We need a detector that meets the following requirements: large field (~10'x10'), high resolution (0.14"), and high cadence (readout time less than 0.5 second), but also very short exposure time (a few milliseconds). We considered CMOS detectors for some of their intrinsic qualities: fast readout, easy control and potentially low cost. Furthermore we plan to build a mosaic that will cover the entire Sun (30'x30') with the same resolution, which requires a 165 Mpixels detector. For low exposure time an electronic shutter is essential as it is impossible to have a large and fast mechanical shutter with a sufficient MTBF for a continuous acquisition. Low power consumption is also advantageous for a mosaic.
We chose the IBIS16000, a 3952x4064 CMOS Active Pixel Sensor from Fillfactory (see Fig. 1). When processed with the tower TS50 TLMDP method this sensor has a pixel size of 12x12 ^m2 and on chip double sampling correction circuitry to remove the fixed pattern noise caused by non-uniformities in threshold voltages of the readout electronics [3,4].
The sensor is read out through four parallel analog outputs, which can be multiplexed to one single output. The gain of the output amplifiers is digitally controllable by the user.
This sensor has three transistor active pixels with maximized fill factor. It allows several options: subsampling readout modes in x and y, rolling shutter, programmable gain and offset output voltage.
Table I shows characteristics given by the manufacturer. We intend to confirm theses with a complete characterization of this detector in the near future.
To control the CMOS sensor we use a customized timing board of a classical ARC Genii controller. We have developed a four-channel analog converter and LVDS interface board (see Fig. 2).
This unique architecture allows us to link the ADC output to the acquisition system (4^10 MHz) in parallel, avoiding the rate limitation of the normal ARC controller link to the host (12.5 MHz).
The DSP code allows us to read the detector with global or rolling shutter and generates the control signals for the frame grabber.
Resolution Pixel size Spectral sensitivity Peak QE * fill factor Fill factor
Photodiode capacitance Conversion gain Output voltage swing Full well charge Linear full well charge Temporal noise at 21°C Signal to noise ratio Dark current at 21 °C Pixel rate Outputs Frame rate Supply Voltage Reset Voltage Power dissipation
4 or 1 (multiplexed) 2.5 frames/s
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