A new "self-calibrated" statistical analysis method has been developed for the reduction of nulling interferometry data. The idea is to use the statistical distributions of the fluctuating null depth and beam intensities to retrieve the astrophysical null depth (or equivalently the object's visibility) in the presence of fast atmospheric fluctuations. The approach yields an accuracy much better (about an order of magnitude) than is presently possible with standard data reduction methods, because the astrophysical null depth accuracy is no longer limited by the magnitude of the instrumental phase and intensity errors but by uncertainties on their probability distributions. This approach was tested on the sky with the two-aperture fiber nulling instrument mounted on the Palomar Hale telescope. Using our new data analysis approach alone-and no observations of calibrators-we find that error bars on the astrophysical null depth as low as a few 10 −4 can be obtained in the near-infrared, which means that null depths lower than 10 −3 can be reliably measured. This statistical analysis is not specific to our instrument and may be applicable to other interferometers.

The design, characteristics, and first test flight results are described of the Portable Remote Imaging Spectrometer, an airborne sensor specifically designed to address the challenges of coastal ocean remote sensing. The sensor incorporates several technologies that are demonstrated for the first time, to the best of our knowledge, in a working system in order to achieve a high performance level in terms of uniformity, signal-to-noise ratio, low polarization sensitivity, low stray light, and high spatial resolution. The instrument covers the 350-1050 nm spectral range with a 2.83 nm sampling per pixel, and a 0.88 mrad instantaneous field of view, with 608 cross-track pixels in a pushbroom configuration. Two additional infrared channels (1240 and 1610 nm) are measured by a spot radiometer housed in the same head. The spectrometer design is based on an optically fast (F/1.8) Dyson design form coupled to a wide angle two-mirror telescope in a configuration that minimizes polarization sensitivity without the use of a depolarizer. A grating with minimum polarization sensitivity and broadband efficiency was fabricated as well as a slit assembly with black (etched) silicon surface to minimize backscatter. First flight results over calibration sites as well as Monterey Bay in California have demonstrated good agreement between in situ and remotely sensed data, confirming the potential value of the sensor to the coastal ocean science community.

We report on high-accuracy, high-resolut ion ( < 20mas) stellar measurements obtained in the near infra red (~ 2.2 microns) at the P alomar 200 inch telescope using two elliptical (3 m x 1.5 m) sub-a pertures located 3.4 m a part . Our interferometric coronagraph, known as the " Palomar Fiber Nuller" (PFN), is located downstream of the Palomar adaptive optics (AO) system and recombines t he two separate beams into a common singlemode fiber. The AO system acts as a " fr inge tracker", maintaining the optical path difference (OPD) between t he beams around an adjustable value, which is set to the central dar k interference fringe. AO correction ensures high efficiency and stable inject ion of the beams into the single-mode fi ber. A chopper wheel and a fast photometer are used to record short ( < 50ms per beam) interleaved sequences of backgro. und, 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, wi th 1 a 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 tradit ional gap between regular coronagra phs, 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 bright ness distribution very close to t he opt ical 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 t he 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 wit h 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 t he immediate vicinity (0.1 to a few A Us) of nearby ( < 50pc) stars.

Phase shifters are a key component of nulling interferometry, one of the potential routes to enabling the measurement of faint exoplanet spectra. Here, three different achromatic phase shifters are evaluated experimentally in the mid-infrared, where such nulling interferometers may someday operate. The methods evaluated include the use of dispersive glasses, a through-focus field inversion, and field reversals on reflection from antisymmetric flat-mirror periscopes. All three approaches yielded deep, broadband, mid-infrared nulls, but the deepest broadband nulls were obtained with the periscope architecture. In the periscope system, average null depths of 4x10(-5) were obtained with a 25% bandwidth, and 2x10(-5) with a 20% bandwidth, at a central wavelength of 9.5 mum. The best short term nulls at 20% bandwidth were approximately 9x10(-6), in line with error budget predictions and the limits of the current generation of hardware.

A rotating nulling coronagraph has been built for use on ground-based telescopes. The system is based on the concept of sub-aperturing the pupil of the telescope with two elliptical apertures and combining the resulting two input beams on a single-mode fiber. By a relative n phase shift of the beams, the starlight can be nulled and a relatively faint companion star can be detected. Rotation of the aperture mask on the telescope pupil results in a signal similar to that expected from a space-borne telescope system such as the proposed TPF/Danvin interferometer. The design of the nulling coronagraph and the ancillary systems that are needed, such as the fringe tracker, are described and the potential for observations on telescopes such as the Palomar 200" is discussed. Results of a nulling experiment using a single mode fiber as a beam combiner for broadband light between 1.50 µm and 1.80 µmare shown.