Thomas Stalcup June 15, 2006 Laser Guidestar System Status.
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Transcript of Thomas Stalcup June 15, 2006 Laser Guidestar System Status.
Thomas StalcupJune 15, 2006
Outline
• This talk– Hardware details of laser beam projector and
wavefront sensor
• Next: Christoph Baranec– On-sky testing results of wavefront sensing
and tomographic reconstruction
• Next: Michael Lloyd-Hart– Expected system performance and science
goals
Thomas StalcupJune 15, 2006
Laser Guidestar Advantages
• Use a laser to create an artificial star• Can point anywhere (at least, anywhere approved
by the FAA and Space Command….)
• Virtually 100% sky coverage
Thomas StalcupJune 15, 2006
System Overview
• Beam projector– Projects five beams, 4 Watts each
• Laser wavefront sensor
• Natural star tip/tilt sensor
• Natural star wavefront sensor to verify laser wavefront data during testing
Thomas StalcupJune 15, 2006
Rayleigh Lasers
• Use relatively inexpensive, reliable doubled Nd:YAG technology
• Uses Rayleigh scattering in atmosphere
• Must operate at lower altitudes than sodium-line lasers
• Use range gating to restrict return to telescope depth of field
Thomas StalcupJune 15, 2006
MMT Beam Projector
FoldMirror
Laser Box
Tip/Tilt Pupil Mirror
Pupil BoxL3
L1
L2
AdaptiveSecondary
6.5m Primary Mirror
Hologram
Optical Axis
Laser Power Supply and Chiller in Yoke Room
Star Imager
Thomas StalcupJune 15, 2006
Laser Box
• Two lasers combined with a polarizing beam splitter– 30 W combined output
• Insulated, temperature controlled enclosure• Beam overlap controls
– Waist imaging camera– Steering mirrors
Thomas StalcupJune 15, 2006
Laser Box Output Window
• Originally, the second steering prism was the output window
• Two moth strikes in a year and a half
• New, rotating, easy to replace window
Thomas StalcupJune 15, 2006
Pupil Box
• First lens of beam expander / projection optics
• Hologram to create five beams– Mounted on rotation stage
• Fast steering mirror at a pupil
Thomas StalcupJune 15, 2006
Hub Optics
• 48 cm diameter fused silica positive element
• Lightweight fused silica fold mirror
• 30 cm diameter SF6 negative element
Thomas StalcupJune 15, 2006
Beam Projector On-Sky Tests
• December 2005 spot quality– Star FWHM of 0.92 arcseconds– Laser FWHM of 1.20 arcseconds
Thomas StalcupJune 15, 2006
Laser Wavefront Sensor
• Dynamic Refocus system
• Prism array instead of lenslet array
• Gated CCD camera
Thomas StalcupJune 15, 2006
Dynamic Refocus
• Use a moving element to keep rising laser pulse in sharp focus to allow longer range gate
• Can collect more photons
• Corrects for spot elongation in subapertures away from the projection axis
Thomas StalcupJune 15, 2006
DR Principles
• A moving mirror adjusts the wavefront sensor focus
• At the native f/15, the mirror must move 81 mm• At f/0.5, the mirror needs to move just 150 µm• Even 150 µm at 5 kHz is not easy
– Mount the mirror on a high-Q mechanical resonator
Thomas StalcupJune 15, 2006
Wavefront Sensor Camera
• CCD is a CCID18 from MIT/Lincoln Labs– Electronic shutter– 16 amplifiers– Split frame transfer– 128 x 128 pixels
• Little Joe controller from Scimeasure• Can not transition shutter while reading pixels
– Needs accurate timing to interleave reading lines in between shutter transitions