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 m 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

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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

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• Even larger telescopes – known as Extremely Large telescopes (or ELTs) are now being designed.

Thirty Metre Telescope (TMT)Thirty Metre Telescope (TMT)

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Adaptive Optics images

Jupiter with Gemini Telescope and adaptive Jupiter with Gemini Telescope and adaptive optics — image processed by Chris Go.optics — image processed by Chris Go.

Titan with ESO VLTTitan with ESO VLT

Uranus and its rings with Keck telescopeUranus and its rings with Keck telescope

Titan

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NIFS ImagesNIFS Images

Gemini AO ImagesGemini AO 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-MVEX VIRTIS-M

AAT IRIS2AAT IRIS2

R ~ 200R ~ 200

R ~ 2400R ~ 2400

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

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

Limb viewing

The resolution of The resolution of GMTIFS at Mars and GMTIFS at Mars and Venus (~3-4km) is Venus (~3-4km) is sufficient to vertically sufficient to vertically resolve the atmosphere in resolve the atmosphere in limb viewing geometry.limb viewing geometry.

Spacecraft can do this, but Spacecraft can do this, but it has never before been it has never before been possible with ground-possible with ground-based telescopes.based telescopes.

Current Performance - UKIRT90km resolution

MGS MOLA data

8m Telescope - DiffractionLimited at 2m - 16km resolution

ELT4km 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 m cloud structure or 1.27 m

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).

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Shack-Hartmann Wavefront Sensor

Correlating Shack-Hartmann

Used in solar adaptive Used in solar adaptive optics systems.optics systems.

Measures image shifts Measures image shifts by correlating structure by correlating structure in individual pupil in individual pupil images. images.

Intel processors contain Intel processors contain specific support for rapid specific support for rapid measurements of image measurements of image shifts in their SSE shifts in their SSE instruction sets (as it is instruction sets (as it is important for video important for video compression algorithms)compression algorithms)

Solar AO Example

Data taken with “low cost” Data taken with “low cost” AO system for McMath AO system for McMath Pierce solar telescope.Pierce solar telescope.

Hardware cost US$25,000Hardware cost US$25,000

Keller et al., 2003, SPIE Proc. 4853, Keller et al., 2003, SPIE Proc. 4853, 351.351.

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.

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