A method for measuring distortion in wide-field imaging with high order polynomials

Yasuda, Naoki

Determination of large optical distortion is one of the challenges in wide-field observations. In general, the large distortion is represented neither by a conventional linear terms nor low order polynomials, but needs high order polynomials. The distortion has to be determined and corrected based on high order polynomials for precise analysis for astrometric and photometric studies. We develop a new algorithm to derive the accurate WCS from a wide-field imaging data, by using the TAN-SIP convention to represent the distortion with the high order polynomials. One notable improvement in our new algorithm is employment of simultaneous fitting of the TAN-SIP parameters for the entire field of view rather than using only one CCD data. Since only one CCD has an insufficient area and rather a small number of reference sources to determine accurate the TAN-SIP parameters. Instead, we increase the reliability of our algorithm by using reference sources of all CCD's in the focal plane at at time to derive the global TAN-SIP parameters, and transforming the parameters to corresponding parameters of each CCD. We report the detailed algorithm of parameter fitting and transformation of the TAN-SIP parameters.

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