Shooting My First Supernova

Back at the beginning of May, a supernova was detected in Messier 61, a galaxy in the Virgo Cluster. For us, this is a recent event. But when you consider that supernova actually occured over 52 million years ago, and that the light from it has only just reached us on Earth, it starts putting into perspective exactly how mind bogglingly huge our universe really is.

But what is a supernova? Simply put, it’s the death throes of a star. It’s when it undergoes a rapid expansion in its last evolutionary stage, and sheds its outer layers in the largest explosion in the known universe. The original star at the centre of it will then either collapse into a neutron star, a black hole, or it will completely destroy itself.

This is my attempt at imaging the “recent” supernova, classified as SN2020jfo, in M61.

Equipment & Software

Mount: SkyWatcher Star Adventurer

Telescope: SkyWatcher 72ED

Camera: ZWO ASI178MC (non cooled)

Capture Software: APT

Weather: 20 deg with high cloud

Finding M61

I’m used to having access to my EQ5 Pro, complete with computerised GoTo and the ability to guide with 5 minute subs. For this though, I was at my kids house, and restricted to using the portable SkyWatcher Star Adventurer, and no ability to guide. I had at least though brought my trusty 72ED, or Miranda as I call her, as well as my own imaging camera, the ASI178MC (non cooled.)

Because I’d transported Miranda to the kids house, the first thing I needed to do was double check the focus whilst I carried out my polar alignment routine. For this I chose the star Vega, my mum’s star, and slipped on the Bahtinov Mask. As you can see, I pretty much nailed it right out of the box.

M61 is one of the Virgo Cluster of galaxies, located at RA 12h 21m 54sec, DEC +04deg 28′ 25″. Which is great if you can tell your mount these coordinates. But I couldn’t. I had two options; either star hop, which I’m not especially good at to the point where I’ve previously lost entire galaxies, or I could manually platesolve it myself by taking short exposures, and then using the “Blind Solve” capability of APT and manually orientating the scope until the numbers matched with target coordinates. I’ve tried this before when previously imaging M81 and M82, and it works. As you get closer to the target the movements you have to make to the mount become smaller and smaller, but it’s a process that works, and again was one I found myself using for M61.

Because I know the rough location by eye of whereabouts the Virgo Cluster is in the night sky, getting myself into the approximate area was easy, and it only took another 10 minutes from there to get M61 in the field of view, or FoV.

The Capture

After having issues using the polar alignment tool in SharpCap and being restricted to a rough alignment with the polar scope of the Star Adventurer, I started off first testing at 60 second exposure lengths. Unfortunately I had some trailing so knocked them down to 30 seconds which seemed to be more or less okay. Gain was set to 200 on the ASI178 and I initiated a 4hr capture plan. I already knew in advance that I would face clouds toward the end of the session, and that it would also be getting pretty low down in the atmospheric murk by then, so I set TeamViewer going in order to monitor the session from inside.

About 90 minutes in I could see some download issues creeping in as well with some bad frames showing an interference pattern. After around 200 frames I called it, and stopped the plan. M61 was too low by this point and the other end of the sky around Cassiopeia was looking quite inviting, so I swung round to start a run on NGC281, the Pacman Nebula, using the manual platesolve technique described above.

Another half an hour on that and the clouds were in, so I shot my dark frames, 50 in total. Darks are part of the calibration frames we use, and you can find out more about them here.

I left the flats and dark flats until the following morning, making sure that I’d made no changes to the imaging train.

Stacking & Processing

I loaded the subs into DSS and worked through them, removing any bad frames. These included the bad downloads from the camera, any that had any trailing and any that were photo bombed by passing satellites. In all these gave me a total of 180 frames, which equates to, with 30 second subs, a total of 90 minutes of usable data.

I then loaded up the subs and the full set of calibration frames (not including the bias ones) into APP. I love APP and it’s interface, but it can seem pretty daunting at first when compared to the ease and simplicity of DSS. But for ease of use the process is broken down into numbered tabs, so I’ll quickly run through what I do. Bear in mind I’m also still getting to grips with APP myself, but there are some pretty good tutorials on YouTube for getting you going with it.

0: tick “Force Bayer CFA”

1: select your lights, darks, flats and dark flats under each section in that tab.

2: I tick “Create Bad Pixel Map”, “Adaptive pedestal/reduce Amp-Glow” “Align Channels” and “remove light pollution.” I untick “Create 32 bit masters”

3: I leave this set to default

4: I make sure “Same camera and optics” is ticked

5: I leave at default

6: I set “Weights” to quality, “Integrate” mode to Bayer Drizzle. The droplet size I set to 0.75 and scale to 1.5

Note: if you’re going to use the drizzle function, it’s worth noting that this will increase the amount of hard drive space that APP needs by a massive amount. As an example, when I was stacking M61 my temporary drive space requirements were upwards of 46Gb.

The Processing

Initially I’ll do a crop whilst still in APP, followed by removal of light pollution, background and stars calibration. Because I shoot with flat frames I rarely feel the need to touch the vignetting removal tool. From there I’ll save as a 16bit TIFF file and exit out before opening up Lightroom and importing that saved file into there. This is partly for catalogue purposes, but I also make some global adjustments in there as well. These include exposure, highlights, lights and shadows, as well as some very minor sharpening and masking, as well as minor luminence and colour noise reduction, before opening the file in Photoshop for the levels, curves and star reduction. The last thing I did was to annotate the image in Photoshop with the location of the supernova itself.

Summary

After first hearing about this supernova, I made it my mission to go after it and image it. I’ve never knowingly shot one before and I was pretty excited to go after it, even though the weather conditions were less than ideal. I may well revisit the image itself at some point and see if I can’t improve it any. The stars have some elongation in them from where I’ve left data in the stack that was less than optimal, and there’s still a fair bit of noise in there as well, although some of that is down to the high cloud. If I can get another crack at imagining this afresh, then I’ll try again, but for now, the image below is my first attempt at shooting a supernova.

Thanks for reading, and clear skies!