Rayleigh scatter rejection for NGAO: A Trade Study

1. Rayleigh scatter rejection for NGAO: A Trade Study NGAO Team Meeting #3 V. Velur 12/13/2006

2. Presentation Outline • Scope of this trade study • Estimation of the amount of Rayleigh scatter using classic LIDAR equations • Simplistic extension of the classic theory to NGAO’s LGS asterism geometry • Thoughts on more complex and elaborate models • Introduction to laser types • Comparison of various laser/ pulse formats • Comparison of various Rayleigh rejection techniques. • Other concerns - atmospheric scatter fluctuations over short and long time scales, effect of sub-visual cirrus, and effect of volcanic eruptions

3. Trade study scope • 3.1.2.2.5 Rayleigh Rejection - Evaluate the impact of unwanted Rayleigh backscatter to NGAO system performance. Consider the relative performance, cost, risk and schedule of various strategies for mitigation of LGS Rayleigh backscatter. Techniques include background subtraction, modulation and optimizing projection location. This issue is closely coupled to laser pulse format, with pulsed lasers generally providing more options for Rayleigh mitigation than CW lasers. Complete when NGAO baseline architecture selected. • NGAO’s baseline architecture requires multiple LGS’s and to enable productive science it is important to reject the scatter from the lower atmosphere or subtract it effectively. This scattered light is unwanted in all parts of the AO system. In particular visible WFSs. This trade study looks at quantifying, mitigate and may be even eliminate the effect of this unwanted scatter.

4. LIDAR equation Where, Assumptions: On-axis projection, U.S. standard atmosphere.

5. Simplistic modeling of Rayleigh scatter effect from a single laser beacon • Write down the LIDAR equation and integrate over z = 4.125-90 Km. • For each z we can associate a disk (not a doughnut) shaped image at the telescope/AO focal plane. The disks become progressively dimmer(overall power) and smaller with increasing altitude. • Generate intensity profile based on above approach assuming a top-hat return and no obscuration.

6. Rayleigh return as a function of altitude • Optical transmission = 0.5 • 1 way atmospheric Tranmission = 0.86 • Quantum efficiency of CCD = 0.9

7. Disk size at focal plane

8. Radial photon density (Nz/area of disk)

9. Intensity profile

10. More complex modeling (yet to be done) • Write down the LIDAR equation and integrate over z = 4.125-90 Km. • For each z we can associate a doughnut shaped image at the telescope/AO focal plane. The doughnuts become progressively dimmer and smaller with increasing altitude. • Generate intensity profile based on above approach assuming a Gaussian return and 1.4 m secondary obscuration. • Depending on the LGS asterism geometry we can add the effect of the doughnuts produced by each of the LGSs over the AO FoV. • The effect on each WFS is calculated by using the overall intensity distribution of Rayleigh scatter from each of the beacons. The Rayleigh background can be approximated to be the intensity over the FoV of the WFS at its position in the focal plane.

11. Laser types and pulse formats

12. Comparison of different lasers.

13. Comparison of Rayleigh rejection techniques

14. Projection location • On-axis projection gives rise to maximum Rayleigh background while it produces the least spot elongation effects on the farther most sub-apertures. • Irrespective of the projection location the BTO path needs to be baffled such that no laser light gets through to the AO system. • Off axis projection has serious problems - up to 1000 nm of WFE [van Dam et. al.], biases that creep in because of the changing Na-layer profile, and truncation because of field-stop size. The spot elongation approximately doubles when moving from 5 to 12 m separation between projection location and sup-aperture. Need for noise optimal estimators [Ellerbroek] (has it been tested on sky?, what factor do we gain?). Need for real-time bias control on extended pixels to deal with WFS non-linearity. • A trade off between sub-aperture selection/WFS noise and laser projection position was not considered in this study. This must be looked at in detail with any baffling options that can be implemented to mitigate the effect of Rayleigh scatter.

15. Problems suppressing Rayleigh scatter due to CW lasers • Currently the only technology mature enough to cater to the needs of NGAO is CW lasers. LMCT is a commercial outfit that is building 50W class lasers for adaptive optics. CW lasers are a much more mature technology to scale powers than untested pulsed laser formats. • If CW lasers were used for NGAO fluctuations in Rayleigh background, cirrus clouds and scattering due to aerosols are to be accounted in the error budget. The following slides show the effect of the 3 factors

16. Other concerns: scattering due to Aerosols, cirrus clouds etc. Micro-pulse LIDAR observations of cloud and aerosol profiles using backscatter. Ref: http://www.sigmaspace.com/sigma/micropulseLidar.php See 2-3X changes in return in matter of minutes.

17. Another example of aerosol and cloud profile over a 3 day period Ref. : J. D. Spinhirne et. al

18. Comparison of modeled Rayleigh scatter and measured scatter presence of sub-visual cirrus and boundary layer aerosols 2X difference between modeled Rayleigh return and that due to sub-visual cirrus. 2.5X difference at the lower atmosphere due to Mie scattering Ref. : J. D. Spinhirne et. al

19. Long term trends Effect of volcanic eruptions on the long term trend of Rayleigh scatter Ref: http://www.mlo.noaa.gov

20. Effect of Mie scatter due to cirrus clouds at Mauna Loa Measured non Rayleigh contribution Ref: http://www.mlo.noaa.gov

21. Conclusion: • If CW lasers were used and if they were projected from behind the secondary mirror, a large part of the HOWFS well depth will go into Rayleigh scatter subtraction. • If projected from the edge of the telescope, the WFSs will see severe spot elongation. • The best solution is to have a pulsed laser. A 1-3 micro-sec laser would be ideal for solving both spot elongation and Rayleigh scatter. A 200 micro-sec. laser with 10%-15% duty cycle may also be used to do away with Rayleigh scatter from the lower atmosphere. • If by the time NGAO is built the default option happens to be CW lasers: • In a matter of minutes the Rayleigh scatter could change by a factor of 2-2.5. • Long term trends show that the detector well depth must be matched to carefully chosen. • An extra error terms must be included in the error budget to account for these effects. • More time is required for studying the effect of projection location and how it affects the WFS’s noise and eventually the overall system WFE.