Photographing the Milky Way with VIEWFINDR. Read how you can easily take photos of the Milky Way and how our app can help you.
One of the goals of many landscape photographers is to photograph the Milky Way. To photograph the Milky Way, you need to consider a few things, not only your camera settings. This blog gives you an overview of all things Milky Way photography. In the blog Milky Way Predictions you will find information about when and where you can photograph the Milky Way. This blog will focus on camera settings and possible tools.
The basic problem for photographing the Milky Way is the Earth’s rotation. Without the Earth’s rotation, however, there would be no real life on our planet. That’s why it’s not stupid per se, but only when you’re taking pictures. Because of the Earth’s rotation, you cannot set an infinitely long exposure time with your camera. If you expose for longer than a certain time, the starry sky will become blurred.
The earth rotates and the stars move in the photo. Combined with the fact that the night sky is very dark, this is a bad mix. It is not possible to simply set a very long exposure time in order to be able to photograph especially much light of the dark Milky Way. There is therefore a maximum exposure time that you can set on your camera so that the stars are just in focus.
First of all, we need the angular velocity of the starry sky. This, of course, simply corresponds to the Earth’s rotation, at least for the stars that can be seen at the equator in the zenith. In 24 hours, the Earth rotates once through 360°. This results in an angular velocity of 0.00417°/s of the stars that are at the equator in the zenith. In Europe, these are the stars that are just above the horizon in the south.
Now the angle of view and the number of pixels (photodiodes) of the image sensor come into play. The angle of view indicates how large an angle the lens can capture. Ultra-wide-angle lenses have a very large angle of view, telephoto lenses a small angle of view. From the image angle/pixel factor, we can calculate how many degrees of angle are imaged per pixel (photodiode). So a pixel does not represent a “point”, but actually a small portion of the total angle of view that the lens sees.
To photograph the Milky Way, point-shaped stars are now needed. In order for a star to remain point-like, it must not move more than an angle during the exposure time that corresponds to the angle of view/pixel factor. This results in the formula with which you can calculate how long you may expose:
Angle of view/(pixel * angular speed)
For an ultra-wide angle lens with an angle of view of 100° (diagonal) on an image sensor of 24MP (4000x6000px, diagonal 7211px) this results in a maximum exposure time of 3.33s, which is quite short.
In reality, lenses have aberrations and no star is really spot imaged, so this time can be tripled as a rule of thumb. At this point, every astrophotographer will cry out “Of course it makes a difference!!!”. Therefore, this rule of thumb applies to anyone who would like to photograph the Milky Way at some point, but explicitly does not call themselves an astrophotographer. This results in the adjacent values for a typical 24MP image sensor for different focal lengths or angles of view.
This makes it easy to see a correlation: The larger the focal length, the shorter the exposure time must be chosen! Which is stupid, because the shorter the exposure time, the less light we can photograph from the Milky Way. The Milky Way can therefore only be photographed well up to 24mm (18mm APS-C).
Now ISO and aperture remain. You have to open the aperture all the way, so set it to the smallest f-number. Fast lenses have a clear advantage here, because they capture more light from the Milky Way. A fast ultra-wide-angle lens has clear advantages over a kit lens. You need to set the ISO sensitivity to a level that is still acceptable for your camera. Here, too, modern cameras that have a good noise behaviour even at high ISO numbers are a great advantage.
If you don’t have either, don’t get frustrated! Try everything first. The right weather forecast and planning for the time of year are much more important for photographing the Milky Way anyway.
If you have successfully photographed the Milky Way for the first time and enjoyed it so much, then there is the next step you should take before buying a better camera or a faster lens: Tracking. In a nutshell, a tracking system is a device that you place between the tripod and the camera that rotates your camera at the same speed as the Earth’s rotation, i.e. a full 360° in 24 hours. With it, you can theoretically expose for as long as you want.
A simple astro guide costs much less than a fast lens and is much more useful for Milky Way photography. The disadvantage is that you have to work a little with Photoshop, because while the starry sky is sharp with an astro guide, the foreground in the photo is blurred. In Photoshop, both exposures have to be put together.
Every tracking system must be aligned with Polaris. Polaris is approximately at the centre of the Earth’s axis of rotation. The tracking system must be aligned with this axis of rotation. Simple trackers have a small telescope or peephole for this purpose. With such a simple tracking system, it is easily possible to expose the Milky Way for several minutes with the ultra-wide angle.
With complex tracking systems for photographing the Milky Way, or better, for so-called deep-sky photography, there is a complex procedure for keeping them on course. No product, no component can be built with a manufacturing tolerance of 0. For this reason, the gears and drives in the tracking system are always subject to deviation. However, if you want to take photographs with a telescope that has a focal length of several metres, the tracking must be constantly corrected. In addition to the actual telescope with which the Milky Way is photographed, there is a second telescope that is only used to correct the tracking. But this is a subject in itself and not really normal photography any more. Astrophotography is a hobby in its own right.