Comet photography
How to produce images of sharp comets against sharp background stars
Your images can contribute to science by helping to document how a comet evolves over time. A good and surprising example was comet 17P/Holmes, which underwent a massive outburst back in 2007, appearing over half a million times brighter than expected. The outburst was accompanied by many changes in the comet’s
appearance that, when viewed in sequence, described the fascinating and unusual evolution of this amazing object.
Many different camera setups are suitable for imaging comets. Brighter comets make ideal targets for general photographic equipment, such as a DSLR fitted with a standard lens. Dimmer targets up the ante, requiring more sensitive equipment, typically a cooled CCD camera fitted to a telescope.
Comets move, often quite rapidly, relative to the background stars. For this reason, mid-range ISOs and relatively short exposures between 30–120 seconds are normally used. Dark and flat-field calibration is also essential to obtain the best and most accurate results. When it comes to registration-stacking, doing it on the stars will result in a blurred comet, while registering on the comet will produce trailing stars.
More sophisticated techniques involve registering on the comet and rejecting maximum pixel values between shots. If a comet’s movement is sufficient, this should remove most of the stars. Subtracting the resulting comet-only image from the original star and comet frames will isolate the star-field. Stacking the star-only images and recombining them with the processed cometonly image then allows you to produce a shot of a detailed comet against the background of a sharp star-field.
Submissions to organisations such as the BAA often require image filenames to be formatted in a certain way. For the BAA, the format cometname_yyyymmdd_ hhmm_observer is preferred, where ‘cometname’ is the identifier of the comet, ‘yyyymmdd’ and ‘hhmm’ are the date and time of the image and ‘observer’ is the imager’s name or initials. Scale and orientation should also be included on the image with the preferred orientation being north up and east to the left.