7.6.2 Analysis of Dithered Data

The software we recommend for combining dithered data is known as "MultiDrizzle" (Koekemoer, et al. 2002), which is based on the "drizzle" program (Fruchter and Hook 2002). This method has been incorporated into the IRAF/STSDAS dither package, and allows effective cosmic ray removal from dithered data.

In order to help users reduce dithered images, we have prepared the HST Dither Handbook (Koekemoer et al. 2002), available from the above WFPC2 dither web site. This document gives a general outline of the reduction of dithered images and provides step-by-step instructions for six real-life examples that cover a range of characteristics users might encounter in their observations. The data and scripts needed to reproduce the examples are also available via the same URL. (This handbook expands upon the original Drizzling Cookbook by Gonzaga et al. 1998.)

Despite all the improvements in the combination of dithered images, users should be mindful of the following cautionary notes:

• Processing singly dithered images can require substantially more work (and more CPU cycles) than processing data with a number of images per pointing.
• Removing cosmic rays from singly dithered WFPC2 data requires good sub-pixel sampling; therefore one should probably not consider attempting this method with WFPC2 using fewer than four images and preferably no fewer than six to eight if the exposures are longer than a few minutes and thus subject to significant cosmic ray flux.
• It is particularly difficult to correct stellar images for cosmic rays, due to the undersampling of the WFPC2 (particularly in the WF images). Therefore, in cases where stellar photometry better than a few percent is required, the user should take CR-split images, or be prepared to use the combined image only to find sources, and then extract the photometry from the individual images, rejecting entire stars where cosmic ray contamination has occurred.

Figure 7.8: 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.

• Offsets between dithered images must be determined accurately. The jitter files, which contain guiding information, cannot always be relied upon to provide accurate shifts. Therefore, the images should be deep enough for the offsets to be measured directly from the images themselves (typically via cross-correlation). In many cases, the observer would be wise to consider taking at least two images per dither position to allow a first-pass removal of cosmic rays for position determination.
• Finally, and perhaps most importantly, dithering will provide little additional spatial information unless the objects under investigation will have a signal-to-noise per pixel of at least a few at each dither position. In cases where the signal-to-noise of the image will be low, one need only dither enough to remove detector defects.