Understanding Flat Calibration Frames

Learning how to take flat calibration frames is an essential step for anyone involved in deep-sky astrophotography. Flat frames correct optical and sensor-related imperfections, significantly improving the quality of calibrated images.

What Are Flat Calibration Frames?

A flat calibration frame, commonly referred to as a flat, is an evenly illuminated image used to correct for vignetting, dust motes, and uneven sensor response in astrophotography images.

Flats record how your optical system and camera respond to uniform light, allowing calibration software to correct brightness falloff and artefacts in your final stacked images.

Key Purposes of Flat Frames

• Correcting vignetting: Flat frames capture brightness falloff toward the edges of the image.
• Removing dust artefacts: Dust particles on sensors or optical elements create visible shadows that flats help eliminate.
• Normalising sensor response: Flats compensate for non-uniform pixel sensitivity across the sensor.

How to Capture Flat Calibration Frames

1. Use Uniform Illumination

Flat frames must be taken using a uniform and stable light source placed over the telescope aperture. Dedicated flat field panels provide the most consistent and repeatable results.

2. Keep the Optical Path Unchanged

Flats must be taken using the same focus position, filters, and optical configuration used for capturing your light frames. Any change invalidates the flat frames.

3. Set Correct Exposure

Adjust exposure and brightness so the histogram peak falls around the middle of the available range (typically 30–50%). Avoid clipped highlights or underexposed frames.

Typical flat exposures range from 2 to 5 seconds. Very short exposures can introduce unwanted artefacts.

Automated Flats Using N.I.N.A. and Flat Panels

N.I.N.A. imaging software includes a Flat Wizard that can automate flat frame capture when used with compatible flat field panels.

When configuring automated flats, care must be taken to select the correct driver and exposure settings. Incorrect configuration can result in visible banding or inaccurate calibration frames.

Important Note on Line Banding

Line banding can occur when flat exposures are extremely short (typically below 0.5 seconds). This effect is caused by LED dimming frequencies interacting with short exposure times.

To avoid banding, ensure flat exposures remain above one second, even when using fast optical systems.

Recommended N.I.N.A. Flat Wizard Settings

• Select the ASCOM Pegasus Astro FlatMaster driver
• Enable Dynamic Exposure
• Set Minimum flat exposure to 1 second
• Set Maximum flat exposure to 6 seconds (increase for narrowband filters if required)
• Set an initial panel brightness value and adjust as needed
• Set Histogram mean target to approximately 40%
• Set Mean tolerance to 10%

Once configured, start the Flat Wizard to capture a complete set of flat calibration frames.