The Green Comet and the Red Planet

Belated post but I have several things to share so playing catch-up....

In my last post, I shared my first attempt to capture The Green Comet, the solar system body with the official name C/2022 E3 (ZTF). By the time I was shooting it, the comet had faded enough that capturing it in the heavily light-polluted urban skies where we live so it was just a dim, green smudge in that shot. 

Much of the challenge in taking that shot was the glare of the Moon. About a week later, the Moon wasn't rising until after midnight so I had an opportunity to try again. Thankfully, the skies were clear on February 11th because in addition to the period of moonless skies that night, the Green Comet was within the same telescopic field of view as the Red Planet, Mars. 

To boost my prospects of capturing a better image of the comet framed in the same skies as Mars, I also went looking for a spot with less light pollution. My initial plan was to shoot from out by Lake Ray Hubbard. The light pollution at home is about as heavy as it can get, classified as 8-9 on the Bortle scale, whereas the skies out at the lake are darker, Bortle class 6. That's a long way from the truly dark skies in Big Bend (Bortle class 1) but I was limited to what was available in the Metroplex and I've had a lot of success over the years shooting things like meteor showers out there. 

Unfortunately, the spots I usually went had too many lights in the direction of the comet so I had to try someplace new. Consulting a light pollution map, I picked a spot a little south of Forney, outside a town called Talty. I found a spot at a turn-out off of an FM road we used to drive to get to our lake cabin at Cedar Creek Lake that was Bortle 5. I set up my telescope and started shooting. After capturing dozens of shots plus calibration shots, I packed it in and headed home. 

After a few days working on post-processing the images (see prior post about the process involved), here is what I ended up with. 

Although not perfect, I am pretty satisfied with the result. With the Green Comet on the left, Mars on the right and the stars of the constellation Taurus the Bull in between (the bright star at bottom is Tau Tauri, in the horn of Taurus), it's a lot closer to what I was hoping for than my last attempt. To do any better than this, I probably would have needed an equatorial mount capable of tracking the comet movement (vs. tracking the stars with my current tracker). If you see other shots of this comet where the tail is more prominent, it's likely it was shot with a telescope on an equatorial mount. 

One thing to note about this image is, yes, that really is Mars, not a star (or the Sun). Because I was taking long-exposure shots to capture as much of the comet as possible, then Mars (far brigher than the comet) is a bit over-exposed. That's why it appears more yellowish/white instead of rusty red. The down-side of an over-exposure like this ("blown highlights") in a digital shot is that the over-exposed area is lost data... there is nothing there to edit so no way to correct it. At least not without adding something to the image that wasn't naturally there. 

I learned a lot planning for and executing this shot. Sadly, it's unlikely I'll ever get another chance to shoot a comet and Mars like this but there are plenty of other interesting sights in the night sky that I'll be able to shoot, applying what I learned shooting the Green Comet and the Red Planet. 

As always, click on the image to see it big!

The Green Comet

Well, the first thing that came to mind seeing news headlines about The Green Comet was a flashback to the 60s TV show The Green Hornet staring Bruce Lee. :-)

Seriously, this has been a newsworthy comet. While it wasn't bright enough to be seen except under very dark skies and/or with optical assistance like a telescope, as you know if you've followed the news, it is a singularly unique solar system visitor. Check out sites like EarthSky.org for more details and some of the more interesting images. 

For my part, I had hoped to capture comet C/2022 E3 (ZTF) with my rig (Evostar 72ED on a Star Adventurer tracker, using a Nikon D750). Unfortunately, the North Texas weather hasn't cooperated. Last week when it was at its closest to Earth (and at its brightest magnitude), we were in the throes of a winter storm. By the time we had a night with relatively clear skies on Friday night, we also had a nearly full Moon. It was also still very cold outside. If it had been a moonless night, I'd have packed things up and headed out east of town to darker skies to shoot but since I knew the Moon was going to make it hard to get really good shots, I just set up in my backyard. At least I could duck inside from time to time to warm up. 

Here is the result. See below for more on how it was created. 

Comet C/2022 E3 (ZTF) captured in my backyard
February 3rd, 2023

Between the light pollution and the glare of the Moon, it was hard for me to pull out much detail but you can at least see the dim fuzzball of the comet's core. The comet was in a region of the sky without any bright stars but the few brighter ones on the right side of the image are in the constellation Camelopardalis.

Still, it was time well spent. It was the first time I've gone through the process of capturing what are called calibration frames (bias, darks, flats), additional shots taken to improve astrophotography images by accounting for things like sensor noise and dust. It is also the first time I have processed using calibration frames; I took 100 images (20 seconds each) and post-processed them and the calibration frames using a software tool called Siril and then finished in Photoshop. 

Capturing a comet is often tricky since it is moving at a different rate than the stars. Using a star tracker (vs. a telescope mount), my only choice was to track the stars and hope to be able to post-process the comet and stars separately then combine them afterwards. I did that but it took a long time and many tries just to get the results above. I wish I had more data but longer exposures weren't possible since the histogram showed that any longer than 20 seconds at ISO 3200 would have resulted in clipping (i.e., the blue was almost lost to the right side at 30 seconds). On top of that, the comet would have blurred if I used too long an exposure given how fast it was moving.

One of the other tools I learned how to use is Astrometry.net which can do something called plate solving, taking an image and finding where in the sky it is a view of. Here is the results of plate solving using one of the images used to create the shot above. I find this raw, astrometrically-labeled view an interesting alternative version of my night's labors since the comet (though not green) stands out a bit more.

The three dots and labels represent stars in the constellation
Camelopardalis identified using plate solving on astrometry.net

Anyway, the next time I am shooting a dim fuzzy (hopefully without the Moon up and away from city light pollution), I'll have had some practice and expect to be able to produce much better results. 

Stay tuned...

Click on an image to see full-screen.