How to Take Bias Frames for Astrophotography
As you continue to make progress in the hobby of astrophotography, you’ll quickly learn that taking proper calibration frames is essential. The most common calibration frames or “support frames” are dark frames, flat frames, and bias frames.
The main reason amateur astrophotographers take bias frames is to reduce noise in their images, more specifically the fixed-pattern noise. No matter what type of camera you are taking your images with, the signal-to-noise ratio (S/N) is often the primary deciding factor between a good image and a great one.
In this article, I will describe what bias frames are, and why it is important to use them to improve your astrophotography images.
What is a Bias Frame?
To start, I think is important to understand what bias frames actually are. Bias is an offset that occurs when a pixel is read from the CCD or CMOS camera sensor. Your camera inherently has a base level of readout noise as it reads the values of each pixel on the sensor, called bias. Basically it’s an inherent pattern to the sensor.
Like the other calibration frames we use, Bias frames are meant to capture this so it can be removed. Below, is an exaggerated look at a bias frame captured using a Canon EOS Rebel T3i (600D) DSLR camera.
A single bias frame captured with a DSLR camera (ISO 800, 1/8000″).
The Bias noise pattern can vary across the image. In many cases, if the bias frame is not corrected then flat-field calibration will not work accurately.
A bias frame is essentially a zero-length exposure or to the shortest exposure time possible on the camera, with the shutter closed. We are interested in the fixed noise pattern of the electronics, therefore keeping the exposure short as possible helps prevent other noise creeping in, such as dark currents.
What makes a good bias frame?
To capture a successful bias calibration frame, make sure the camera is in a dark place or room (I take mine at night). Take several pictures (20-40) and then stack them together in your pre-processing software such as DeepSkyStacker. Each pixel will have a slightly different value, except for a small amount of noise.
The value for any one pixel will be consistent from image to image. Since the bias is consistent from image to image it can be subtracted. The bias frame itself will contain a small amount of readout noise. This readout noise is produced inside the electronics which read the pixels. It can be very low in sophisticated cameras, but never fully absent. This noise can be easily suppressed by combining a number of bias frames together.
Bias/offset frames were subtracted from my light frames in this photo of the Orion Nebula.
The bias for a camera is generally consistent over a large period of time. This means one can take bias frames just once, and use them on all images for many months to come. Note that some cameras may have a small bias dependency on temperature. Small bias offsets are not important of themselves, but they can degrade the effectiveness of the flat-fielding calibration.
Ideally, the other types of calibration frames (dark and flat-field) should themselves be bias frame calibrated. Software such as MaxIm DL does this automatically when bias frame files are selected.
It is important to mention bias is also included in dark frames. As such, it actually is possible to perform accurate calibration without using bias frames. For certain cameras, you may find that bias frames are not necessary (as was the case with my ZWO ASI294MC Pro). You’ll need to research your particular camera model to find out whether bias frames are recommended or not.
When shooting with a DSLR, I always include bias frames in the stacking process. They are incredibly quick and easy to capture, so I usually capture a brand new set of bias frames for each and every imaging session.
How to take Bias Frames for Astrophotography
That said, bias frames are extremely easy to acquire, so there is little reason to skip them. You can quickly acquire 40 frames on average, so the read noise in the bias frames will not be contributing to your overall noise level.
Here is an overview of the steps required for Bias Frames for Astrophotography.
- Use the fastest shutter speed your camera has (often 1/8000″)
- Keep the lens cap on your camera or telescope (and cover the viewfinder)
- Use the same ISO as your lights
- The temperature is not important
- Do not debayer your bias frames
- Capture about 20-40 bias frames
I usually capture my bias frames at the end of my imaging session. Whether I am using a remote shutter cable on the DSLR, or am running a sequence of image in Astro Photography Tool or BackyardEOS, collecting bias calibration frames takes less than 5 minutes.
Because each exposure is less a second, you can quickly accumulate 20-40 bias frames for stacking purposes. Other than racking up the shutter count of your DSLR camera, there is very little reason not to shoot new bias frames after each session. Camera control software such as Sequence Generator Pro can completely automate the process for you if you want.
Once you have collected at least 20 bias frames, you can load them into your project in DeepSkyStacker, Astro Pixel Processor, PixInsight, or whichever pre-processing software you like to use. In DeepSkyStacker, I simply select all of the bias frames from the folder I created using the “offset/bias files…” button at the top left of the user interface.
Loading bias frames into your project in DeepSkyStacker.
Your stacking software should create a master offset file of your bias frames, which is essentially an average of the pixel information. This creates an accurate noise profile of the bias signal, and it is automatically subtracted from your light frames in the final image.
I always use the default settings in the stacking mode parameters for bias frames, which happens to be “median” mode. Your stacking software should allow you to adjust these settings manually if you want.
I recommend leaving the stacking mode set to median for bias/offset calibration files.
I hope this article has shown you how quick and easy it is to take bias calibration frames with your camera. Astrophotography is all about collecting as much signal as possible, and reducing noise. By subtracting the bias readout noise from your camera sensor, your images will benefit from a stronger S/N, and the image processing stages will be much more enjoyable.