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Introduction

The Taiwan American Occultation Survey (TAOS) aims to detect small (~1 km diameter) objects in the Kuiper Belt and beyond. More than 1,000 small bodies with diameters larger than 100 km have been detected beyond Neptune over the past decade using large telescopes. Pluto and its satellite Charon are two of the largest members of this family. The goal of the TAOS project is to measure directly the number of Kuiper Belt Objects (KBOs) with diameters in the range of 500 m to 30 km.

Such objects are expected to have magnitudes larger than 30, and are thus impossible to detect directly, even with the largest telescopes. However, when such objects pass between the observer and a distant star, the resulting occultation signature is detectable, even with a small telescope. The rate of such occultation events is extremely low (we expect less than 10^-2 events per star per year), so we need to monitor hundreds of stars for a long period of time to detect any events. Such events are very short in duration (typically less than 0.2 seconds), so high speed readout is needed. Furthermore, the diameters of the objects we are searching for are in the Fresnel regime, so the occultation shadows will exhibit significant diffraction effects.

TAOS operates four small (50 cm) telescopes at Lulin observatory in central Taiwan. Each telescope is equipped with a 2kx2k CCD camera manufactured by Spectral Instruments. These cameras take 2.5 seconds to read out an entire image, which is much too slow to detect occultation events. We have thus developed a new readout technique which we call zipper mode. A detailed description of this readout method is given here.


A CCD camera used by TAOS
A CCD camera used by TAOS.

The telescopes TAOS A and TAOS B at Lulin Observatory
The telescopes TAOS A and TAOS B at Lulin Observatory.

TAOS began collecting data with three telescopes in February, 2005, and the fourth telescope came online in August of 2008. As of that time, the survey has collected over 500,000 star-hours of image data, which corresponds to a total of 27 billion individual photometric measurements. No events were found in the data set, allowing the survey to place strict upper limits on the surface density of small objects in the Kuiper Belt.

A CCD camera used by TAOS
The telescope TAOS A.
The TAOS telescopes in operation

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