The Messenger 115
March 2004

Correcting spatial gradients: The case of Wide Field Imager photometry

A. Koch [1,2], E.K. Grebel [1,2], M. Odenkirchen [2], J.A.R. Caldwell [2,3]

[1] Astronomisches Institut Basel, Switzerland; [2] MPIA, Heidelberg, Germany; [3] STScI, Baltimore MD, USA

Abstract: We present a method to significantly reduce large-scale photometric variations seriously affecting imaging data from the Wide Field Imager (WFI). The primary source for these gradients is non-uniform illumination, which cannot be corrected by standard flatfielding techniques. Comparison of our observations with well-calibrated multi-colour photometry from the SDSS enabled us to characterize and quantify these variations and finally to model them using a second-order polynomial. Application of the model to our observations and an independent dataset consistently reduced the large-scale gradients and thus provides a generally valid and simple tool for improving WFI photometry.

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The Messenger 115March 2004

Correcting spatial gradients: The case of Wide Field Imager photometry

A. Koch [1,2], E.K. Grebel [1,2], M. Odenkirchen [2], J.A.R. Caldwell [2,3]

[1] Astronomisches Institut Basel, Switzerland; [2] MPIA, Heidelberg, Germany; [3] STScI, Baltimore MD, USA

The Messenger 115
March 2004