|
Feature: Laser Guide Star, LGS
Can be propagated at a fixed altitude-azimuth position (telescope stationary)
or a specified RA-Dec position (telescope tracking). It is NOT possible to
set on a star, then stop tracking and keep the laser propagating.
The mean extinction at Lick Observatory for 584 nm is 0.166 mag/airmass
(i.e. at zenith the transmission is cicra 85 per cent at 589 nm for our
laser beacon). The detailed table of information is at the following URL:
https://mthamilton.ucolick.org/techdocs/standards/lick_mean_extinct.html
Seeing conditions do not affect the total number of photons, but in poor
seeing the photons are spread out over a larger area, so the surface
brightness will be less. Our LGS in good seeing tends to be at least
1.5 arcseconds FWHM, but will expand in bad seeing conditions to be
whatever the seeing FWHM is.
LGS Pulse
Pulsed at a fixed rate of 13 kHz, and currently there is no syncing possible
to eliminate the rayleigh scatter.
The beam does get modulated but this is mainly for spectral broadening to
better match the sodium profile at the mesosphere.
While the laser is on, "tuning off" is possible, which extinguishes
the beacon and allows for sky background measurements.
LGS Magnitude
The LGS spot is approximately the brightness of an 11 or 12 magnitude star.
The magnitude variability depends on the power of the laser and the sodium
abundance in the atmosphere.
We do not routinely measure atmospheric conditions such as Cn^2, usually
just measuring the seeing occasionally during the night. We do track
general weather outside the dome (temperature, wind speed, humidity,
particle counts). The brightness of the laser guide star is affected
by particles (smoke, pollen, water vapor, etc.), reducing the brightness
of the laser guide star due to more Rayleigh scattering from the particles
before the light gets to the sodium layer.
LGS Zenith Distance Limits
FAA regulations limit us to Zenith distances less than 45 degrees.
LGS Cone
Permitted USAF Space Command LGS cone: 6 arc minutes
(±3 arc minutes from target position)
Radar Cone
Radar cone: 7.5 degrees (as specified in 2018 Notice of Proposed Outdoor
Laser Operations Document).
LGS Power
Experience (2017-08-04 and 2017-08-05) under otherwise good conditions
(i.e. clear transparency, seeing circa 1 arcsec, source zenith distance
less than 5 degrees) shows that ShaneAO system receives insufficent
LGS signal in WFS in order to close AO loops in 16x mode when
the LGS power falls in the range 2.75--2.30 W.
Change Log:
- 2021-08-23: Updated.
- 2017-08-05: Updated.
- 2017-01-14: Updated.
- 2015-03-07: First version.
|