How to Take Bias Frames

Bias Frames are very easy and quick to take so there is no excuse for skipping on these!

Bias Frames are a type of calibration frame that astrophotographers use to reduce noise in their stacked image. On this page, I will discuss what Bias frames are, what type of noise they eliminate and how to take them to improve your astrophotography.

What are Bias Frames?

No matter what type of camera you’re using for your astrophotography, when the camera sensor reads the data that’s being gathered from an exposure, it will introduce a level of noise.

The noise is a consistent pattern that is produced by your camera, and we can isolate that noise pattern by taking Bias frames to reduce that noise from our final stacked image.

By taking lots of frames (50), stacking software such as DeepSkyStacker can read the pattern that is created from your Bias frames. This will then be subtracted from your final stacked image when DSS goes through the stacking process, reducing the noise level in your image.

This results in a cleaner final image with a higher signal to noise ratio, which will help in post processing when we stretch the data in our image.

Bias frames are captured in darkness, with your lens cap or telescope dust cap on and setting the exposure time to the fastest possible setting for your camera. On my Canon 650D, 1/4000 is the shortest exposure time possible, yours may vary slightly.

Taking the shortest possible exposure time ensures that you’re only capturing the noise that is produced by the sensor electronics and not noise from other variables.

Capturing successful Bias frames is a very simple and easy process. I take mine at night for two reasons. Bias frames need to be shot in the dark, and they’re very quick to take so they don’t eat into your imaging time like Dark frames.

However, if you forget or simply don’t want to take them at night, then you can just shoot them in a dark cupboard or similar to achieve the same results.

To ensure you’ve collected enough data for your stacking software to know the pattern of your camera sensor noise, I recommend taking 50 Bias frames.

50 may seem like a lot but they’re so quick to take and the more you take, the easier it is to isolate that read noise pattern of your camera. Some people choose to take more than 50, and taking more certainly won’t do any harm.

The good thing about Bias frames is that your camera noise will stay consistent over time so there’s no need to take these after each imaging session. It is however good practice to refresh them every 4-6 months to account for any changes in your camera.

Camera Settings

Use the fastest shutter speed your camera has (1/4000 for my 650D)

Use the same ISO as your light frames
Do not debayer
Captured 50+ frames

Lens cap or telescope dust cover must be on

Capturing Bias frames will only take a few minutes but your astrophotography will benefit greatly by taking them!