel (Santa Clara, CA, USA) is chosen as the main processor since it has sufficient on-chip memory and user-configurable I/O pins, which can deliver high performance and low power consumption at a reduced cost. To detect ultraviolet (UV) photon from TLEs, the Hamamatsu H7546B MaPMT [30] is adopted because of its high responsivity and sensitivity on a single UV photon. The MaPMT has a wide spectral range from 185 nm to 650 nm and a peak response at 420 nm as specified in the datasheet [31]. It will be triggered by any flash lights that exceed the threshold of the detector with similar spectral characteristic. Nonetheless, the intensity level of a UV photon due to TLEs is known to be much higher than the level of other similar phenomena [32], and the MaPMT is assumed only to detect the TLEs by capturing the UV photon derived from a TLE occurrence. Furthermore, as shown in Figure 3, the MaPMT is a 2-dimensional (2D) sensor providing continuous UV intensity information of 64 cells arranged in an 8 \(\times\) 8 array. A pin-hole lens with an aperture of 300 \(\upmu\)m is utilized to get focused over the specified field of view (FOV). The aperture of the pin-hole lens is selected to allow sufficient photons to pass through. In Section 4, an analysis on the number of UV photon arriving at the MaPMT taking into account the size of aperture of the pin-hole lens is discussed. Because the output of the MaPMT is the 64 channels of analog current signal, an external analog-to-digital converter is required to obtain digital data. The sensitivity of the MaPMT output can be configured externally by adjusting the negative supply voltage to the MaPMT. Since the main power supply of VisionCube is relatively low voltage, a DC to high-voltage DC converter unit Q10N by XP Power (Singapore) is used to supply the variable voltage down to negative 1000 V. The output of the high-voltage converter is directly proportional to the analog command input provided by an external digital-to-analog converter (DAC) in order to set the sensitivity level of the MaPMT. The DDC264 [33] is a 64-channel, current-input analog-to-digital converter from Texas Instruments (Dallas, TX, USA), which provides simple and reliable conversion of consecutive channels in a small form factor. Because the DDC264 uses the dual switched integrator front-end at each channel, continuous current integration is possible without charge leakage. The integration times can be adjusted ranging from 160 \(\upmu\)s to 1 s, enabling currents from femto-amperes to micro-amperes being continuously measured with outstanding precision [33]. As the mission requirements of VisionCube is to acquire an image of a TLE occurrence, a CMOS image sensor MT9P031 from ON Semiconductor (Phoenix, AZ, USA) is chosen to capture the digital images of TLEs. The imager is an 1/2.5-inch image sensor with an active 5 Mega pixel array of 2592 H \(\times\) 1944 V. Besides the merit of size, cost, and integration advantages, the CMOS sensor achieves CCD image quality in terms of signal-to-noise ratio and low-light sensitivity at the maximum pixel rate of 96 MHz [34], which allows the frame rate of gray-scale VGA images (640 \(\times\) 480 px) to be up to 40 frames per second (fps). The spectral response of this sensor is specified from 350 nm to 1050 nm. However, because the spectral response of blue pixels of the imager has a common response from 350 nm to 550 nm with the MaPMT, the TLE observation system is designed only to collect the blue pixels for a clear distinction of TLEs. Finally, the system has two SDRAMs, one with 512 Mbit and the other with 64 Mbit.