Comet processing

Forums Comets Comet 46P/Wirtanen Comet processing

Martin Mobberley

Hello James,

There’s no simple answer to the question of how to process comet images as every comet is different. Some move fast, some move slowly, some have very little tail detail and some have loads of complex ion tail detail. Also, individual equipment, and skies, and detectors vary considerably. My own recent 46P images have all been remote, via Siding Spring. The comet has been almost overhead there, in a very dark sky. In addition the system I’ve been using has a sensitive FLI 16803 cooled detector. I take LRGB images because the L image goes deep and produces a very smooth background, so the noise in the RGB images is not obvious. With the system you are using it would be best to take as many frames as possible to improve signal-to-noise. Even exposing for 1 hour (eg 60 x 60s) is not excessive providing you are happy with long star trails.

Modern CMOS (eg ZWO ASI) devices have sensitive low noise detectors which would outperform your Canon 6D. The Sony A7s cameras have remarkable sensitivity too. Your images look very dark. What’s needed is some sort of log stretch to brighten up the outer coma without burning out the comet’s core. A straight histogram stretch won’t work. All modern astro packages have a variety of stretches which can be experimented with for best results.

I used to use AIP4Win but now (as AIP V2 doesn’t like Windows 10) use AstroArt 6. Weeks of endless experiments with various stretching routines and tweaks will deliver the results. For final labelling and jpeg production I use an old copy of Paintshop Pro!

I could go on, but it’s an endless subject!
Some experts use k-sigma type routines to separately stack on the stars and stack on the comet resulting in an image with stars and comet stationary, but that is VERY complex and labour intensive. Some even image the stellar background separately when the comet has departed so the comet doesn’t mess up the starfield! Obviously at some point science drifts into art if you take that approach too far!!

As a first step I’d simply stack as many short exposures as possible with the comet as high up as possible, in as dark a sky as possible, and then experiment with every non-linear stretch routine you have at your disposal to see which works best!

Email me direct, off-forum, if you want any detailed info!