When I first started taking photographs of the universe, one of the terms that was thrown about was “Dark Frames”. As a beginner, this can cause a lot of confusion – we have light frames, dark frames, flat frames, calibration frames — does it ever end? In this post we’ll delve into Dark Frames, why they’re important, and how to take them.

What are Dark Frames?

In short, Dark Frames are a type of calibration frame. Calibration frames are images we take to help remove noise and artefacts from our light frames – the photos we take of the night sky. Dark frames capture the noise profile that a sensor experiences at a specific temperature and exposure time. The purpose of using dark frames is to reduce the amount of noise in our final image.

How do Dark Frames remove noise?

This can get a bit complicated so I’ll try to make this as simple as possible. Pretend you took a photo of a football and a tennis ball, but you only wanted the tennis ball. What would you do? Well, we could give the computer a photo of the football on it’s own, and then ask it to remove it from our image.

What we’ve done is teach the computer what a football is, and ask it to remove anything that looks like it from our image, leaving only the tennis ball behind.

The same principle works in astrophotography, except we have photons from deep space, and noise from the sensor.

More detail

We need to tell the computer exactly what noise we want to remove, and therefore need to be very specific about how we take our dark frames.

The noise in your dark frames (which is used to tell the stacking software what to remove) MUST match the noise in your light frames.

Sensor noise is based on two things:

1. Exposure time
2. Temperature

This means, if it was 20 °C when you took your light frames, and your frames were 300 seconds long, your dark frames must also be taken at 20 °C and 300 seconds in exposure. This will create an image that contains only the noise from the sensor, and will teach the stacking software exactly what noise to remove from your light frame. In general, it’s best to take around 30 dark frames to give the stacking software the best chance of working properly.

How to take Dark Frames

Taking Dark Frames is very easy. I’ve broken this down into the steps below:

1. Place the lens cap on your telescope (or better yet, remove your camera and cover the sensor with a dark cap). Either way, absolutely no light must hit the sensor.
2. Get the sensor to the right temperature. if you’re using a DSLR, you’ll have to take your dark frames on the same night as your light frames, or wait until another day comes along where the temperature is similar.
3. Take 20-30 frames at the same temperature and exposure as your light frames with the camera covered.

Challenges

Dark Frames require the exposure time and temperature to match the light frames. Matching the exposure time is simple – but how do we match the temperature?

This is the biggest problem with DSLR cameras. You can take some amazing photos with them, but taking dark frames is a big challange as there is no way to control the temperature of the sensor – it simply depends on the outdoor temperature of the night.

Wasted time: If you can’t control the temperature of the sensor, then it’s usually best to take your dark frames on the same night to give you the best chance of them matching your light frames. Unfortunately, this means precious time is taken away from capturing your target – not ideal!

The outdoor temperature also changes throughout the night, meaning even if you did take your darks on the same night, they would never perfectly match your lights. The light frames taken at the beginning of your session would probably be warmer than those at the end.

Solutions