Measurement and Calibration

Gerard van Belle (lead, Jet Propulsion Laboratory)
David R. Ciardi (University of Florida)
M.J. Creech-Eakman (Jet Propulsion Laboratory)
Robert R. Thompson (Jet Propulsion Laboratory)

Abstract:

An interesting recent result from the Palomar Testbed Interferometer (PTI) has been the direct observation of photospheric oblateness for the known rapid rotator Altair. This initial foray into the regime of higher-order stellar surface characterization with long baseline interferometers is a harbinger of things to come with the more capable multi-aperture instruments currently coming online. The morning talks will progress from the theory of two telescope interferometry, to its incarnation at PTI, to the application of PTI to the specific observational program of measuring stellar oblateness. In the afternoon, two labs will give the students hands-on experience with the observational experience: first, the process of observing program creation will be explored, from target selection to vetting of suitable calibration objects to be observed along with the science target. Second, reduction and analysis of visibility data to produce the physical parameters of a hypothetical rapid rotator will be demonstrated through use of PTI data reduction tools.

Interferometry with Two Telescopes

The basic theory of operation for a two telescope interferometer will be presented. Concepts such as the coherent recombination of radiation, optical delay lines, and the point response function will be discussed. Geometric considerations, such as (u,v) plane coverage and source geometry, will also be presented. Overall, the student will be given a walking tour of the mathematical tapestry that underpins the operation of a modern two element optical interferometer.

The Palomar Testbed Interferometer

The Palomar Testbed Interferometer (PTI) is a long-baseline infrared interferometer located at Palomar Observatory, California. It was built as a testbed for interferometric techniques applicable to the Keck Interferometer. First fringes were obtained in 1995 July. The three fixed 40 cm apertures can be combined pairwise to provide baselines to 110 m. The interferometer actively tracks the white-light fringe using an array detector at 1.6 and 2.2 microns and active delay lines with a range of +-38 m. Laser metrology of the delay lines allows for servo control, and laser metrology of the complete optical path enables narrow-angle astrometric measurements. The instrument is highly automated, using a multiprocessing computer system for instrument control and sequencing. PTI implements a dual-star architecture, tracking two stars simultaneously for phase referencing and narrow-angle astrometry. In addition to the technical aspects of the instrument, a brief review of the scientific results from the instrument will be presented, highlighting the potential uses of a simple two-telescope interferometer.

Observing Strategy & Rapidly Rotating Stars

Not infrequently an interesting perturbation of the interferometric visibility data is nothing more than a instrumental artifact; careful selection of both science targets and accompanying calibration objects can help identify and isolate bad data from the good. The fine art of such source selection is a large part in earning one's interferometry black belt and will be expounded upon in nauseating detail. Application of this art to the example program of rapidly rotating stars will be discussed. Rapidly rotating main sequence stars represent an interesting application of the high resolution capabilities of long baseline interferometry. The otherwise compact photospheres of these objects are equatorially elongated due to centripetal acceleration arising from their rapid rotation; this elongation may be directly detected via interferometry.

Observing Lab: Target Selection

In this lab, the process of target selection will be put to the test as we attempt to construct example observing programs.

Observing Lab: Data Reduction & Modelling

Gerard van Belle et al.
Viewgraphs PDF 280k Bytes.

This lab will walk the student through the process of data reduction, and interpretation of that reduced data in the context of a rapidly rotating star.

References:

• Altair's Oblateness and Rotation Velocity from Long-Baseline Interferometry
  G.T. van Belle, D.R. Ciardi, R.R. Thompson, R. L. Akeson, E.A. Lada
  Astrophys. J. 559, 1155 (2001).

• Fringe Visibility Estimators for the Palomar Testbed Interferometer
  M.M. Colavita
  Pub. Astron. Soc. Pac. 111, 111 (1999).

• On the Near-Infrared Size of Vega
  D.R. Ciardi, G.T. van Belle, R.L. Akeson, R.R. Thompson, E. A. Lada, S. B. Howell
  Astrophys. J. 559, 1147 (2001).

• The Palomar Testbed Interferometer
  M.M. Colavita, et al.
  Astrophys. J. 510, 505 (1999).

• Predicting Stellar Angular Sizes
  G.T. van Belle
  Pub. Astron. Soc. Pac. 111, 1515 (1999).