Drizzle (image processing)

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Drizzle is a method for the linear reconstruction of undersampled images. It is normally used for the combination of astronomical images and was originally developed for the Hubble Deep Field observations made by the Hubble Space Telescope. The algorithm, known as Variable-Pixel Linear Reconstruction, or informally as "Drizzle," preserves photometry and resolution, can weight input images according to the statistical significance of each pixel, and removes the effects of geometric distortion on both image shape and photometry. In addition, it is possible to use drizzling to combine dithered images in the presence of cosmic rays.

[edit] The goals of Drizzle

On the left a single 2400s F814W WF2 image taken from the HST archive. On the right, the drizzled combination of twelve such images, each taken at a different dither position.

Camera optics generally introduce geometric distortion of images. Undersampled images are, for example, common in astronomy because instrument designers are frequently forced to choose between properly sampling a small field of view and undersampling a larger field. This is a particular problem for the Hubble Space Telescope (HST), where the corrected optics may provide superb resolution, but the detectors are only able to take full advantage of the full resolving power of the telescope over a limited field of view. Fortunately, much of the information lost to undersampling can be restored. The most commonly used of these techniques are shift-and-add and interlacing.

Drizzle was originally developed to combine the dithered images of the Hubble Deep Field North and has since been widely used for the combination of dithered images from both HST's cameras and those on other telescopes. Drizzle has the versatility of shift-and-add yet largely maintains the resolution and independent noise statistics of interlacing. Drizzle has the advantage of being able to handle images with essentially arbitrary shifts, rotations, and geometric distortion and, when given input images with proper associated weight maps, creates an optimal statistically summed image. Drizzle also naturally handles images with "missing" data, due, for instance, to corruption by cosmic rays or detector defects.

Drizzle is freely available as an IRAF task as part of the Space Telescope Science Data Analysis System (STSDAS) package and can be retrieved from the Space Telescope Science Institute (STScI) web site. In addition to Drizzle, a number of ancillary tasks that assist in the combination of Hubble Space Telescope imaging data are available as part of the "dither" package in STSDAS.

Drizzle was developed as a collaboration between the Space Telescope Science Institute and the Space Telescope European Coordinating Facility. A detailed account of the method can be found in the paper referenced below.