Daniel Edward Potter

Imaging polarimetric observations of a new circumstellar disk system

Few circumstellar disks have been observed directly. Here we use sensitive differential polarimetric techniques to overcome atmospheric speckle noise in order to image the circumstellar material around HD 169142. The detected envelope or disk is considerably smaller than expectations based on the measured strength of the far-IR excess from this system.

A Hartmann Differential Image Motion Monitor (H-DIMM) for Atmospheric Turbulence Characterisation

We describe the use of a multi-aperture Hartmann mask coupled to a slightly out-of-focus focal plane array imager to monitor atmospheric turbulence (‘seeing’) produced by refractive index fluctuations. The imager (a CCD) is located inside or outside the focal surface of the imaging system so that each sub-aperture of the Hartmann mask produces an image well separated from all of the other images produced by the mask. Since the depth of focus of the sub-apertures is an order of magnitude larger than that of the parent optical system, the individual images are still diffraction-limited. We obtain short (10 to 100 msec) exposures and monitor the position fluctuations of the images. Analysis of the position and intensity fluctuations of the images can be used to determine the atmospheric parameter r 0 , the wind direction and velocity, and, under some circumstances, the distance of the turbulent layer from the observing site.

High-Resolution Mid-Infrared Imaging of the Asymptotic Giant Branch Star RV Bootis with the Steward Observatory Adaptive Optics System

We present high-resolution (~01), very high Strehl ratio (0.97 ± 0.03) mid-IR adaptive optics (AO) images of the asymptotic giant branch (AGB) star RV Boo utilizing the MMT adaptive secondary AO system. RV Boo was observed at a number of wavelengths over two epochs (9.8 μm in 2003 May and 8.8, 9.8, and 11.7 μm in 2004 February) and appeared slightly extended at all wavelengths. While the extension is very slight at 8.8 and 11.7 μm, the extension is somewhat more pronounced at 9.8 μm. With such high Strehl ratios, we can achieve superresolutions of 01 by deconvolving RV Boo with a point-spread function (PSF) derived from an unresolved star. We tentatively resolve RV Boo into a 016 FWHM extension at a position angle of 120°. At a distance of 390 pc, this corresponds to a FWHM of 60 AU. We measure a total flux at 9.8 μm of 145 ± 24 Jy for the disk and star. Based on a dust thermal emission model for the observed IR spectral energy distribution and the 9.8 μm AO image, we derive a disk dust mass of 1.6 × 10-6 M☉ and an inclination of 30°-45° from edge-on. We discuss whether the dust disk observed around RV Boo is an example of the early stages in the formation of asymmetric structure in planetary nebulae.

Astronomical coronagraphy with high-order adaptive optics systems

Space surveillance systems have recently been developed that exploit high order adaptive optics systems to take diffraction limited images in visible light on 4 meter class telescopes. Most astronomical targets are faint, thus driving astronomical AO systems towards larger subapertures, and thus longer observing wavelengths for diffraction limited imaging at moderate Strehl ratio. There is, however, a particular niche that can be exploited by turning these visible light space surveillance systems to astronomical use at infrared wavelengths. At the longer wavelengths, the Strehl ratio rises dramatically, thus placing more light into the diffracted Airy pattern compared to the atmospheric halo. A Lyot coronagraph can be used to suppress the diffracted light from an on axis star, and observe faint companions and debris disks around nearby, bright stars. These very high contrast objects can only be observed with much higher order adaptive optics systems than are presently available to the astronomical community. We describe simulations of high order adaptive optics coronagraphs, and outline a project to deploy an astronomical coronagraph at the Air Force AEOS facility at the Maui Space Surveillance System.

PEPPER: a photometer designed for the direct detection of extrasolar planets

PEPPER, a high-speed differential Polarization-Encoded Photometer and Polarimeter, is designed to perform self-calibrated shot noise-limited photometry from the ground to directly detect the light from the phase changes of close-in extrasolar planets. This is accomplished by using high-speed electro-optical switching techniques coupled with zero-read noise photon counting detectors to eliminate errors due to sky and detector gain drift variability. Here we present the design concept behind the photometer mode of PEPPER as well as some initial results from lab and on-sky engineering tests of the prototype instrument. The polarimeter mode of the instrument is presented in a seperate paper.

High-Contrast Imaging Science with Adaptive Optics

Adaptive Optics (AO) has become an integral part of almost all present and future large telescope initiatives, and will be essential in exploiting the full potential of the large optical interferometers currently under construction. This contribution focuses on the limitations of AO with respect to dynamic range, and on the potential of high-contrast imaging techniques.