The OPTICON JRA2 science working group set the top level requirements after carefully considering the needs of AO systems for future instruments and their science programs. The following detailed requirements were established:
1. Big pixels, square 24 ^m (goal), to ease the optical system design, but not too large to produce excessive Dark Current (DC) or CTE problems.
2. Versatility of a 100% fill factor and 240x240 square grid array of pixels that can be used by any WFS systems: SH, curvature, or pyramid, with or without gaps (guard bands) between subapertures.
3. Format size of 240 pixels being a number that is divisible by the number of output nodes, 8, and binning factors and aperture sizes of 1, 2, 3, 4, 5, 6 and meets the minimum pixel requirement of 40 subapertures x 6 pixels/subapertures.
4. Low read noise of <1 e-/pixel and goal of 0.1 e"/pixel.
5. Range of operating frame rates from 25 frames/s (fps) for use when photon starved with faint-NGS (Natural Guide Star) to highest sampling rate of 1,200 fps for use with bright-NGS and LGS (Laser Guide Star).
6. Easy to use; eight output nodes each operating at maximum pixel rate of 15 Mpixel/s, that provide a good compromise between the number of connections between the detector and the outside world and operational practicalities such as power dissipation, pixel rates and clocking rates.
7. Low image smearing (<5%) when transferring from the image to store area; an undesirable effect that can be corrected.
8. Cosmetic defect free, as every defect will either complicate the centroiding or make it impossible to centroid a subaperture.
9. Good spatial characteristics, PSF <0.9 pixel FWHM over 460 to 950 nm, to accurately determine where the photons are detected.
10. Very low DC, of <0.01 e-/pixel/frame at 1,200 fps and <0.04 e-/ pixel/frame at 25 fps to minimize large errors introduced by the quantum nature of DC; as the electron is the smallest unit. A single electron of DC creates a large error when centroiding on a small number or single photon event. DC includes contributions from the clock induced charge (fixed amount per frame readout), the image area during exposure (^ exposure time), the frame store and the serial register during readout (^ frame read out time).
11. Peltier cooled package for small compact size, maintenance free, and minimal support equipment so that the final assembled camera system can fit in the small volumes usually reserved for AO systems.
12. Detection signal limit of 5 ke-. In normal operation, the system will be photon starved without too many bright NGS and LGS will be operated at as low a power as possible. Well depth and output amplifier dynamic range can be traded to improve other parameters such as higher gain of output amplifier and lower clock amplitudes to transfer charge.
13. Linearity of <2%. Analysis shows that this level of nonlinearity introduces insignificant errors. Nonlinearity can be corrected by a look-up table.
Was this article helpful?