Thomas Stalcup June 15, 2006 Laser Guidestar System Status

Thomas Stalcup June 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 Stalcup June 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 Stalcup June 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 Stalcup June 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 Stalcup June 15, 2006 The Laser…. Image Credit: Gabor Furesz

Thomas Stalcup June 15, 2006 MMT Beam Projector Fold Mirror Laser Box Tip/Tilt Pupil Mirror Pupil Box L3 L1 L2 Adaptive Secondary 6.5m Primary Mirror Hologram Optical Axis Laser Power Supply and Chiller in Yoke Room Star Imager

Thomas Stalcup June 15, 2006 MMT Beam Projector

Thomas Stalcup June 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 Stalcup June 15, 2006 Laser Box Optics

Thomas Stalcup June 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 Stalcup June 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 Stalcup June 15, 2006 Pupil Box

Thomas Stalcup June 15, 2006 Hub Optics 48 cm diameter fused silica positive element Lightweight fused silica fold mirror 30 cm diameter SF6 negative element

Thomas Stalcup June 15, 2006 Hub Optics

Thomas Stalcup June 15, 2006 Beam Projector On-Sky Tests December 2005 spot quality –Star FWHM of 0.92 arcseconds –Laser FWHM of 1.20 arcseconds

Thomas Stalcup June 15, 2006 Projected Pattern

Thomas Stalcup June 15, 2006 Laser Wavefront Sensor Dynamic Refocus system Prism array instead of lenslet array Gated CCD camera

Thomas Stalcup June 15, 2006 Pupil Sampling Natural Star Laser Spots Telescope Turbulence

Thomas Stalcup June 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 Stalcup June 15, 2006 DR Effects

Thomas Stalcup June 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 Stalcup June 15, 2006 DR System Mechanics

Thomas Stalcup June 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

Thomas Stalcup June 15, 2006 The Team