After weeks of Bavarian cloudy nights, I finally had a chance to photograph the comet 46P/Wirtanen. Icing on the cake: the sky cleared up right when the comet was passing the Pleiades!

Comet 46P/Wirtanen and the Pleiades
Mount: iOptron SkyGuider Pro (without counterweight)
Camera: Fujifilm X-T1
Lens: Fujifilm XF 55-200mm f/3.5-4.8 R LM OIS, at ~150mm
Aperture: f/5.6
ISO: 1600
Lights: 40 x 30 seconds
Darks: yes
Flats: no
Bias: yes
Post-processing: DeepSkyStacker, Photoshop

The acquisition

Following my last journal entry, my SkyGuider mount was back from repair, thanks to the folks over at Teleskop Service. All I needed was a clear sky, and I have been lucky!

I wanted to photograph the comet as it was passing near the Pleiades. It makes a much better composition, especially with the blue color of the reflection nebula. The only window to capture this fly by was Saturday night, and luckily, that’s also the only clear night I had for weeks.

I wasn’t really sure about which focal length to use. Comets are fast moving targets, and to have the Pleiades in the frame as well, I needed something wide. I ended up using my Fujifilm 55-200mm zoom lens, as I was also curious to see how it performs for astrophotography.

The weather was quite cold, close to 0°C and with a bit of wind. I went to Munich’s city park, which doesn’t have any annoying street light. However, the light pollution here is still heavy (Bortle 6/7).

Finding the comet wasn’t difficult, due to its proximity to the Pleiades. I still had to take a few pictures to compose the shot properly. I could barely see 46P with my binoculars.

I settled for rather short exposures (30 seconds), due to the heavy light pollution and the speed of the comet. With a longer exposure, I was afraid 46P would appear elongated.

To give an idea of how fast it’s moving, I made a timelapse with all my subs, which spans over about 30 minutes.

Timelapse of comet 46P/Wirtanen

I had some trouble finding the right aperture. I tried f/5.6 and f/6.3, because the Pleiades are much brighter than the comet. I didn’t want to overexpose the stars. Finally, I ended up using only the subs at f/5.6.

It was the first time I used a zoom lens for astrophotography, as well as the SkyGuider mount without its counterweight. And of course, I remembered the hard way that attaching a lens hood to a zoom lens pointing up, makes the lens retract…

In the end, I realize that I wasted some precious time fiddling with the gear and settings. Given the very cold weather, I didn’t really want to spend the night outside, so I ended up without a lot of data. But overall, happy to have a few pics of the comet!

The processing

The acquisition wasn’t easy due to poor logistics, but the processing was a nightmare!

First of all, I didn’t have a lot of data to play with. I only used the best subs, about 40 pictures of 30 seconds each.

Secondly, I had a lot of issues stacking all the subs. I made about 10 different attempts, with different settings, and more or less subs. The main problem is that, the stacking program has to align and stack the stars, but also the comet, which are all “moving” at different speeds.

There are several methods to do it in DeepSkyStacker:

I tried the Stars + Comet Stacking method first, but couldn’t get any decent result. I even tagged the comet manually on all my subs, using the comet editor in DSS, but that didn’t really help.

I finally opted for two stackings: one in Comet mode, and one in Standard mode. Then, I processed both files similarly, and pasted the area around the comet into the main image with the Pleiades. I’m not very good at this, and the result isn’t very clean…

What I learned

Stacking and processing a comet is very difficult! When imaging DSO, my comfort zone isn’t very comfortable yet… But having both a comet and a DSO in the same frame is even harder. It required a lot of trials and errors, and I don’t exclude to have another go at 46P/Wirtanen some day!

Also, as always, the logistics are key. Trying out a new lens system on a new type of object, that’s not going to be here tomorrow, isn’t a very good idea. Perhaps I should have sticked to a lens I know better, like the Samyang 135mm.

However, despite the problems I had, I did learn a lot. And I’m quite pleased with the result overall. What matters, in the end, is that I was able to photograph this little comet!

About comet 46P/Wirtanen

Comets are really fascinating objects, and I’ve wanted to photograph one for a long time. Sadly, they’re quite rare and often travel far from the Earth.

But sometimes, all you need is a bit of luck! Comet 46P/Wirtanen is orbiting the Sun, roughly between the orbits of Jupiter and the Earth (its perihelion is about 1.06 AU!).

With an orbital period of about 5 years, 46P is considered to be a short period comet. It is also a small object, according to a recent radar observation, with a nucleus about 1.4 km wide. As a comparison, comet Halley’s nucleus was 15 km wide!

Interestingly, the ESA considered landing on this comet, as part of the Rosetta mission. But due to a technical issue, the launch was delayed and the scientists had to choose another target, 67P/Churyumov-Gerasimenko.

Photographing comet 46P/Wirtanen (December 2018)

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