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Appendix A: Image cleaning procedure

With the aim of removing cosmic ray hits (CRH) and stellar profiles elongated by non-sidereal tracking, we grouped consecutive images into pairs and compared the first frame with the second frame in each pair. We took advantage of the statistical independence of the CRHs in different frames, and that in two consecutive frames a given stellar trail occupies separable regions (the 40-s CCD readout time corresponds to three arcsec of cometary motion). We subtracted and averaged images in pairs, obtaining two series of images: one with differential frames and an other with mean frames. For each differential image, we correlated its noise statistics with the mean signal in the corresponding average frame. Using the relation between the noise level and mean signal, it is possible, for a given pixel, to estimate the dispersion in the signal in the differential frame. When the pixel value in the differential image was higher than Kσ (i.e. a factor of K above the local noise), we flagged it in an auxiliary binary mask frame. As the stellar patterns in the auxiliary mask frames contain regions with disconnected pixels (external envelopes of bright stars and whole patterns for stars at the noise limit), we produced compact patterns improving pixel connectivity. We used the closing-like procedure based on the local surface density of the flagged pixels. For a given pixel, the density was probed in a small circle surrounding this pixel and compared with the limiting value. This value was determined using the trial-and-error procedure. Above this limit, the pixel was kept flagged and vice versa. This stage of image processing was completed by synthesizing clean stacks. In the regions where neither CRHs nor stellar trails were detected, the mean values of both

frames were taken. Otherwise, the values from the image with no detection were accepted. As the comet was observed in relatively dense stellar fields, the profiles of different stars occasionally overlapped in the images from one pair. In these situations, the stellar trails were masked and filled using interpolation. By stacking two consecutive images, we increased the S/N except for the regions occupied by the stellar profiles or CRHs. We note that in rare cases one and the same frame was used to create two consecutive pairs. Obviously, such pairs are not independent, but let us use the entire nightly material, even if the number of single images is odd. Figure A.1 presents one example of how our cleaning procedure is able to remove unwanted objects from the comet image.

	Fig. A.1 Result of removal of CRHs and stellar profiles by the cleaning procedure (left panel). For comparison, the right panel shows the result of simple stacking of the same two frames. Date of observation is UT 2010 Nov. 7.1375. The field of view is 6.4 arcmin. North is up and east is to the left.
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Movie 2 (Access here)

© ESO, 2012