Caption: These two pictures of P/2010 A2 were taken UT 2010 April 02 from the HST and are included here to show how badly affected is HST by "cosmic rays", how faint a target is P/2010 A2 and how, despite all this, something fantastic can be pulled out of the data. [click on the panels for larger versions].

The first panel shows a "raw" image from WFC3. It is peppered with cosmic rays (the points and short lines, which are cosmic rays or other charged particles passing diagonally through the thickness of the CCD detector). The longer lines, some sharp and some fuzzy, are field stars and galaxies that are trailed because the HST was tracked to follow the motion of the object. P/2010 A2 itself is a faint shadow in the raw image. Step back from the computer to see it better. Can you pick out the main nucleus in the first panel without checking the second panel to see where it is? No, I didn't think so. Neither could I.

The second panel shows the "cleaned" image, in which cosmic rays and star and galaxy trails have been removed. The latter process is crucial to allow measurements and science from the HST data. As the image shows, there are limits....some faint background objects cannot be completely removed from the data without compromising the science.

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David Jewitt

Comet	Jewitt	Kuiper