I managed to dig out C/2017 K2 in CFHT images taken in 2013 May. The search was thought to be hopeless at the beginning, because the SSOIS of the CADC did not show any useful serendipitous images within the past three years before its discovery. The comet was discovered already at a heliocentric distance of AU after all. But after noticing a long exposure was used for those CFHT images, I decided to look into the data anyway.
The 3-sigma uncertainty region computed by the JPL HORIZONS based on then-available astrometry was pretty big, ~10′. My search was focusing on images taken from May 12, since that night there were more images. But soon I found out that the last two images were kind of useless, as the limiting magnitude was worsening and the brightness of the sky background rose up, which made my visual search more difficult.
When I was about to finish my last scan of the images from a common chip, I suddenly noticed something comet like moving at the expected direction, and also at a right speed. I measured the positions from three best images, added the astrometry to all observations about C/2017 K2 from this year, and then loaded the file into Find_Orb. Excitingly, the software successfully produced an excellent orbital solution with pretty small astrometric residuals.
With the new orbital solution, looking for C/2017 K2 in images from the second night was way easier. Unfortunately there were only two images, and one of them suffered from bad quality. I managed to find a faint blob of light at the expected pixel coordinates based on the new orbital solution.
The orbital solution by MPEC 2017-N26 puts the comet onto a heliocentric hyperbolic orbit. These CFHT serendipitous observations of C/2017 K2 turned out to be a record of the most distant comet ever observed during the preperihelion stage ( AU). The equilibrium temperature at such a distance would be as low as ~59 K, which means that the activity must be driven by supervolatiles rather than water ice. Hmm, interesting…