The equitable comparison of sodium laser guide star photoflux return is a notoriously difficult calibration. Dominated by uncertainty in the sodium atom density (know to fluctuate by factors of a few (hourly) to factors of several (seasonally)), but exasperated by many other observational parameters, it has been difficult for the LGS community to directly compare return and thus make progress in understanding the specific interactions between different laser pulse and line formats and the sodium layer (particularly affecting optical pumping effects.)
On March 26, 2007, at a workshop hosted by CfAO?, an ad hoc group consisting of Caltech, Chicago, ESO, Gemini, Keck, LMCT, NSF, and UC researchers suggested the definition of uniform observational guidelines for Na LGS photoflux return. The following guidelines have not yet been endorsed by this group:
Facility | Altitude (m) ASL | Laser Properties | Laser Power at Source | Total Transmission to Mesosphere (0.0 - 1.0) | Date | r0 @ 0.5 um (cm) | Observing Zenith Angle (deg) | Spot FWHM (arcsec) | Physical Spot Area at Mesosphere (4 ln 2 * 3.14 * FWHM^2) in cm^2 | Measured scoring photocount (ph/ms/cm^2) | Total Downlink Transmission | Photoreturn at Mesosphere (ph/ms/W/cm^2) | Comments |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Palomar | 1760 (TBC) | 1 ns micropulse inside 300 microsec bursts at 600 Hz repetition rate; ~98 MHz line comb | 5.5 W (TBC) | 0.50 (TBC) | 9/21/06 (TBC) | 0.09 (TBC) | 8 (TBC) | 1.84 arcsec (TBC) | TBD | 45 (TBC) | 0.50 (TBC) | 30 (TBC) | Variable seeing |