A CCD based curvature wavefront sensor for adaptive optics in astronomy
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A CCD based curvature wavefront sensor for adaptive optics in astronomy |
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| Advances in Charge-Coupled Device (CCD) technology motivated an investigation of the use of a specially designed CCD as the wavefront sensor detector in a 60-element curvature AO system. A CCD has never been used before as the wavefront sensor in a low light level curvature adaptive optics system. A CCD can achieve nearly the same performance as APDs at a fraction of the cost and with reduced complexity for high order wavefront correction. Moreover the CCD has higher quantum efficiency and a greater dynamic range than APDs. A readout noise of less than 1.5 electrons at 4000 frames per second was achieved. A back-illuminated thinned version of this CCD can replace APDs as the best detector for high order curvature wavefront sensing. |
Curvature AO-systems have traditionally used Avalanche Photo Diodes (APDs) as detectors. APDs are photon-counting devices that produce a pulse on their output whenever a photon is detected. APDs have no read-out noise and read-out is almost instantaneous. However, they have some serious drawbacks compared to Charge-Coupled Devices (CCDs):
Small dynamic range. For the duration of the avalanche of electrons - typically around 40 ns - the APD is blind to any new photons arriving. This limits the dynamic range of the APD and calls for the use of neutral density filters to adapt the incident photon flux to the dynamic range of the APD. CCDs, on the other hand, have an enormous dynamic range and thus require no filters.
Low quantum efficiency. The quantum efficiency, which is the probability that an incident photon is detected, is lower for APDs than for CCDs. APDs typically have a peak quantum efficiency of 70 % at 700 nm, while the same number for CCDs is over 90 %.
High dark current. APDs generate dark current, or false photon counts, of 100 to 250 counts per second depending on the cost of the APDs, while a well-cooled CCD generates a negligible amount of dark current. The dark current affects the performance of the system when faint guide stars are used.
High cost. A 60-element AO-system using APDs is significantly more expensive than one using a CCD. The drawbacks of the CCDs are their read-out delay and read-out noise.