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A New Instrument for the UH 2.2-meter Telescope

by Joseph Masiero

Dual-Beam Imaging Polarimeter (DBIP)

The Dual-Beam Imaging Polarimeter (DBIP) was built at the Institute for Astronomy. Photo by Joseph Masiero.

Light is the primary connection astronomers have with the Universe they study. Quantum mechanics tells us that light can be described using wave-particle duality, where it behaves as both a photon and an electromagnetic wave at the same time. The color of light can be determined from the wavelength, while the number of photons can be counted to find light's intensity. Beyond color and intensity, light can also have a polarization, or an alignment of the vibrations of the electromagnetic waves. This property of light is used by many sunglass manufacturers to make antiglare polarized glasses. Using special optical components, astronomers can also make use of the polarization of light to investigate the physical properties of objects with strong magnetic fields or the scattering surfaces of planets and asteroids.

Some astronomical sources, such as the centers of active galaxies, can be very strongly polarized, but most objects are only polarized at the 10 percent or even 1 percent level. To investigate these objects, we need instruments that have polarization measurement errors less than 0.1 percent. This can be accomplished by splitting the light from an object into two complementary, side-by-side images that have opposite polarizations. By comparing the difference between the number of photons in each image, polarization can be measured to better than 0.1 percent.

With the help of IfA astronomers Klaus Hodapp, Dave Harrington, Haosheng Lin, and generous support from the IfA and the Friends of the IfA, I have recently completed building a new instrument for the University of Hawaii 2.2-meter telescope that will measure polarization in just such a fashion. The instrument, called the Dual-Beam Imaging Polarimeter (DBIP), uses a calcite crystal to split light into two beams of opposite polarization. Along with the calcite are optical elements called retarders, which can alter the polarization of light passing through them, to change what direction of polarization is measured by DBIP. Using these retarders, we can measure light that is polarized linearly in any direction, as well as light with circular polarization (the waves spiral around the path of motion in a helical fashion).

Two calibration runs were performed with DBIP in March and August to test the instrument against standard stars with known polarization levels. Both of these runs had clear weather and excellent viewing conditions that allowed us to take many important calibration measurements. Results for these runs have confirmed that, as designed, DBIP is able to measure polarization to better than one part in a thousand. A final night of calibration in January will test our ability to operate the instrument remotely from Manoa before it becomes available to the entire IfA community for science operations on February 1, 2008.

DBIP on the telescope.

The DBIP mounted on the UH 2.2-meter telescope atop Mauna Kea. Photo by Joseph Masiero.

The precision of DBIP will allow IfA scientists to measure the polarization of asteroids, stars, and distant galaxies in a way that had not previously been available, and on a personal note will allow me to obtain much needed data for the completion of my PhD research. In particular, my work with DBIP will focus on measuring the polarization signatures of asteroids in the Main Belt and inner solar system.

As asteroids and Earth orbit the Sun, the angle between Earth and the Sun as seen from the asteroid changes. This angle, called the phase angle, is one of the main variables that determines the polarization of the light we see coming from the asteroid's surface. The effect of the phase angle is also modulated by the mineralogical makeup of the surface. Because the phase angle for any observation is known, measuring the polarization allows us to investigate the surface composition of our target.

Polarization can be used to determine an asteroid's albedo, the fraction of light that is reflected off its surface. For example, the albedo for a mirror is 100 percent, while for snow it is up to 90 percent, and for new asphalt about 4 percent. Most asteroids have albedos between 4 and 20 percent. Knowing the albedo allows me to determine the diameter of the target body, even though the asteroid looks like a single point in the images.

I will also be looking for changes in the albedo of asteroids as they rotate. Such changes mean that the composition of an asteroid's surface varies from place to place. This can be the result of either the breakup of a very large body or the collision and combination of two asteroids with different compositions. By using this information in my research, I will help to develop a better picture of how the solar system formed and evolved to its present state.

Scientific article: http://arxiv.org/abs/0708.1335