Photographing the Milky Way

In this post, I'm going to show how I created this image of the Milky Way, all the way from planning to processing.

Hartung-Boothroyd Observatory under the Milky Way on an August summer night

This post comes after a long break. The reason for the break is that my photography workflow has stagnated. Therefore, there really wasn't too much matter besides what had already been published on this blog. However, since the last time I posted a tutorial on Darktable, the software has advanced sufficiently – this warrants a new tutorial to show off some of the cooler features of the software – to become my primary workhorse. In fact, I really haven't used GIMP in the recent past.

Photographing the Milky Way requires planning. The Milky Way can be seen in the northern hemisphere only in the summer months. Therefore, August was really the last opportunity I had until March when the Milky Way would reappear. This is compounded by the difficult Ithaca weather, which presents rather gloomy overcast skies most of the time. The lucky break was just this past Thursday with a clear sky.

I envision a photograph of the Milky Way with an interesting foreground. I think it is the foreground that helps set the context for the Milky Way in the background, otherwise I'm just looking at a splash of colour against a black background with some pinpoints of light. Luckily, Hartung-Boothroyd Observatory is ideally located. It is an observatory, so it has been built on a relatively dark mountain and also makes a meta statement when photographing the galaxy. The fact that it is just a fifteen minute drive from my home is also very attractive.

To plan the shoot, I used Stellarium, a free and open source planetarium. It has an extensive database of cities and observatories, so I set the location to Hartung-Boothroyd Observatory, and the time to when I planned to be there. I was looking for the Milky way core being visible at the time, which it was – thank god – and that the moon would not be in the sky at the time. Now, it is possible to take a great photograph of the Milky Way with the moon behind the camera and low towards the horizon, that way it lights up the foreground just right and can be very interesting. However, having no moon at all makes it much easier. The information I gained from Stellarium helped me visualise the photograph in my head, so that I new exactly what I had to do once I reached the location. This helped me a great deal, because as you'll soon see, the Milky Way wasn't even visible to the naked eye.

I knew that the Milky Way would be due South, and the core of the galaxy would be down close to the horizon. This meant that I could not attempt a photograph with the arc of the Milky way above the observatory or something similar. I had to take a photograph with the Milky Way descending upon the observatory, or close to it. The next thing I had to decide was on how I planned to light up the observatory to make it stand out in the foreground. I recently bought a Metz 58-AF2 speedlight; I'd obviously use that, but in what colour? To decide this, I spent a few minutes taking photographs of a white wall in a dark room illuminated only using the flash and various colour gels. I knew that I would set the white balance close to tungsten in order to get a gorgeous blue sky, so I checked for each of the colours corrected for a tungsten white balance. I really liked pinks because they would appear a blue-magenta shade under the tungsten white balance setting; your creative opinions may vary.

So, the gear

  1. Pentax K-5 with 18–55 mm kit lens
  2. Tripod
  3. Metz 58-AF2 speedlight
  4. Flash gels
  5. Torch
  6. Remote shutter release cable (not used)
  7. Heavy backpack to stabilise the tripod in case the wind was very strong (not used)
  8. Some friends for company. It is really annoying to be up on a dark hill all alone, having people to keep an eye out for traffic and other contingencies is a good idea.

The rule for photographing stars is that you divide 500 by the focal length (adjusted to 35 mm format) to get the maximum shutter time. Photographing faster than this shutter time will prevent star trails. Don't ask me for the math, I'm sure that it should also include the sensor PPI or something, but I've found it to be a useful rule of thumb. Shooting at 18 mm on my Pentax K-5 gives me an equivalent focal length of around 27 mm, which gives me a shutter time of 18 s. This means that I could shoot either at 15 s or 20 s on my Pentax K-5. (A remote shutter release in bulb mode could have helped me get the sweet spot, but I'm an utter idiot and I forgot about this until after the fact.) To maximize the light entering the camera, I had my lens wide open at F/3.5 (the widest at 18 mm), and my ISO set to 1600 initially for a test shot.

Upon reaching the observatory, I was disappointed as I could not even see the Milky Way with my naked eyes. Turns out there are some factors that are not simulated by Stellarium. Always the optimist, I still set up my gear and pointed it in the general direction of where I expected to find the Milky Way. This is where the careful planning helped. The sky was just too dark to even see the stars through the viewfinder or in live view, so I resorted to dead reckoning, using my best estimates of what would be in the field of view for a 18 mm lens. I hoped to have the observatory to the camera right and the milky way dead centre. There was a tree as well to the left, I wanted that out of the frame. Once I had this approximately set up, I set my camera to manual focus, set to infinity, then took a test shot, ISO 1600, F/3.5, 15 s. This showed me that my framing was spot on, but the sky was still too dark. Therefore, I had to push up the ISO to 3200. I did not want to go all the way up to 6400 as I would have to deal with all the noise. While I'm okay with ISO 6400 for concerts and performances, the resulting noise looks a lot like salt-pepper (i.e. very white and black spots) that make it indistinguishable from stars in the night sky and much more difficult to clean up in post. My camera was also set to automatic long exposure noise reduction, which means that the camera takes a second frame immediately after the picture is taken, this frame is taken with the mirror down in order to get a black field. The idea is that the camera sensor develops characteristic noise which is not random when it gets hot, subtracting this black photograph from the original in camera is a great way to reduce noise caused by the sensor overheating. While this can also be done in post, I need to remember to take a photo with the lens cap on, which is much harder to do in the field.

