The Milky Way is the galaxy in which our solar system evolves. It contains around 250 billions of stars, a mesmerizing amount. When it comes to visible objects, more than stars it contains dust clouds and nebulae. A simple human eye cannot resolve the position of each individual star composing this giant structure, thus its cloudy appearance and its name, "Milky Way": a white band spanning accross the night sky. If you live in a civilized area, chances are you were never able to truly observe the details of the Galaxy. Too bad, it is a nice sight! It is worth travelling once to a darker area to be able to observe it.

In the nothern hemisphere during summer times, its center nears the south horizon. This is precisely the region at which we will be aiming our camera below.

Choice of a location

I live in Switzerland, a small but very dense country where the next town is never further than a few kilometers. In other words the sky is never truly dark on the swiss plateau. Luckily, around 60% of the territory of Switzerland is mountainous: there is were we are headed. Because I needed to carry my telescope mount, I chose a location that is easily reachable with a car but is still decently dark, and there is one spot that satisfies these two conditions quite nicely: the Gurnigel pass. (The location of the swiss star party). Well it also is a pretty location, as seen in the image below, captured just after dawn. Panorama from the image taking site From here, the nearest town in the south direction is 30km away. Perfect, no light pollution is to be feared from there, hopefully the sky will be transparent and some details of the Milky Way will reveal.

The amount of light pollution also depends on the quality of the atmosphere. The warm colours (sodium: 590 nm) emitted by the public lighting are mostly diffracted by small particles of dust or droplets of water. However, as the sodium lamps are being replaced by LEDs for the sake of their lower power consumption, shorter wavelengths are also emitted. Those can be scattered by the molecules composing the air, thus causing a light pollution that never diminishes in magnitude.

General strategy

We know since the beginning of the 20\(^\mathrm{th}\) century that light is quantized. It can furthermore act as a particle or as a wave depending on the situation. When it comes to the sensor of a camera the particle nature of light matters the most, thus we can see a photograph as an accumulation of counts of particles. Unfortunately a sensor cannot be perfect, and a certain amount of particles will be counted even though they never existed at the first place ("read noise" and "dark noise"). When it comes to taking daylight pictures this a non-issue as the number of relevant counts will be much higher than the noise related counts. In astrophotography however, the light sources are extremely weak and we need to wait for a long time before we get a picture with a decent signal to noise ration (SNR). With modern sensors, the read and dark noises are quickly overcome, but there still is the natural variability of the incoming photons.

The number of counts recorded by a certain pixel follows a Poisson distribution because the incoming photons are completely independant from eachother. Thus, the uncertainty on the pixel value \(\sigma\) is equal to the square root of the pixel value \(N\): \(\sigma = \sqrt{N}\).

As a rule of thumb we can assume that the SNR of a pixel in our picture is proportional to the square root of the number of counts the pixel recorded. How do we accumulate counts? We simply set longer exposure times on our camera. I set this on my camera to the maximum allowed value: 30 seconds. Moreover, why stop after a single picture? I will set the camera to continuously take 30 seconds pictures until I get bored.

Tracking

One of the hard parts of astrophotography is the compensation of the rotation of the earth. With a 50mm lens on a full frame camera, the stars will start to trail after around 10 seconds of exposure or less depending on the atmospheric conditions. The longer the focal length, the shorter the allowed time before starting to observe star trails. I could take 5 seconds exposures during half an hour, and align the portion of the sky that is common to all the pictures later on the computer. This is a valid strategy if you do not own a telescope mount. Just be prepared to occupy a lot of storage space. I used in this case a motorized equatorial telescope mount and 30 seconds exposures. The main axis of an equatorial mount is parallel to the rotation axis of the earth, which allows for an easy compensation of the rotation of the earth.

Setting up

I will be using a DSLR camera, a cheap zoom lens lens and an equatorial tracking mount for telescope.

  • Nikon D750
  • Nikon 28-80mm f/3.5-5.6 AF-D
  • Skywatcher NEQ6

The procedure is simple:

  • arrive on the location, install the mount.
  • When the first stars start to appear, station the mount on the polar star (aligning the main axis of the mount with the rotation axis of the earth)
  • Focus on a bright star (or on a planet) using the camera autofocus, then disable it.
  • Camera settings: record RAW files, noise reduction disabled, f/2.8, 6400 ISO, 30 seconds.

The video below shows the scene as the night falls on the mountains, just for the context.

This second one starts right after dusk and shows the object of our trip rotate with the rest of the night sky. At the end, the planet Mars rises over the mountains. Added to that, planes and artifical satellites cross the frame regularly.

Results

I took two series of 50 images as showcased in the lighttable below. The stacking and further processing of these images is discussed in another post, as this one starts to be long already.

lighttable

Now before postponing the actual processing, let us assemble all the images into two timelapses. This time, the equatorial mount guarantees that the camera points steadily in the direction of the center of the Milky Way while the earth keeps rotating. Peculiar sensation right?