Adaptive optics with a Laser For Astronomy

On the left is a short video showing ALFA's laser guide star on the wavefront sensor during June 1999. To the right is an image from March 98 for comparison. In both cases the 5x5 lenslet array was used (giving 18 subapertures), and the sampling rates were 100Hz. The video has 24 frames repeated, and so represents only 1/4 sec of actual time.

LGS: June 99
LGS video
LGS: March 98
LGS image

The pixel scale is about 0.82 arcsec and during a particularly stable period of the night such as this, the spots do not move more than this even while the AO loop is open.

The average jitter of the entire spot pattern is entirely atmospheric, a consequence of launching the laser off-axis from the main telescope. This can be seen better when the laser is imaged with a single lenslet.

To the lower left of the right-most spot it is just about possible to see some stray light. It is most likely from the top of the Rayleigh cone, and although faint it has a bad effect on the spot centroiding. It can probably be removed by careful adjustment of the field stop, which is currently positioned for viewing natural guide stars.