Anyway, back to photographing. I set my ISO to 3200, F/3.5, 15 s. The shot looked good on the LCD, I could see the Milky Way faintly and I knew I could pop that out in post. The next step was to take photographs with the observatory (foreground) lit using my flash so that it would be more interesting that a silhouette. For this, I used my speedlight with a pink gel, positioned to the right and behind the camera. The speedlight has a guide number of 58 m when fully zoomed at ISO 100. Let me briefly explain the guide number and how that can be used to set the speedlight power.

Unlike the standard exposure triangle where the key metrics are ISO, aperture and shutter speed; a flash unit fires for so short a time as to make the shutter speed irrelevant. What is important is the flash power, as this is the light output of the flash. A guide number is simply the product of the F-number and the distance at which a subject is correctly illuminated by the flash firing at full power. This means that my flash unit firing at full power can illuminate a subject 58 m away when I'm shooting at ISO 100 and F/1. I was shooting at ISO 3200 and F/3.5, so the distance between the flash and the observatory should have been 58 * (3200 / 100) / 3.5 = 530 m away, which is clearly ridiculous. The way to fix this is to reduce the power of the flash. At 1/32 power, the distance is approximately 16 m, which is more like the distance I had in mind originally. The next part was a lucky fluke, but I forgot to account for the reduction in flash power caused by the gel. I used a Selens Follies Pink gel, which reduces the flash power by two stops, so my actual flash power should have been 1/8 and not 1/32. However, the lower power actually prevented the observatory from being too bright and distracting in the final image. In hindsight, I could have gone for slightly brighter flash, but these decisions are much harder when it is too dark to even see a single button on your camera. I handed my flash to a friend, asked him to aim it at the observatory and fire it on my mark. My friend was positioned so as to only light up the front face of the observatory and leave the side in silhouette, approximately 15–20 m away (more reasons for increasing flash power, with 20/20 hindsight). I set my camera on a 2 s self timer to prevent camera shake, and when the shutter opened, gave my friend a verbal cue to fire the flash at the observatory. Then review the image.

I just took four images, two each at 15 s and 20 s exposures, and with a Follies Pink and a Smokey Pink gel. I finally liked the version with the Follies Pink gel at 15 s, so that formed the basis of my final image.

We start off with the base image imported in Darktable. Darktable automatically makes some adjustments such as orientation, sharpen, and base curve upon import.

The first thing I want to do is set the white balance to tungsten to get the cool blue sky. That way, I can check all other adjustments while keeping an eye on the colour.

Then I push up the exposure by around 0.7 stops.

And add some contrast and saturation.

The next part is my favourite. Darktable recently added support for drawable masks, similar to the other software from the clay-company that shall remain nameless, but sounds a lot like the opposite of Darktable. I duplicated the exposure module, chose a large, soft brush and drew over the core of the Milky Way. I then pushed its exposure up by 0.7 stops and also deepened the blacks slightly

Next, I used the tone curve module to increase contrast even further in the galaxy core. Darktable saves masks, so I could re-use the same mask I used to correct the exposure on the core.

There's a lot of noise in the image, which may not be apparent in the screenshots, but it had to go away. I used the "denosie (profiled) tool, which is my go-to tool, but there was still a lot of noise. So I used the equalizer module set to the preset "sharpen and denoise".

Still not good enough, so go all the way, turning on "raw denoise", "hot pixels" and "chromatic aberrations"

Lens corrections!

Now that the image is coming together, I felt like the drawn mask for the galaxy core was just not wide enough. So I went back and fixed it by using a larger brush

Velvia is my favourite module to pop colours and get a look somewhat closer to film.

Some colour correction to make the highlights (the galaxy core) a bit more orange and the rest of the image a little more blue.

Second colour correction, just on the galaxy core (using the same mask as the exposure module), this time making the highlights just a bit green. I also used the mouse wheel to increase saturation, this has the added effect of increasing colour saturation on just the core.

Finally, something I forgot to do initially, adjust the shadows and highlights. This is, however, exactly the opposite of what I would normally do, and the defaults in Darktable. Typically, one would like to lift the shadows and make them brighter, or have the shadows positive, and bring the highlights down, or in the negative. However, in this case, I want the less bright stars to disappear and give me a cleaner image, so I push the shadows down to the negatives, and the highlights to the positives until I get an image that I like.

Remember when I said that I messed up accounting for the reduction in flash power caused by my gel? This is where I feel that the observatory is a bit too dark, so I need to pop the exposure on that as well. Create a new mask, and push the exposure up by 0.7 stops.

All in all, I pushed the exposure on the observatory up by 1.4 stops (0.7 + 0.7), and it is still a bit dark. My flash gel took up 2 stops of light. Guide number calculations work perfectly if you are smart enough to use them (unlike me).

That's it. I have no idea what the source of the spotlight at the bottom of the image is, but it sure looks cool right below the brightest part of the Milky Way.

This has been a longer post than most others on this blog, I hope you enjoyed it. Please do share your feedback in the comments below. Until next time…

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