If you're having trouble with noise in your long-exposure astro images captured using a DSLR camera, you're not alone!
There is a good technique you can use to avoid noise spoiling your DSLR images, and we'll cover that in this guide.
The techniques we'll cover include frames and stacking, but you can also read our guide to astrophotography for beginners and image processing for more advice.
Fixed noise
Fixed noise, such as a ‘hot pixel’, can be removed by a technique called dark frame subtraction.
Following a normal exposure, called a light frame, take an image using exactly the same settings but with the lens cap on.
Then subtract this frame from the light frame to remove the fixed noise.
Find out more about this in our guide to flat frames.
Thermal noise
However, the most common type of noise is thermal noise.
This is reduced by averaging light frames in a process called stacking.
Typically, multiple dark frames are also averaged before subtracting them from light frames.
A program such as DeepSkyStacker will perform these stacking procedures automatically for you.
Long exposure noise reduction
Some DSLR cameras have a function called 'long exposure noise reduction'. Does this mean you don’t need to take dark frames?
This function is essentially an automatic dark frame subtraction routine.
When it’s switched on, each long exposure you take is followed by another of the same length taken with the shutter closed.
This gives you a record of the static noise (eg hot pixels) present in the main image.
The camera automatically subtracts this noise from the main shot and, hey presto, the static noise disappears.
In theory this means you don’t have to take separate dark frames… but there are several reasons why you still should.
The normal method of deep-sky astrophotography is to take lots of exposures of your subject, which are then calibrated (including dark frame subtraction) before being stacked to reduce random noise.
With the camera’s automatic routine active, you’re forced to wait for the dark-frame to be exposed and subtracted for every shot you take. This effectively halves your imaging time.
A single dark frame image also contains random noise as well as the predictable static noise it’s designed to remove.
Subtract just one dark frame and you may well introduce additional noise in the main image.
Taking several dark frames and stacking them together helps reduce this effect.
The routine is a good idea, but for astrophotography, taking your own dark frames gives you more overall control and allows you to make better use of your imaging time.
