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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:

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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