NGAO Calibration/Simulation Source

1. NGAO Calibration/Simulation Source T. Stalcup, M. Pollard

2. Requirements • Relay NGS and LGS point sources into AO image plane • Wavelength range • NGS 500 nm to 2500 nm • LGS 589 nm or 594 nm laser line • 120 arcsecond field • Resolved and unresolved sources at NGS and LGS 85 km and 180 km conjugates • Flat field and spectral line sources • 30 nm rms wavefront error • Estimated specification. Need to produce a calibration budget • Turbulence generator • Optional at summit, decided this shouldn’t drive design • Ground layer at cal unit secondary mirror • 11.5 km conjugate at cal unit primary mirror • More details later in optical design 2

3. Design Goals • Minimize motion requirements using beam splitters and multiple source points • Include enough focus range for NGS/LGS sources for image sharpening and IF dichroic compensation • Fiber-fed sources to minimize heat and access in AO cold enclosure • Simultaneous operation of NGS/LGS sources • Minimize surfaces • More surfaces requires tighter figure error specification 3

4. Design Status • Concentrated on difficult part – the optical design • Choice of source sizes, brightness, and wavelengths still in progress • Very flexible design, can put anything in input plane for calibration unit • Flat field source still under development • Optomechanical design in progress 4

5. Optical Design • Explored a few alternatives – refractive, TMA, etc. • Too complicated or costly due to wavelength range and wavefront quality constraints • Chose an Offner relay • All spherical surfaces • Excellent image quality • Pupil at mirror, not free space 5

6. 250 mm Pupil at secondary mirror Primary mirror Fold down to AO image plane LGS 85 km and 180 km conjugates NGS conjugate NGS/LGS beam splitter Flat field beam splitter Flat Field and spectral line source Optical Design 6

7. Calibration Unit Location 7

8. nm rms 30 15 0.5 NGS Performance 8

9. NGS Wavefront Error 9

10. 30 30 18 16 6.5 3.5 LGS Performance nm rms nm rms 90km conjugate 180km conjugate 10

11. LGS Wavefront Error 90km conjugate 180km conjugate 11

12. Unresolved Resolved Sources • Point sources are fibers mounted in a fixed plate • Same pattern for NGS and LGS • Mixture of unresolved and resolved sources • Unresolved fiber core is 9 µm, same as current Keck system • Resolved fiber core is 400 µm, or 0.55 arcseconds, also same as current Keck system • Roughly 60 fibers total • Include 589 nm notch filter in NGS to prevent crosstalk 12

13. NGS Source • Detailed analysis of fiber coupling and throughput pending… • NGS • Tungsten Halogen or Arc Lamp • Some electronic intensity control possible • If electronic dynamic range is insufficient, would need motorized filter wheel • Need electronically controlled shutter for dark images during calibration • Newport Oriel lamp source • RS232 on/off and intensity control • Lamp elapsed hours • Available high stability controller, uses optical feedback to stabilize output 13

14. LGS Source • DPSS lasers at 589 nm are now available • 10 mW to 2 W output • $4k to $22k • Some electronic intensity control possible • Filter wheel needed if intensity control has insufficient dynamic range • Shutter not needed, can turn on/off electronically • HeNe at 594 nm also an option • Lower power, 2 mW • $2k 14

15. Flat Field and Spectral Line Source • Uses pupil stop in simulator to define beam • Based on woven fiber optic backlight • Compact, Efficient • Still talking to manufacturer about spectral properties • Use broadband tungsten halogen or arc lamp for flat field • Need to choose spectral lamps – probably hollow cathode type 15

16. Diffuse lambertian reflector Sources Output plane Alternate Flat Field Source • If woven fiber panels are not suitable • Pattern of several fibers and diffuser • Integrating sphere • Davinci requested uniformity is much less strict than Contour requirements • 10% vs. 0.2% 16

17. Astrometric Grid • Aluminized glass plate with micromachined 100x100 grid of 3.6 µm holes on 360 µm centers • 5 milliarcseconds diameter, 0.5 arcsecond spacing • Requirement is only for 80x80. Added some for margin at edges when shifting, but if expensive per hole could reduce. • Design very similar to current NIRC2 plate • Backlit with either woven fiber optic or large core fiber with projection optics • On in/out stage with NGS point sources • If flat field arm extended to image plane, could place there • Still need in/out stage • Possible aberrations from flat field beam splitter • Current plan includes a rotation mount for grid • Chosen to keep aberrations from system as constant as possible, but may not be necessary 17

18. Turbulence Generator • Phase plates near mirrors, two copies of phase distribution separated by ~2-3 mm • Place for 11.5 km phase plate, but it would need to be very large • Wouldn’t fit in AO enclosure with top on • Could be used for warm checkout of tomography 11.5 km conjugate phase plate 250 mm Ground Layer Wheel 18

19. Optomechanical 19

20. Elevation Ring Clearance Planar design Increased clearance design 20

21. Optomechanical • All components mounted to common baseplate except AO rotator fold • Slide in/out of elevation bearing on rails mounted to AO bench • Use overhead crane to lift on/off AO bench • Not sure if commerical mounts will provide wavefront error performance and stability required • Mike Pollard is now past interviews, DAVINCI, etc., and will be filling out design for PDR 21

22. Questions? 22