Intro

A comet can be both an astrophotographer's nightmare and treat. Comets never stay put with respect to the background stars, but move through the field at a speed that depends on the geometric circumstances (e.g., distance of the comet to the Earth and the Sun, position along its orbit, especially with respect to its perihelium, and so on).

When observing visually through binoculars or a telescope, a comet appears fixed in the field, but it will move noticeably between frames taken even just a few minutes apart. So, if the frames are aligned on the stars, the comet will be trailed, but if they are aligned on the comet's false nucleus, this will yield trailed stars. What if we want to produce an image more similar to the visual view, where both the comet AND the background stars are nicely fixed in the field without any hint of trailing? This it not impossible, but is often quite of a challenge for astrophotographers.

Procedure

Several astrophotography applications have tools for aligning comet frames, but still, producing a composite that gets rid of any kind of trailing can be quite challenging. Back in 2018, renowned astrophotographer and PixInsight team member Edoardo Radice gave a nice webinar which describes a procedure for obtaining untrailed comet images in PixInsight:

Unfortunately the webinar is available only in Italian and is relatively old (late 2018), but most of the content is still valid today with the latest available PixInsight release (1.8.9-2).

Since following the webinar step by step can be quite difficult, especially for non-Italian speakers (the majority I guess), I took the hassle of turning the 50-min video into a detailed tutorial. For the sake of compactness, this tutorial contains almost no figures, so it is assumed that the user is already familiar with PI's user interface, workflow, approach, and processes.

I am sometimes under the impression that PixInsight is not the fastest tool for specialized applications :-), but the procedure works very nicely for me. I processed all untrailed images of comets published on my website with this procedure.

The tutorial is available at this link in PDF format. Please write directly to me (by e-mail or in a comment) for questions, suggestions or any type of feedback, always appreciated.