Subaperture Design

The requirements above are further detailed in the subaperture design:

• Each subaperture will have image and frame transfer areas.

• Each aperture will have a dump drain for rapid flushing of the image and frame storage areas.

• Both image and frame storage areas will have 3-phase clocks that are independently programmable as a function of annulus (distance of the subaperture from the laser).

• Two-stage amplifiers will be placed at each subaperture. The second stage has the drive capacity to run at 1 MHz and faster readout rates, and can be turned off when not in use to save power.

• The center of the image area in each subaperture lies on a square 2-D grid.

• The orientation of the pixels in each subaperture lies along a radial vector from the center of the pupil.

• All image array pixels in the entire chip are square and of the same size - corresponding to 0.5 arc sec on the sky.

• A summing well will be provided to allow noiseless binning.

• An output select amplifier will be used to multiplex the signal on the video out "column bus".

Combining these requirements, we have developed the subaperture design shown in Fig. 3.

Figure 4. The subaperture design of the CCD we are developing to sense pulsed sodium laser guide stars on an ELT. Not shown in this schematic is the orientation of the pixels of each subaperture. Each subaperture will align the image array pixels with the direction of laser spot elongation, i.e. along the radial direction from the center of the telescope pupil (assuming laser projection from behind the secondary). The center of the image array of each subaperture will lie on a square 2-D grid. We expect that there will be 4^16 pixels in the image and frame storage arrays; only 4x8 pixels are shown in this figure. A computer simulation has been developed to model the performance of the wavefront sensor as a function of atmospheric seeing, pixel size, readout speed, readout noise and light level. This simulation is being used to optimize the number of pixels per subaperture and size of pixels projected on the sky (arc sec per pixel).

Figure 4. The subaperture design of the CCD we are developing to sense pulsed sodium laser guide stars on an ELT. Not shown in this schematic is the orientation of the pixels of each subaperture. Each subaperture will align the image array pixels with the direction of laser spot elongation, i.e. along the radial direction from the center of the telescope pupil (assuming laser projection from behind the secondary). The center of the image array of each subaperture will lie on a square 2-D grid. We expect that there will be 4^16 pixels in the image and frame storage arrays; only 4x8 pixels are shown in this figure. A computer simulation has been developed to model the performance of the wavefront sensor as a function of atmospheric seeing, pixel size, readout speed, readout noise and light level. This simulation is being used to optimize the number of pixels per subaperture and size of pixels projected on the sky (arc sec per pixel).

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