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Sodium Laser Guidestar Return Flux Measurement Guidelines

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:

Basic Observation and Reporting Guidelines

  • The units of reporting shall be collected photons per collecting millisecond per delivered Watt per collected square centimeter (ph/ms/W/cm^2); where delivered Watt is average power delivered to the mesosphere, and collected photons are calculated to be also collected (e.g. just below) the mesophere. (This takes out atmospheric and optics transmission losses in both directions.)
  • Measured photoflux should be made with a high-quality scoring camera, used at or near Nyquist sampling, corrected for sky background and calibrated to at most a few % residual flat-fielding variations
  • Downlink transmission should be calibrated and reported using known photometric standards (preferrably UKIDSS standards)
  • Observers should endeavor to constrain basic observations within 5 and 10 degree zenith angle (80 - 85 degree elevation angle)
    • Investigations of optical pumping vs. Earth's magnetic field angle should be reported as close as possible to either 0, 30, 45, 60, or 90 degree projection-to-local magnetic flux line angle
  • Any external estimates of sodium abundance should be noted as a comment to each measurement

Seasonal Average Reporting

  • Each participating observatory should collect uniform data over an entire annual cycle to obtain an annual average
    • A result shall be considered provisional until at least 10 separate nights of photoreturn are measured, calibrated, and averaged
    • Results shall be reported including the standard error over the discrete night ensemble used in the calculation (such as 15.2 +- 2.1 ph/ms/W/cm^2)
    • Following the >10 measurement rule per bin, the following seasonal bins should be compiled (Jan-Mar, Apr-Jun, Jul-Sept, Oct-Dec).

Return Worksheet

The following table may be helpful in regularizing photoreturn measurements:

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
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