PERFORMANCE OF THE COROT CCD FOR HIGH-ACCURACY PHOTOMETRY

摘要

The focal plane of the COROT instrument is made of four CCDs, two dedicated to asteroseismology and two dedicated to the detection of telluric planets. The detectors are provided by E2V (4280 series), each having 2k x 4k pixels. They work in AIMO and frame transfer mode, at a working temperature of -40 ℃. As the COROT photometer will have to detect fluctuations expressed in ppm (part per million), a specific calibration of the whole photometric chain has to be achieved. Moreover, we have ten CCDs available and therefore need to calibrate them in order to select four CCDs for the flight focal plane. A specific test bench is dedicated to the calibration of these CCDs, and five have already been tested. The main characteristics of interest are: 1. cosmetics (black and white pixels or columns): zero white pixels, less than 6 black columns; 2. pixel response non uniformity (PRNU) versus wavelength: 1 % rms at 650 nm. 3. gain versus temperature: -900 ppm K~(-1) relative fluctuation 4. absolute quantum efficiency: 95 % max at 650 nm; 5. quantum efficiency versus temperature: 2000 ppm K~(-1) relative fluctuation at 650 nm; 6. full well capacity: 85 to 100 ke~-; 7. dark current at -40℃: < 0.5e~- pixel~(-1) sec~(-1). Comparing the measured characteristics to those provided by E2V, we first check that CCDs meet specifications. Then, when all CCDs are tested, we will select the best CCDs for each scientific program (asteroseismology or planet finding). Specific tools have been developed to compare the performances of the detectors using the images acquired on the test bench and the shapes of the PSF for the two scientific programs. The detectors have also been tested under irradiation; the results strongly depend on the specific orbit of COROT (irradiation dose and related particles) and have to be compared to the global performances of the instrument.