Beginning Astrophotography: The Wide Field

Photo of the night sky showing the core of the Milky Way on the horizon
Photo of the night sky showing the core of the Milky Way on the horizon, taken 22 Jul 2017 at 10:44 p.m. PDT.

“You know, ‘galaxy’ means ‘milky,'” I said, still looking up.

“What? No way,” my friend, who was stargazing with me with her own camera, said.

“Totally. ‘Milky Way’ is directly from Latin, ‘via lactea.'”

“So it’s not from the candy bar?”

“No.”


I was taking photos with a new friend at her farm south of Portland. I remain extremely grateful to her for allowing me to do so because they allowed me to my first photos of the core of the galaxy unaffected by light pollution.

Photo of the Milky Way in the sky
Wide-field view toward the core of the Milky Way, taken on 22 July 2017

The photo above was processed somewhat delicately to improve the white balance and the colors and brighten things up a bit, but that’s more or less how it came out of the camera. Taking photos of the sky at large is a very different activity than taking photos of individual objects through a telescope.

Chiefly, there is no telescope. None of this post will discuss using a telescope. I took all these photos with my same mirrorless camera, the Sony α6300, and a tripod. To adapt this camera to wide-field night sky images of the Milky Way, there are two big differences from ordinary photography: for one, using a long exposure and high ISO, and for two, using a suitable lens.

When I started last year, I was practicing blind, experimenting in wintry months, guessing at settings, and using a 32 mm lens with significant shortcomings for night-sky photography. To make improvements, I’m grateful for information I got from Lonely Speck, which I adapted to suit me.

Exposure settings

First, most of the job of collecting a night-sky image is accomplished by exposing with a high ISO and a long exposure period. This means trucking out to a dark site—this activity is absolutely impossible anywhere near a city and impractical in a suburb. You also have to have a camera capable of manual control over its ISO and exposure length, among other things.

Detail of the night sky, looking into Cygnus, taken with ISO 800.
Detail of the night sky, looking into Cygnus, taken with ISO 800.

For my early wide-field attempts, I was afraid to raise the ISO higher than about 1600. I took some experimental shots with the ISO as high as I could go, but few were in the middle ground. I assumed these photos would be unusably noisy. Therefore, the photos I which turned out best were at ISO 800, but to bring out any detail, I had to push them dramatically, such that they looked artificial.

The most important thing I read was an article on Lonely Speck about finding the best ISO which explained that ISO doesn’t increase sensitivity so much as it provides amplification of the underlying signal. ISO can be thought of as a gain control for the sensor signal. Quoting,

It’s a (very) common misconception that increasing ISO increases the sensitivity of a camera sensor. ISO doesn’t change sensitivity. Increasing ISO simply increases the brightness of a photo by amplifying the sensor signal. In the electronics world, amplification is sometimes called “gain.” …[W]e can “gain” brightness if we increase our ISO. … Higher ISOs won’t increase the visible noise in a photo. …A higher ISO will decrease the total dynamic range of the image…And, in many cases (like astrophotography), a higher ISO will actually decrease the visible noise[.]

I was amazed to learn this. The article goes on to explain the conditions under which this occurs and how. This meant that I was free to amp up the ISO on my photos considerably.

The other consideration was exposure length. Mostly, the goal is to expose as long as possible before stars stop being points of light and start being streaks. How long this takes is entirely a function of the focal length of the camera—that is, the wider the field of view, the smaller the points of light are, so the less noticeable it becomes when stars seem to “move” across the field of view.

The lens I had used before was a bit longer than typically used for Milky Way photography. It’s only able to capture about the size of a constellation. That meant that stars would appear to move if I exposed longer than about fifteen seconds.

Add these together, and I was taking in a lot less light than my camera was capable of. On top of that, my lens was not designed for astrophotography, meaning that it introduced significant distortions, called aberrations, to each photo around the edges.

