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3.0 Dithering with WFPC2
To be read in conjunction with Section 7.6 of the WFPC2 Instrument Handbook, Version 4.0
Dithering is the act of displacing the telescope between observations either on integral pixel scales (to assist in removing chip blemishes such as hot pixels) or on sub-pixel scales (to improve final image resolution). The present Handbook provides a good introduction to dithering strategies in Section 7.6; however, as our experience with processing dithered WFPC2 data is now substantially greater than when the Handbook was written, we can afford to be somewhat less conservative in our recommendations. In particular, at the time of the last writing, our experience in removing cosmic rays from singly dithered data (i.e., only one image per pointing) was quite limited. We are currently preparing IRAF scripts for beta distribution, which in conjunction with the variable pixel linear reconstruction (Drizzle) method (Fruchter and Hook, 1997), can be quite effective in processing such data. Nonetheless, users should consider the following cautionary notes before proposing to take singly dithered data:
- 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 a 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 to 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.
- One must be able to determine the offset between dithered images. The jitter files, which contain guiding information, can not always be relied upon to provide accurate shifts. Therefore, one should have images that are sufficiently deep such that the offsets can be measured by observing the offsets in features in the image (typically through cross-correlation of the images). 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.
Further information on the software in development to process dithered data can be found in two papers in the 1997 HST Calibration Workshop Proceedings: "A Package for the Reduction of Dithered Undersampled Images," by Fruchter et al., and "Dithered WFPC2 Images-A Demonstration," by M. Mutchler and A. Fruchter.
Figure 3: 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.
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Last updated: 06/16/98 10:38:00