thomas stalcup june 15, 2006 laser guidestar system status

Thomas StalcupJune 15, 2006

Laser Guidestar System Status


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


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


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


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


The Laser….

Image Credit:Gabor Furesz


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


MMT Beam Projector


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


Laser Box Optics


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


Pupil Box

• First lens of beam expander / projection optics

• Hologram to create five beams– Mounted on rotation stage

• Fast steering mirror at a pupil


Pupil Box


Hub Optics

• 48 cm diameter fused silica positive element

• Lightweight fused silica fold mirror

• 30 cm diameter SF6 negative element


Hub Optics


Beam Projector On-Sky Tests

• December 2005 spot quality– Star FWHM of 0.92 arcseconds– Laser FWHM of 1.20 arcseconds


Projected Pattern


Laser Wavefront Sensor

• Dynamic Refocus system

• Prism array instead of lenslet array

• Gated CCD camera


Pupil SamplingNatural Star

Laser Spots

Telescope

Turbulence


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


DR Effects

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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


DR System Mechanics


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


The Team

thomas stalcup june 15, 2006 laser guidestar system status

Documents

thomas stalcup

pupil slide

window slide

laser box optics slide

arcseconds slide

testing slide

pupil box slide

hub optics slide