Data acquisition and Analysis of the Taiwan-America Occultation Survey

J. F. Chen (陳睿甫) & W. P. Chen (陳文屏)
Institute of Astronomy, National Central University, Taiwan, R.O.C.

Abstract

The TOAS (Taiwan-America Occultation Survey) project aims to perform an inventory of Kuiper Belt objects by monitoring chance stellar occultation events.  The short duration of a typical event, a few tenths of a second, poses stringent requirement in data acquisition, processing, and analysis.  We describe how fast-photometry is achieved by reading out the CCD chip sequentially a few rows at a time (pause-and-shift), and how these row-blocks of data, with temporal and sky-background confusion, may be analyzed to derive stellar light curves.  From data taken at Lulin Observatory, we show how image motion is correlated across the 3 square degree field of view of the telescope.  We also present simulated row-block data --- taking into account of instrument and site characteristics --- of proposed target star fields to be monitored for the survey.

Data Acquisition

TAOS is a collaborative project (Chen 2000) that will monitor the frequency of chance stellar occultation events to estimate the total number and spatial distribution of Kuiper-belt objects (KBO).  A total of 4 wide-field (D=0.5 m, f/2) telescopes --- contributed by Lawrence Livermore National Laboratory of the USA, Academia Sinica of Taiwan, National Central University of Taiwan, and Yonsei University of Korea --- will be put atop Lulin Observatory in Taiwan.  A few thousand stars are to be monitored simultaneously for photometric variations characteristic of an occultation event.  A typical KBO (diameter of a few kilometers, at a distance of 40-100 AU) stellar occultation will dim a background star for about 0.2 s.  Thus data acquisition and photometric measurement have to be done in a timely fashion.  

Fig. 3:  Two stars 10’ apart on the sky do not seem to correlate on their image motion vectors.  
We have developed a data acquisition technique called the 'zipper method' due to the appearance of the stellar images.  The CCD camera (AP-10, 2048x2048 pixels) continuously reads out a sequential block of rows (e.g., 32 rows at a time) of the chip after each pause (integration), while the shutter remains open and the telescope keeps tracking on the same target field of the sky.  This electronic pause-and-shift scheme essentially produces a light curve for each star in the field with very high time resolutions.  Except in the very beginning when the shutter has just opened, the image at any time will be filled with dots; the image of a star, as time goes on, will trail like a zipper (Fig. 1), thus the name.   The readout continues until the CCD computer RAM buffer is full.  Such a ‘zipper’ image will then be divided into row-blocks, each of 32x2048 pixels, for stellar photometric analysis.  Fig. 2 illustrates the data acquisition process, with different colors denoting sequential pause-and-shift steps.     
Fig. 1: ‘Stare-mode’ star field image and its ‘zipper-mode’ image

Fig. 4:  A simulated TAOS target field (a) stare-mode image, (b) zipper-mode image, and (3) one of the row-blocks.  

Fig. 2: Each row-block presents a time-series sampling of flux for each star.  With a fast pause-and-shift  (each shown with a different color) high-speed photometry is obtained.  

Image Motion & Photometry

Stellar flux is measured by the total count within a specified distribution of pixels encompassing as complete as desirable of a star’s radial profile, less background contribution from sky, instrument, etc.  For the TAOS data, sky background is a major concern.  This is so because adjacent pixels to a star, unlike in traditional ‘staring mode’ imaging, accumulate exposures from different parts of the sky at different times in the pause-and-shift process, therefore cannot serve as a fair measure of the true sky background.   

To assess the sky background and to investigate how image motion is spatially correlated across the 3 square degree field of view, we analyzed data in ??? (month, year) at Lulin Observatory with the TAOS instruments.  We cross-correlated how the photocenters of two stars changed, in terms of motion vectors (amplitude and position angle), with respect to each other, and found no apparent coherent motion for stars 10’ apart (Fig. 3).  
Simulated Data

To provide an aid to photometric analysis software, we have developed an IDL code to generate TAOS data.  A selected star field (e.g., from a star catalog, with position and brightness information of each star) can be used to produce a ‘zippered’ images, with a corresponding set of row-blocks, by taking into account of instrumentation responses, various noises, and image motion.  Fig. 4 shows an example of a simulated image, with a typical star image of an FWHM of 2 pixels (each ~3”) , and an image motion with a Gaussian distribution of an FWHM of 0.1 pixel.  Photon noises for the sky and for the star are assumed Poisson.  

References

Chen, W. P., 1999, Census of Kuiper Comets --- The Taiwan-America Occultation Survey (TAOS) Project,  East Asia Meeting of Astronomy, Kunming, Yunnan, China

King, S. K. _(IAUC 183??