1. LGS WFS Design Status & Issues Dekany, Delacroix, & Velur Caltech Optical Observatories

2. LGS WFS concept at SF review 2

3. LGS WFS parameters 3 LGS HOWFS (9) after WF relay 589 nm727 16x16, 32x32, 64x64 1.45 no filter 64x64, 32x32, 16x16 174 Theta-phi OSM LocationTypeRange of travel Min. step size (est.) LGS channel (individual unit focus) delta Z~10 mm10 um LGS lenslet changer deltaX, Y delta X = 20 mm, delta Y = 2 mm 1 um LGS channel relay and camera focus for pupil sampling change delta Z5 mm0.1 um Pick off (theta mech) delta theta 360 deg. 0.2 (UP)/ 115" (LA) Pick off (phi mech.) delta phi 360 deg. 0.2" (UP)/ 300" (LA) LGS WFS unit overall travel delta Z130 mm1 mm Rotation of the WFS (sans the central WFS) delta (theta- phi) 360 deg.100" WFS type Length in Z [mm] Breadth X [mm] Width Y [mm] # of sensors Pick off mech.Cooling LGS WFS850150 9LGS theta-phi 3 stage Peltier WFS type (# of sensors) Location Sensing wavelength (nm) Input platesc ale (um/") # of sub- apertures Detector platescale( "/pixel) Filters Pupil sampling Field of regard (arcsec) Comments

4. SF review LGS WFS conceptual design 4

5. Post SF review changes to LGS WFS architecture 4 fixed + 3 (PnS) LGS sensors (as compared to 1 fixed + 8 mobile sensors) Field of regard of PnS sensors is 120 arcsec dia. and the fixed sensors are positioned to accept LGS light at 10 arcsec. radius (as compared to 10-150 arcsec continuous FoR (dia.) for sensors in the pentagon formation and 10- 174” arcsec continuous FoR (dia.) for PnS sensors) Only 2 pupil sampling scales – 64x64, 32x32 (no 16x16). LGS TT mirror to be implemented on the downlink – not appraised as part of WFS costs, neither have the motion controls for these mirrors defined as part of LGS WFS (this was a recent change). LGS WFS’s are not refrigerated and are fed through a window (or a pair of windows) – sensors are not required to operate -17C but need to work at -5 C. Lesser motion control Total laser power has been reduced and the sodium return numbers have been scaled accordingly for performance in Mauna Kea. 5

6. Current optical design (Lick) 6

7. LGS WFS design concept (needs refinement) 7 Fixed frame Camera Rotation axis Camera Translation 4 Fixed Cameras Reflective tetrahedron LGS WFS TT stages are not implemented yet (we realized the need to accommodate TT sensor last week).

8. FLAMINGOS-2 OIWFS 8

9. LGS WFS issues The most extreme RMS wavefront error in the LGS path is 1.5-1.7micron, and the largest spot size is about 250 miliarcsec - these static aberrations on the LGS caused a shift of the centroids of 5% of a sub-aperture (at most) [Kupke] 9 Laser Guide Star Size Calculation Finite Object Size Intrinsic guide star diameter 0.00arcsec Uplink formation of the beacon(s) Perfect Uplink AO?NO Inherent aberrations in the uplink beam:0.90arcsec Beam movement contribution to uplink0.33arcsec Residual seeing contribution to uplink0.60arcsec Diameter of point source laser at Na layer:1.08arcsec Seeing Natural seeing FWHM at GS wavelength 0.58arcsec Subaperture Tip/Tilt corrected FWHM 0.47arcsec Contribution due to seeing 0.47arcsec Elongation Distance from LLT to telescope axis:0.00m Use Max. Elongation?NO1.27arcsec Avg. Elongation0.85arcsec Contribution due to elongation0.85arcsec System Aberrations Aberrations in AO thru to WFS 0.25arcsec Atmospheric Dispersion ADC in HOWFS?NO RMS blurring due to atmospheric dispersion0.000arcsec Total size of detected return beam: 1.47 arcsec Sensing Approach Pyramid WFS?NO Charge Diffusion 0.25pixels Contribution due to Charge Diffusion0.40arcsec Subaperture Diffraction Lambda/d (for sensing) 0.66arcsec Spot size used for centroiding 1.66 arcsec We have started working with Lick to understand what WF quality is required at the LGS input (Gemini and TMT may have over- specified their sensor).

10. LGS WFS design concept 10 Pick off arms search area Pick off arms effective 120” search area Tetrahedron to direct light to fixed WFS’s.

11. Issues & to-dos: Need to update all relevant requirements and nail down as many #s and parameters as possible. Refine the LGS sensor concept to work as 4 fixed + 3 PnS (conceptual work has already started). Redesign pick off arm with pupil access and TTM. Now that a 256x256 pixel CCD is being talked about in meetings – should we make it our baseline? If so, what is the pixel size? We need to model 2 more windows in zemax as the sensor resides outside the refrigerated chamber. 11

12. LGS spots with sensor in the current optical design 12 The AO design team and the WFS team are trying to hash out details to be able to build a integrated optical model with version control.

13. Other WFS related changes No narrow field TWFS. No ADC on NGS WFS. No ADC in the TWFS. In general, the system has undergone many changes, any design work without nailing down requirements might mean re-work. Next 2 weeks will mostly be spent in updating the requirements database. Alex is already obtained a Contour account and is getting familiar with the software, the requirements and the PBS. 13