This is an old revision of the document!
Judit email of 8 Sep, summarising previous discussion:
First thing to decide is whether we want to use the beacon for slow WFS in the lab, or not.
MJI: If NO for bluer stars and during system on-sky calibration and YES at other times: All questions from YES plus (4) from NO.
Laszlo email of 7 Sep:
On the other hand if John is right and the lab seeing is fast and we want to correct that, a laser beacon could provide much faster WFS. These two approaches are not mutually exclusive, of course. Initially we will probably use starlight because the beacon won't be ready. Laser diodes are available from about 400 nm to 2000 nm for under $1k and the most commons are less that $40. With driver and optics the beacon could be as little as $1.5-$2k.
MJI notes that the most expensive part is probably a collimator to get a high-quality wavefront with 125mm beam.