CCD performance compared to APDs

A common measure used to quantify the performance of an AO-system is the Strehl ratio of the corrected Point Spread Function (PSF). To compare the performance of the CCD and APDs, a computer model of the atmosphere, telescope and AO-system was developed in MatLab. Using the computer model, the Strehl ratio in K-band was measured for guide stars of magnitude 10 to 18 and compared for the different detectors. The CCD performs as well as APDs over the entire range of magnitudes down to very faint guide stars at magnitude 18. The plot shows a small difference of the performance of the CCD of 3 % at magnitude 15 compared to APDs.

Property APD CCD

Sky background 19.0 19.0

Field of view diameter 2" 2"

Quantum efficiency 70 % 80 %

Dark current [electrons/s] 250 0

Read out noise [electrons RMS] 0 1.5

Read out delay [盜] 0 250

Seeing [arcsec] 0.65 0.65

Property	APD	CCD
Sky background	19.0	19.0
Field of view diameter	2"	2"
Quantum efficiency	70 %	80 %
Dark current [electrons/s]	250	0
Read out noise [electrons RMS]	0	1.5
Read out delay [盜]	0	250
Seeing [arcsec]	0.65	0.65

The CCD achieves nearly the same performance as APDs.

Thinned versions have the potential to work as well as APDs with reduced cost and reduced complexity.
No neutral density filters needed (simpler).
The CCD provides a much greater integration area per subaperture (360 痠 x 360 痠)
CCD has a higher quantum efficiency than APDs
CCD has a greater dynamic range than APDs (factor 1000)
Read out noise of less than 1.5 electrons at 4000 frames/s has been demonstrated
Curvature CCD combines high order curvature sensing with the possibility of separate tip/tilt sensing in one sensor

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Last update: Jan 27, 2003
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