Proceedings Article | 15 July 2010
Proc. SPIE. 7735, Ground-based and Airborne Instrumentation for Astronomy III
KEYWORDS: Near infrared, Diffraction, Telescopes, Stars, Interferometry, Adaptive optics, Coronagraphy, K band, Nulling interferometry, Visibility
We report on high-accuracy, high-resolution (< 20mas) stellar measurements obtained in the near infrared (
2.2 microns) at the Palomar 200 inch telescope using two elliptical (3m x 1.5m) sub-apertures located 3.4m
apart. Our interferometric coronagraph, known as the "Palomar Fiber Nuller" (PFN), is located downstream
of the Palomar adaptive optics (AO) system and recombines the two separate beams into a common singlemode
fiber. The AO system acts as a "fringe tracker", maintaining the optical path difference (OPD) between
the beams around an adjustable value, which is set to the central dark interference fringe. AO correction
ensures high efficiency and stable injection of the beams into the single-mode fiber. A chopper wheel and a fast
photometer are used to record short (< 50ms per beam) interleaved sequences of background, individual beam
and interferometric signals. In order to analyze these chopped null data sequences, we developed a new statistical
method, baptized "Null Self-Calibration" (NSC), which provides astrophysical null measurements at the 0.001
level, with 1 σ uncertainties as low as 0.0003. Such accuracy translates into a dynamic range greater than 1000:1
within the diffraction limit, demonstrating that the approach effectively bridges the traditional gap between
regular coronagraphs, limited in angular resolution, and long baseline visibility interferometers, whose dynamic
range is restricted to 100:1. As our measurements are extremely sensitive to the brightness distribution very
close to the optical axis, we were able to constrain the stellar diameters and amounts of circumstellar emission
for a sample of very bright stars. With the improvement expected when the PALM-3000 extreme AO system
comes on-line at Palomar, the same instrument now equipped with a state of the art low noise fast read-out near
IR camera, will yield 10-4 to 10-3 contrast as close as 30 mas for stars with K magnitude brighter than 6. Such
a system will provide a unique and ideal tool for the detection of young (<100 Myr) self-luminous planets and
hot debris disks in the immediate vicinity (0.1 to a few AUs) of nearby (< 50pc) stars.
