Objectives

The Planet Detection Testbed.

The Planet Detection Testbed (PDT) was developed to demonstrate the feasibility of four-beam nulling, the required null stability, and the detection of faint planets using an approach similar to the ones contemplated for a flight-mission. The most promising architectures for TPF-I (the X-Array and the Linear Dual-Chopped Bracewell) are four-beam nulling interferometers that use interferometric chopping to detect planets in the presence of a strong mid-infrared background.

By ensuring that the starlight is suppressed at an adequately stable and controlled level, and by modulating a sensitivity/fringe pattern around the star, TPF-I should detect a planet signal at a frequency of a few Hertz. This modulation technique is in many ways similar to the use of a chopper wheel that allows the detection of infrared sources against a thermal background and/or drifting detector offsets. In this case the thermal background on the sky includes the local and exozodiacal light. To achieve this modulation the interferometer uses two nullers each phased to null out the starlight, and a second beam combiner which takes the output from the nullers and phases it to form the moving sensitivity pattern. A dark null fringe is fixed over the star and the bright fringes move alternately on and off the planet. If there are other planets in the field of view, their signals will also contribute depending on their locations, and by rotating the fringe system around the star the whole planetary system can be observed. Signal processing is then used to determine the location of the planets orbiting the star.

The primary objective of the Planet Detection Testbed is to simulate this observing scenario and demonstrate the instrument stability needed to make this process work. The PDT therefore has the following main components: a star and planet source to generate a planetary system to be observed, a pair of nullers to null out the starlight, and a cross-combiner to allow modulation of the detected planet signal.

The PDT will demonstrate active control of two nulling beam combiners (four input beams in total), yielding a null depth of 1 part in 1,000,000 that is stable to 0.1%, the detection of a planet signal that is 1,000,000 times fainter than the simulated starlight, and interferometric chopping to 0.1%.

Testbed Description

The Planet Detection Testbed combines four mid-infrared beams representing inputs from the four telescopes of the interferometer. In the testbed, these beams contain bright starlight and faint planet light that are separated by a pair of nullers and a phase-chopping cross combiner in a process that reproduces the operation of the flight beamcombiner. An important element of the testbed plan is to demonstrate control of the nullers and the cross-combiner at levels close to those needed for flight, and to show realistic faint planet detection within a period of about two hours in the presence of ambient laboratory noise and optical disturbances. The Planet Detection Testbed therefore includes servo loops and control systems necessary for deep and stable nulling.

An artificial star is formed from the output of a carbon dioxide laser and a small thermal source. The carbon dioxide laser light provides the 10 micron radiation which is to be nulled and the thermal source provides 2 to 4 micron radiation that is used for fringe tracking on the star. The starlight is passed through a pinhole and chopper and then split into two beams. These beams are split again forming four beams and, at this second splitting stage, simultaneously combined with beams from a second thermal source. This second source, band-limited to radiation between 10 and 11 microns, forms the artificial planet.

Figure 2: Four-beam nulling results.

The PDT includes numerous control loops that periodically degrade the null in the search of the best alignment for pathlength and tilt control of the four beams. The time-series of data from the PDT, plotted on a log-scale in Figure 2, show the signature of these modulations and yet the null degradations are so slight that deep nulling is maintained. The Planet Detection Testbed demonstrated four-beam nulling with null depths of 250,000:1 and the detection of a simulated planet at a contrast level of two million times fainter than its star.

System alignment, control, and calibration techniques needed for flight are being developed and tested as necessary parts of the testbed. By controlling the planet phase, the testbed will simulate a complete rotation of the telescope formation around the line of sight to the star over a 5000 s period and will demonstrate reconstruction of the planet signal from the data.

The principal investigator of the Planet Detection Testbed is Dr. Stefan Martin at the Jet Propulsion Laboratory.

State of the Art

As shown in Figure 2, the PDT has demonstrated 4-beam nulling using a 10 µm laser with a null depth greater than 1 part in 100,000. This is one milestone towards achieving its objective of demonstrating null stability and control.

Progress to Date

Following successful tests in 2005, the PDT has been rebuilt to include tilt and shear sensors that in February 2007 demonstrated intensity stability in each arm of the interferometer to better than 0.2%. Experiments are underway in 2007 to achieve the principal objectives described above.

Table 1 Planet Detection Testbed Schedule Planned Activities Performance Targets Demonstrate 4-beam nulling Null depth of 10-5 Demonstrate planet detection Null depth of 10-5 Demonstrate amplitude stability of 4-beam nuller Stable to 0.1% Demonstrate 4-beam nulling and phase chopping Null depth of 10-6; control of chopping to 0.1% Demonstrate planet detection at high level of nulling performance Null depth of 10-6; strong planet signal (10-4 of star) with fringes stationary with respect to planet. Demonstrate planet extraction at high level of nulling performance Null depth of 10-6; with fringes rotating through the position of the planet.