Many comets constantly travel across our solar system and, from time to time, one may come close enough to earth to be seen with naked eyes in the sky. This happened last with comet C/2020 F3 (Neowise) in 2020.
While such occasions are great enough to observe, revealing finer details require longer focal lengths and exposure times, as usual in astrophotography. Then a green glow surrounding the comet's core will get visible. This is caused by carbon (C2) which is ionized by our sun's strong ultraviolet radiation. While traveling through the outer solar system a comet collects a lot of debris and ice particles which are now left on its path and visible as a bright tail. And there may be a secondary thin, blueish tail, facing away from the sun. These are again ionized gases evaporating from the core.
On its path towards the sun comet C/2022 E3 (ZTF) came near earth in January 2023, so why not point a telescope to that comet and capture light for some hours. What could possibly go wrong?
First of all: Comets move. And they move quite fast, so that their position in relation to the stars change quickly. The image above demonstrates this movement during a period of two hours. Details from the comet are not visible at all, except from the green glow.
Most stacking applications provide tools to align frames not only on stars but on moving targets as well, like comets. Such an integration will reveal finer details of the comet, like the thin secondary tail, but now we have startrails.
Looking more closely on the trails, they resolve into smaller, colored dots. I captured the images with my monochrome setup, so the color channels were taken in sequence. I also had to drop several subs due to clouds, so some star dots are incomplete regarding the color channels.
But in this quite special image, taken on February, 10th in 2023, there is yet another object moving in a different direction, the planet Mars. So the diffraction spikes look blurred. When registering on Mars the integration will look like this:
This leads to a simple conclusion: The three different movements (stars, planet, comet) need to be processed separately and recombined at the end. But for this image I simply reduced the number of background subs to minimize the movement of Mars and so the blur of its diffraction spikes to an acceptable level. Dark nebulae contained in that region will no longer be visible.
Our goal will be to obtain one background image with as little of the comet as possible, and a proper image from the comet without the stars.
I intentionally will not go into processing details, as there are plenty good tutorials around, for example on YouTube. But I'd like to point out the primary steps in my personal workflow, which seems to work fine for both color and monochrome processing.
- First of all I register the good subs on stars and save a new set of images. From habit this is done in Astro Pixel Processor but should work straightforward in other tools like PixInsight as well.
- These subs are then integrated as usual, separate for each channel for monochrome images. As long as the subs are of constant quality, a simple average integration should work fine. The more subs are integrated in this step, the comet glow gets dimmer but wider as well.
To stack the comet some more efford is required, for me this works best in PixInsight:
- From the registered subs I use the process CometAlign in PixInsight to create another set of images, now with the comet at a fixed position. Since CometAlign refers to the image's timestamp, this works well even if there are gaps in the subs. And since the source subs are already registered for stars, guiding errors and dithered exposures do not harm this process.
- Now the comet aligned subs get their stars removed. This should work with StarNet++ but I personally prefer StarXTerminator. This works quite well and the integrated batch processor simplifies this step further. Depending on your computer you may do a break until all images are processed.
- Finally the starless comet images are integrated. A simple averaging without any kind of rejection works fine (again, separated per channel for monochrome images).
For monochrome processing the separated stacks for each color channel are now combined.
The RGB images may be color calibrated as preferred.
The comet image is probably not that clean like the one above. The original stack had a bright red glow of Mars in the bottom left corner, which I removed in Photoshop. The faint trails left from star removal were eliminated with a gradation curve where I slightly emphasized the secondary tail as well.
By the way: Since the faint secondary tail is oriented away from the sun, its orientation changes over time as well. Taking images for too long and with a larger focal length, the tail will loose detail and gets blurred. Depending on the comet's trajectory and focal length even 30 minutes may already be too long. This image was taken using a focal length of 380mm and only a short part of the secondary tail is visible, so the blur is kind of acceptable. For best results a very fast telescope is preferable.