Learn how to shoot Starscapes
There are some challenges when you aim to create a starscape. First, it’s a typical low light situation, so you have to use a tripod and understand the relationship between shutter speed, aperture, and sensitivity. There are also astronomical phenomena that are very obvious: the rotation of the earth around its axis.
We will look at one of the most common objects to incorporate in a starscape: our own Milky Way galaxy.
Milky Way is a rather large barred shaped spiral galaxy made up of approximately 100 billion stars. Together they form a large disk with a diameter of about 100 000 light-years. Our own solar system is located around 25 000 light-years from the galaxy’s center on the galaxy Orion-arm’s inner edge. The almost countless number of stars in the center of the galaxy forms av bulge, and in the very center, there is substantial evidence for the existence of a supermassive black hole corresponding to the intense radio source Sagittarius A.
In areas with little light pollution during dark nights, we can observe some aspects of this impressive object that we are part of, as a faint band of light that stretches over the dark sky. With a digital camera, a tripod, knowledge, and patience, you can create stunning starscape images with the Milky Way. So how do you do?
Camera: I recommend to use a DSLR or mirrorless camera with interchangeable lenses
Lens: Wide-angle lenses. I often use the Irix 15 mm f/2.4 on a Nikon D810 camera.
Tripod: Because you will use long shutter speeds
Star-tracker: Optional, but it makes it possible to extend shutter-speed before you get star trailing.
Almost all starscapes are composites that consist of sets of exposures for the foreground and set of exposures for the night sky that are stacked together and masked in postproduction.
I will also mention the concept of stacking since it’s an important method in all astrophotography. Stacking is a method to get a better signal-to-noise ratio and hence reduce noise and helps you to get more information out of the image files. I will not get into detail here, and for starscapes, there is some software that is very helpful; for example, Starry Landscape Stacker for Mac is also quite east, to begin with.
First thing – find a dark spot that has little light pollution. The darker, the better. If you don’t know the area, it’s easier to find it when it’s still light outside. Think of how you want to compose the image. Large water surfaces could make striking results when the stars reflect in the water.
It’s almost all about collecting light, which is important: You want to have a high signal-to-noise ratio; this increases the quality of the image and helps you reduce noise. This is one reason why we use stacking techniques.
ISO is probably the most misunderstood concept in astrophotography.
First: The ISO-setting is not influencing your camera’s sensitivity to light!
The ISO-setting is determining the amplification factor that your camera applies to the photons that hit the sensor; it does NOT, however, make the sensor more sensitive, so it collects more photons. I will not go into the details, but this article is very informative.
To have a brief understanding of this and the concept of Signal-to-Noise ratio is important however, when you should decide the ISO-setting for your camera because it will affect the dynamic range:
“The best ISO for astrophotography for any DSLR is the lowest ISO level from which either a.) the upstream noise swamps out the downstream noise OR b.) the amplification will be done digitally in the camera, whatever value of both is the lowest.”
Or, to be concise: The best ISO for astrophotography depends on your camera. Here are some guidelines for Nikon, Canon, and Sony-users.
Startracker or not?
As we all know, the earth rotates around its axis once per 23 hours and 56 minutes. That has significant consequences for starscape photography as you will get star trailing in your images after just a few seconds, depending on the focal length of your lens.
400-rule: The 400-rule is a method that helps you decide when there is a risk for star trailing. Simply divide the focal length of your lens by 400 and get the maximum exposure time you can use before star trailing occurs.
For example, with my Irix 15 mm lens, I can choose a shutter speed of 400/15=about 26 seconds.
A star tracker has its clock drive and is a simplified equatorial mount that makes it possible to use longer shutter speeds. With a good polar alignment, probably up to maybe 1-2 minutes.
There are pros and cons to both methods:
Not using startracker
– You are limited to the 400-rule to avoid star-trailing
– You have to take more exposures to get the corresponding signal-to-noise-ration (SNR) as with one longer exposure
+ The postprocessing is a little bit easier. Starry Landscape Stacker is a very useful software.
– Because the star tracker moves, you will get motion blur in the foreground. Therefore you have to take two sets of images, 1. the night sky with the tracker on. 2. the foreground with the tracker off.
– A little bit more advanced postproduction with stacking and masking
+ You minimize the risk for star-trailing
+ Better SNR when compared to shorter single exposures
Integration time is the total exposure time. With a longer total integration time, you will get a higher Signal-to-Noise-Ratio (SNR) and hence reduce noise and increase the light information from faint stars. How long depends on the situation, but I would recommend beginning with at least 5-10 minutes for the night sky.