Glossary - Calibration Frames

Flat Frames

updated: 2025-10-15

Flat Frames (or white frames) are used to capture flat calibration frames and belong to the group of calibration frames.


To capture flat frames, allow the camera to automatically select the optimal exposure time. Point the lens at a bright, homogeneous white object, such as an LED Light Pad directly in front of the lens, or at a homogeneous section of the sky.


Important: 

  • Flat frames should be taken during every imaging session because they depend on the alignment of the entire imaging train. 
  • At the very least, the flat frames should be taken immediately before or after the light frames so that any contamination of the sensor can be taken into account during calibration.
  • Flat frames normally target a mean ADU of 22,000 to 25,000, but this will depend on camera and user preference. 
  • If your flats data is defined (per filter), selecting a flats event will automatically populate the saved exposure time (for the selected binning).
  • GAIN must be set to an identical value as the light frames you want to calibrate
  • Filters: Colour light frames from an ASIAIR OSC camera never contain any filter information, even when a filter has been used. 
    • Therefore, the Master Flat Frames must be renamed, with the string e.g.: '_FILTER-NoFilter' added somewhere in the file name. 
    • The files also need to be converted from .fit to .xisf in order to be accepted by the WBPP or FBPP batch pre-processing scripts in PixInsight.
    • The name of a Flat Frame master file should always include the abbreviation of the filter used. This Must be set manually if no filter wheel is used. 

Examples: _FILTER-NoFilter, _FILTER-OLeN, _FILTER-UHC2 or _FILTER-NoFilter

  • Focal Length of the optical equipment must be the same as for the light frames
  • Focus must be identical as for the light frames
  • Exposure: set exposure time to AUTO or to any other value depending in the brightness
  • Temperature:  no temperature requirements
  • No. of Flat frames: typically the same amount of flats as light frames (but 20 photos are typically enough)
  • Use a evenly-lit light source, e.g. twilight sky, laptop screen, light panel, light box, TV screen, a white T-shirt to cover your telescope  or similar.
  • Use the Calibration Frames Naming Convention to name the flat master file.
  • In addition, at least the flat frames should be stored together with the light frames in the same folder structure associated with the observation in the “Calibration” subdirectory to facilitate later post-processing.

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