Ccd Optimized For Pulsed Laser Guide Star Wavefront Sensing

Our project team is developing a CCD that is optimized for the detection of short laser pulses that illuminate the sodium layer for wavefront sensing. The combination of this CCD and a pulsed sodium laser can overcome the problem of spot elongation that limits standard LGS AO systems. The spot elongation problem is shown graphically in Fig. 1. The LGS return is stretched in one dimension, due to the viewpoint of the subaperture. The laser guide star spot elongation is proportional to the distance that a subaperture is off-axis. Since the error of the tip/tilt measurement is proportional to the width of the LGS spot, the quality of wavefront measurement is significantly degraded for large off-axis distances.

Figure 1. Geometry of laser spot elongation, where L is the lower altitude of the sodium layer, AL is the thickness of the sodium layer, and d is the lateral separation between the laser and the subaperture. The horizontal scale is greatly exaggerated to demonstrate the geometry. The sodium layer is approximately 10 km thick, centered at 90 km altitude above sea level. An observatory is typically located 3-5 km above sea level. A laser guide star is generated by a thin cylinder of excited sodium atoms: ~50 cm diameter and ~10 km long. The extension of the sodium guide star return is ~2.5 arc sec for every 10 m that a subaperture is separated from the axis of laser projection.

Figure 1. Geometry of laser spot elongation, where L is the lower altitude of the sodium layer, AL is the thickness of the sodium layer, and d is the lateral separation between the laser and the subaperture. The horizontal scale is greatly exaggerated to demonstrate the geometry. The sodium layer is approximately 10 km thick, centered at 90 km altitude above sea level. An observatory is typically located 3-5 km above sea level. A laser guide star is generated by a thin cylinder of excited sodium atoms: ~50 cm diameter and ~10 km long. The extension of the sodium guide star return is ~2.5 arc sec for every 10 m that a subaperture is separated from the axis of laser projection.

Deanna Pennington and her colleagues at the Lawrence Livermore National Laboratory have been funded to develop a pulsed sodium laser with 1-3 ^sec pulse length. Other groups are also pursuing development of pulsed sodium laser guide stars. While the pulse travels through the sodium layer at the speed of light, it can easily be tracked on a CCD since the two-way path through a 10 km layer will take 60 ^sec at the wavefront sensor. The combination of a pulsed laser and a specially designed CCD will eliminate the laser spot elongation problem as shown in Fig. 2.

To track the laser pulse in the focal plane of the subaperture of a Shack-Hartmann wavefront sensor, we will use rectangular arrays of square pixels, but will align the orientation of the pixel array within each subaperture so that charge transfer follows the direction of spot travel (see Fig. 3).

Figure 2. A pulsed laser will significantly reduce laser guide star spot elongation. The diagram on the left shows the effect of perspective elongation on a continuous wave (CW) laser return, while the diagram on the right shows the tenfold reduction in spot elongation achieved by using a short (1-3 |isec) pulse of laser light.

Figure 2. A pulsed laser will significantly reduce laser guide star spot elongation. The diagram on the left shows the effect of perspective elongation on a continuous wave (CW) laser return, while the diagram on the right shows the tenfold reduction in spot elongation achieved by using a short (1-3 |isec) pulse of laser light.

Figure 3. The image array pixels for one subaperture of a pulsed laser guide star Shack-Hartmann wavefront sensor. Pixels within a subaperture are aligned with the orientation of the laser pulse travel. The laser pulse will travel in a radial direction away from the location of laser projection. Thus, the pixels of each subaperture will need to be aligned to different directions. An image array of 4 by 8 pixels are shown as an example, but most likely we will use 4^16 pixels per subaperture to accommodate variations in the sodium layer that can be up to 20 km thick. The size of each pixel is not finalized, but a possible scale is 0.5 arc sec square, in which case the image area per subaperture will be 2^8 arc sec.

Figure 3. The image array pixels for one subaperture of a pulsed laser guide star Shack-Hartmann wavefront sensor. Pixels within a subaperture are aligned with the orientation of the laser pulse travel. The laser pulse will travel in a radial direction away from the location of laser projection. Thus, the pixels of each subaperture will need to be aligned to different directions. An image array of 4 by 8 pixels are shown as an example, but most likely we will use 4^16 pixels per subaperture to accommodate variations in the sodium layer that can be up to 20 km thick. The size of each pixel is not finalized, but a possible scale is 0.5 arc sec square, in which case the image area per subaperture will be 2^8 arc sec.

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