Collaborators
• R. Paul Butler (Carnegie Institution of Washington)
• Hugh Jones (University of Hertfordshire)
• Geoff Marcy (University of California Berkeley)
• Brad Carter (University of Southern Queensland)
• Simon O'Toole (Anglo-Australian Observatory)
• Jeremy Bailey
• Jade Bond (University of Arizona)
  

Over two hundred planets are now known to be orbiting stars other than the Sun (see http://exoplanets.org/almanacframe.html for a list). Most of these planets were detected using the radial velocity method, which involves measuring (using spectroscopy) the "wobble" of the star due to the planet's gravitational pull. An alternative used by an increasing number of planet searches is the transit method (see figure below). This involves measuring the small decrease in apparent brightness (of order 1%) of the star, as the planet eclipses (or transits) it. This method is thus only sensitive to planets with edge-on orbits. However, if such a planet is detected, its size, mass and orbital characteristics can be determined (with follow-up spectroscopy), constraining models of its structure and formation.

High-precision spectroscopic observations of a host star during a known transit may reveal something about the chemical composition of the planet's atmosphere. The first such observation was recently made by Charbonneau et al. (2001). Under highly favourable circumstances, it may even be feasible to search for the presence of atmospheric oxygen (Webb & Wormleaton, 2001).

For the transit method to be effective, two main difficulties need to be overcome. Firstly, the probability that a given star hosts a planet in an edge-on orbit and the transit occurs while the star is being observed is low. Thus, a reasonable detection rate can only be achieved by continuous photometric monitoring of a large sample (tens of thousands) of stars. A wide-field automated telescope is ideal for this task.

The second challenge is the high level of photometric precision required. Changes in brightness on timescales of a few hours need to be measured to better than 1% precision. This is difficult to achieve with CCD photometry on ground-based telescopes (the only feasible method for most transit searches). Above the fundamental limits set by Poisson statistics (since measurement involves counting photons in the image), systematic errors become important.

The Automated Patrol Telescope

The Automated Patrol Telescope (APT) is a 0.5m telescope owned and operated by the University of New South Wales, and located at Siding Spring Observatory, Australia. The CCD camera images a 2x3 degree field with 9.4 arcsecond pixels. The telescope is entirely computer-controlled, with the possibility of remote or fully automated observation. An important drawback of using a wide-field telescope is that the images tend to be undersampled (as is the case with the APT). Because the sensitivity of a CCD is not uniform over the surface of a single pixel, the total brightness measured for a star is dependent on where the star falls relative to pixel boundaries. This becomes significant in undersampled images, where the light from a star is spread over only a few pixels. For the APT, this effect proved to be the limiting factor, causing photometric errors of several percent.

We have developed a new observing technique specifically aimed at minimising the effects of intra-pixel variations. We systematically move the telescope during intergration, in a raster-type scan covering 1x1 or 2x2 pixels. Although this does broaden the PSF slightly, it effectively eliminates the intra-pixel variation problem. The resulting PSF is fairly flat topped with a very rapid falloff. We therefore developed an optimised aperture photometry package to process the resulting images, with co-located apertures for each object positioned to better than 1/100th of a pixel repeatability in each frame.  Using this system we are obtaining differential photometric precision of ~2mmag down to V=11 in 150sec exposures. The lightcurves are further processed to find and remove periodic signals from variable stars and searched for transit signals using a variant of the Gregory-Loredo algorithm optimised for transit detection.

Current Members
• Prof John Webb (Faculty)
• Prof Michael Ashley (Faculty)
• Dr Andre Phillips (APT Support and Instrumentation Scientist)


• YOU? There is room for more postgraduate students on the team. Please contact John Webb if you are interested in joining us.

Collaborators

• Dr. Marton Hidas (University of California-Santa Barbara)
• Dr. Mike Irwin (IoA, Cambridge)
• Dr. Suzanne Aigrain (IoA, Cambridge)

The Large Antarctic Plateau Clear-Aperture Telescope (LAPCAT) is a preposed 8.4 metre off-axis optical/IR telescope to be located at Dome C, Antarctica. One of the primary science drivers for LAPCAT will be to directly image exo-planets. The high thermal infrared sensitivity, the high level of correction achievable with its adaptive optics system, and the telescope optical configuration make LAPCAT uniquely capable for such science.

The primary source of noise for exo-planet imaging at close angular separations arises from scattered starlight at the position of planet. Traditional coronagraphic and apodisation techniques can suppress the stellar PSF, but have significant limitations in the achievable resolving power and throughput. Techniques currently being investigated, such as the Phase-Induced Amplitude Apodization Coronograph (PIAAC) proposed by Guyon et al.(2005), should allow stellar suppression to inner working angles of several times the diffraction limit with very high efficiency. This equates to 3.5 AU at 10 pc for M band imaging with LAPCAT. Although apodised pupil masks can be used to suppress spider vane diffraction in Lyot coronographs, residual spider diffraction increases noise and can effect speckle statistics. The PIAAC technique is likely to suffer from similar limitations, and thus the un-obscured primary of LAPCAT is advantageous.

In a 24 hour integration, LAPCAT should detect a 1.2 Mjup 1 Gyr old planet at 10 pc, or a more mature 5 Gyr planet of 4.2 Mjup. These are significantly lower masses than are detectable with existing facilities, and arecomparable with the detection limits of future Extremely Large Telescopes at mid-latitude sites. Younger (100 Myr) Jupiter mass planets at very wide (20 AU) orbits should be detectable with LAPCAT out to 60 pc. LAPCAT thus provides a significant extension of the parameter space of other planet detection methods. The higher spatial resolution of ELT class telescopes will enable imaging of planets at closer orbits than LAPCAT. An ELT located in Antarctica would thus be an exceptionally powerful facility for this science.

THE LAPCAT TEAM

Current Members
• Prof John Storey (Faculty)
• Tui Britton (PhD student)
• Dr Jon Lawrence (Postdoc)
• Prof Michael Ashley (Faculty)
• Prof Michael Burton (Faculty)
• Daniel Luong-Van (Instrument Scientist)
Collaborators
• Dr. Roger Angel (University of Arizona)
• Dr. Phil Hinz (University of Arizona)

Planet Search Links

• Sample image of an APT field
• A list of transit search projects around the world
• California & Carnegie Planet Search
• The Extrasolar Planets Encyclopedia

We are grateful to SUN Microsystems Australia for generously providing computing equipment in support of the APT project