Choosing a lens

Corner detail of a wide-field photo showing significant coma and astigmatism aberrations
Corner detail of a wide-field photo showing significant coma and astigmatism aberrations

I had noticed from the first images I took that I had weird comet-looking distortions around the edges of my photos, but I didn’t know why. All the bright stars ended up looking this way.

I figured I might be able to avoid these distortions by stopping down the lens somewhat (and I would have been right, as I later learned), but that would have meant blocking even more light.

Luckily, there was another post on Lonely Speck that explained all about these distortions, called aberrations. I learned that these shapes were a combination of coma (which caused the light from the star to smear inward toward the center of the photo) and tangential astigmatism (which butterflied the distortion apart parallel to the radius running from the center to the star).

These were in-built distortions of the lens. It’s not necessarily that I had a bad lens—indeed, this was a Zeiss Touit f/1.8, an extremely good portrait lens. It just wasn’t designed for work where spots of light in the periphery were meant to be precise dots.

Picture of a Rokinon Cine 12mm lens
Rokinon Cine CV12M-E 12mm T2.2 Cine fixed lens

I found out there are classes of lenses built by Samyang (also known as Rokinon lenses, among others) designed to minimize these aberrations, also having extremely short focal lengths (meaning, really wide fields of view). For my birthday in June, I treated myself to a Rokinon Cine CV12M-E 12mm T2.2 Cine fixed lens. This is the lens I’ve used for all the photos of the Milky Way since then.

The First Batch: Learning What’s Possible

I’ve taken two batches of photos of the Milky Way since getting the lens and figuring out the right direction for settings.

For the first batch, I went to Stub Stewart State Park and waited till about eleven at night. It’s summer, so that’s when astronomical dusk occurs, and you can look up and see the Milky Way (which is visible from that site, though a bit washed out). Being summer, as well, the core of the galaxy is visible in the south, which I’ve wanted to photograph for a long time.

Photo of the Milky Way in the direction of the core of the galaxy, taken 18 July 2017 from Stub Stewart State Park at 11:01 p.m. PDT
Photo of the Milky Way in the direction of the core of the galaxy, taken 18 July 2017 from Stub Stewart State Park at 11:01 p.m. PDT

I followed the instructions from Lonely Speck rather closely, with respect to ISO and exposure, and I found I got wonderful results. In this case, I exposed for twenty-five seconds, and I used ISO 3200. The results exceeded my expectations.

As I processed them later, I found that I captured a lot of the light pollution from the city (which was in the distance in the southeast), and that presented difficulties in processing the photos without bringing out splotches of unnatural color.

I consider my attempts from that night now to be middling, and my ability to process them have evolved considerably as well.

The Second Batch: Finding What Worked

Photo looking up along the Sagittarius arm of the Milky Way near the zenith, taken from the farm
Photo looking up along the Sagittarius arm of the Milky Way near the zenith, taken from the farm

I was extremely lucky enough to have a very helpful and happy friend who let me come to her farm and do more night-time photography. Because her farm was south of Portland, the core of the galaxy was facing away from all the light pollution. The photos at the top of the post represent some taken from this attempt.

Here at the farm, I decided to lessen both the exposure length of time (down to twenty seconds) and the ISO (down to 2000). The earlier settings, I had found, seemed almost too aggressive for the conditions, though I may revisit them if I’m at a darker site. But twenty seconds and ISO 2000 turned out to be perfect. The photos looked gorgeous right off the camera, almost without editing at all. The results had delicate bands of dust and light in them that were considerable easier to work with as I processed them on my computer.

Photo of the core of the Milky Way taken, from the farm
Photo of the core of the Milky Way, taken from the farm

I took enough that night that I’ve been able to find lots of different ways to process each and experiment with what I like. For some, I’ve tried wild color combinations and gradients. I’ve tried delicate forms of processing or pushing others as far as they’ll go. I’ve learned to duplicate a photo many times over so I can manipulate it in many different directions and compare the results.

This post has been about changes I’ve introduced to the photography process, and in a future post, I’d like to talk about processing a bit more (basically editing the RAW photos to make them pop). I’d like to get better at that first, though.