The magic of long exposure astrophotography reveals so much more of our universe: a plethora of colours and structures that tend to go unseen by the naked eye. From wide field lens photography to imaging through a telescope, astrophotography remains the pinnacle of long exposure photography.
With advances in DSLR and telescope technology astrophotography has become more accessible to amateurs, but it still requires a great willingness to learn, and healthy dose of dedication to perfect. Astrophotographers will also have to combat many technical hurdles to attain images of round stars, colours and the detail of elaborate interstellar structures through their photography.
Astrophotography is all about getting as much light (“signal”) to your DSLR sensor as you can, especially in deep space imaging. The more signal you collect, the better your photos will be. So what is signal? Photons of light travel a long way to get to your camera sensor. Signal is your camera interpreting these photons to record this light, and the light frame is the photo of the object that you are photographing. Unfortunately, light is not the only signal recorded by your sensor. Thermally-induced signal and electrical interference are also recorded by your sensor, even when the sensor is not exposed to light. These signals appear as noise on your light frame. Increasing the signal-to-noise ratio is therefore the primary aim in astrophotography. Two of the most common ways to achieve this are:
a) Increasing the exposure time of your image: The exposure required for astro photos often ranges from 30 seconds to ~10 minutes for a single exposure. Keeping the shutter open for longer allows your sensor to collect more light.
b) Taking multiple exposures: Combining (“stacking”) many exposures of the same target object with the aid of astronomical image-processing software averages out the noise and boosts the signal. Simply put, this involves overlaying and aligning multiple images on top of each other to increase the signal, and decrease the noise. A single exposure is referred to as a sub-exposure. As such, you will see references to the complete exposure time of an astro photo as a sum of all the sub-exposures used to create the final image.
A combination of these two methods will reveal faint data that goes unseen by the human eye, as well as some world-class telescopes. The more exposures you are able to combine, the better the outcome of your photo will be.
DSLR’s light gathering capabilities are much better than that of the human eye. When you view any of these structures via a telescope, you will not see colour or most of the detail contained in these nebulas or galaxies.
However, gathering light comes with its own set of challenges. Firstly, you’re on a planet that’s spinning while it hurtles through space, and you’re trying to take photos of other objects that are lightyears away and are moving independently of you. Secondly, earth rotates around its axis. Looking at a star trail image will show how, over the course of a night, stars revolve around a centre point in the sky – the celestial pole. As such, to take pin-point round photos of stars you need to counteract earth’s rotation, which involves setting up a telescope that tracks the sky as it moves.
Best imaging conditions
Dark skies: Get away from city lights to escape light pollution offers the best opportunity for astrophotography.
Stable skies: Seeing conditions describe the steadiness or turbulence of the atmosphere. The less a star twinkles, the more stable the atmosphere is, meaning the light travelling to your sensor is less disturbed. This results in a sharper image.
Colder temperatures: The colder your environment, the less thermal noise your camera will create. This aids greatly in improving the quality of your photos because it increases the signal-to-noise ratio.
More in the series of DSLR Astrophotography:
Part 2: Equipment CLICK HERE
Part 3: Connecting your DSLR to a telescope COMING SOON
Part 4: Solar system and planetary imaging COMING SOON
Part 5: Image processing COMING SOON