Observing Venus (and Mars) with Adaptive Optics Jeremy Bailey (UNSW)

Adaptive Optics Many large ground-based telescopes are equipped with adaptive optics. These allow diffraction limited imaging at near-IR wavelengths. At 2 mm diffraction limit of a telescope is: But currently we are limited by seeing to resolutiond of 0.5–1.0 arcsec (100-200 km). B Aperture Arc sec Km at Venus/Mars 4 m 0.13 32 8 m 0.07 16 32 m 0.016 4

Extremely Large Telescopes Even larger telescopes – known as Extremely Large telescopes (or ELTs) are now being designed. Thirty Metre Telescope (TMT)

Adaptive Optics images Titan with ESO VLT Jupiter with Gemini Telescope and adaptive optics — image processed by Chris Go. Uranus and its rings with Keck telescope

Titan Gemini AO Images NIFS Images

Current AO Systems Require a guide “star” close to the science object. With a planet like Mars or Venus … Too big to be used as a guide star itself. Too bright to allow a nearby star to be used as a guide star (scattered light). e.g. Attempts to use Phobos (mag 10.4) as a guide star for Mars (mag -2.8) have not been successful. Laser guide stars don’t help.

VEX VIRTIS-M R ~ 200 AAT IRIS2 R ~ 2400

Resolution at Mars/Venus (Near-IR Imaging Spectrometers)

Resolution at Mars/Venus (Near-IR Imaging Spectrometers)

Limb viewing The resolution of GMTIFS at Mars and Venus (~3-4km) is sufficient to vertically resolve the atmosphere in limb viewing geometry. Spacecraft can do this, but it has never before been possible with ground-based telescopes.

Current Performance - UKIRT 90km resolution MGS MOLA data

8m Telescope - Diffraction Limited at 2mm - 16km resolution

ELT 4km resolution

Solution We need an AO wavefront sensor that can work on the extended structure of the image of Mars or Venus (rather than a point source). For Venus use the 2.3 mm cloud structure or 1.27 mm airglow (or perhaps the sunlit crescent in the visible). For Mars use the surface albedo features. We know this is possible because solar AO systems work on extended structure (e.g. solar granulation).

Shack-Hartmann Wavefront Sensor

Correlating Shack-Hartmann Used in solar adaptive optics systems. Measures image shifts by correlating structure in individual pupil images. Intel processors contain specific support for rapid measurements of image shifts in their SSE instruction sets (as it is important for video compression algorithms)

Solar AO Example Data taken with “low cost” AO system for McMath Pierce solar telescope. Hardware cost US$25,000 Keller et al., 2003, SPIE Proc. 4853, 351.

Options Build a new instrument designed for planetary AO. Retrofit “correlating” capability to an existing AO system. Use a solar telescope (that already has this capability) to observe Venus. Shoud work very well – these planets are bright sources and should be capable of provding good (high Strehl) AO correction.