Calibration frames are a crucial part of astrophotography. They help to remove unwanted noise and improve the quality of your final image. There are four main types of calibration frames: bias, dark, flat, and dark flat. Here’s a cheat sheet for creating each one:
- Bias frame: A bias frame is taken with the lens cap on to capture the camera’s read noise. It removes the camera’s offset, which is the same for every image with the same camera and setting.
- Use the fastest shutter speed available
- Cover the lens of your camera or telescope with the lens cap.
- Set the ISO to match that of your light frames.
- Take 20-40 bias frames to accurately represent the camera’s offset (read noise).
- Dark frame: A dark frame is taken with the lens cap on and the same exposure time as your light frames (astrophotography images). It removes the camera’s thermal noise (hot pixels).
- Replace the dust cover on your telescope or camera lens.
- Cover the viewfinder to prevent reflected light from hitting the camera sensor.
- Take the images with the same settings and conditions as your light frames.
- Ideally, capture dark frames immediately after your imaging session when:
- The camera still has the same settings loaded
- The ambient temperature outside is still the same
- This is important because dark frames are pure noise, which your stacking software (such as Deep Sky Stacker) will use to offset against the noise in your light frames.
- Flat frame: A flat frame is taken with the lens pointed at an evenly lit, uniform surface (like a white card). It’s used to correct for vignetting and other variations in the camera’s sensitivity across the image field.
- Set the ISO to the same value used during your imaging session.
- Use Aperture Priority (Av on Canon DSLR) to determine the right exposure time. A flat frame should have an evenly distributed and centered histogram, with most of the data points near the middle of the histogram. The shape of the histogram should be a flat line with minimal spikes or valleys. The goal is to capture a neutral, gray image without any areas of overexposure or underexposure. If the histogram is too far to the right or left, it indicates that the flat frame is either overexposed or underexposed, resulting in an ineffective flat frame that cannot be used to correct the light frames.
- Do not rotate the camera or adjust focus after the imaging session
- ISO/Gain settings must match those of your light frames
- Focus level and camera orientation must be the same as for light frames
- Capture 20-30 flat frames.
- Good flat frames improve even illumination and remove dust spots, simplifying image processing.
- Dark flat frame: A dark flat frame combines a dark frame and a flat frame. It’s taken with the lens cap on and the same exposure time as your flat frame. It removes both the thermal noise and the vignetting/sensitivity variations in your images.
For a DSLR, both bias and dark frames can be taken in a controlled environment, like a home or observatory. Flat and dark flat frames should be taken in the field under the same lighting conditions as your light frames.