1. AST3 detector properties
Ma Bin (NAOC)
2015 AST3

2. OUTLINE
1 AST3 CCD
2 CCD test results
3 Nonlinear PTC
4 Current status

3. 1 AST3 CCD STA 1600FT 10560 * 10560 pixels Pixel size 9μm -> 1”
thermoelectric cooling (TEC)

4. Frame transfer -> FOV 4.3deg2
16 readout amplifiers
Slow: 40s; fast 2.5s

5. 2 CCD test results To check the overall performance
Fridge: producing different environment temperatures down to −80C
Light source: a LED lighting through several layers of white paper

6. Linearity Signal level .vs. Exposure time
Full well capacity > 100,000 e−

7. Photon Transfer Curve (PTC)
gain: e- -> analog-to-digital units (ADU)
pairs of flat frames with various signal levels
Variance: photon shot noise + readout noise σ2= N/g + σ2rd/g2
1/g is the slope of the variance-signal plot
g ~ 1.64 e-/ADU

8. Readout Noise RMS of the overscan Slow: 4 e-; fast: 9-12 e-
sky brightness (AST3-1 in 2012) 8e-/sec for 60sec exposure, photon shot noise 22 e-
Fast mode is used for observation

9. Dark Current thermal electrons
decreases by half as the temperature is lowered every 7.3C

10. Charge Transfer Efficiency
CTE: the fraction of charges transferred from one pixel to the next during readout
Extended Pixel Edge Response (EPER): excess charges found in the overscan

11. 3 Nonlinear PTC
Downing+06 reported this effect, and found it was caused by signal correlation between pixels
Downing & Sinclaire (2013): charge diffusion due to the Coulomb force of stored charges (charge sharing)
Antilogus+ 14: effective pixel boundaries shift; predicting brighter-fatter effect from PTC
Downing & Sinclaire (2013)

12. We proposed a simple model, named charge sharing PSF, assuming charge sharing fraction as a function of signal level
AST3 CCDs show significant signal correlation between a pixel and its neighbors: (0,±1)(0, ±2)(±1,±1)
Deriving charge sharing PSF from PTC, then estimating the effect on real image

13. Profiles of stars (FWHM, elongation) depend on their brightness, biasing photometry and shape measurement.

14. 4 Current status CCD#1 (engineering grade) on AST3#1 in 2012
In Jan 2015, 31th Chinese Antarctic Research Expedition (CHINARE) team deployed AST3#2 with CCD#2, and replaced CCD#1 with CCD#3
Realtime status is shown on website

16. CCD temperature control
Original TEC control makes t_CCD oscillate around setpoint with a amplitude of 4 degrees
Prof. Ashley has done a great job to keep it much more stable (~0.2 degree)

17. The heat from CCD chip by TEC is not removed efficiently, so TEC cannot cool CCD very much
T_CCD is about 10 degrees above ambient temperature
Dark current 1.7e-/sec (CCD#1), 0.29e-/sec(CCD#2)
sky brightness (AST3-1 in 2012) 8e-/sec
Noise from dark current is expected to be insignificant

18. Thank you !