If you've ever stacked a set of light frames and noticed uneven brightness, dark corners (vignetting), or faint donut-shaped shadows from dust on your sensor, you've seen exactly what flat frames are designed to fix. Proper calibration is a cornerstone of astrophotography automation — and flats are one of the most impactful steps you can take to improve your astrophotography stacking results.

Flat frames are one of the three calibration frame types in astrophotography — alongside darks and biases — and they play a critical role in producing clean, evenly-illuminated final images. Yet for many beginners, flats are the most confusing and error-prone part of the calibration workflow.

This guide covers what flat frames are, why they matter for your stacking pipeline, and how to shoot them correctly with AsiAir.

What Are Flat Frames?

A flat frame is a photo taken through your telescope's optical train with a uniform, evenly-lit light source in front of the aperture. The camera, telescope, filter, and focus position should all be identical to your light frames — the only difference is the subject: instead of the night sky, you're photographing a flat, featureless white surface.

The resulting image captures every optical imperfection in your system:

Vignetting — the natural darkening at the corners and edges of the frame caused by the optical design. Every telescope has some degree of vignetting.
Dust shadows — specks of dust on the sensor, filters, or optical window cast soft, out-of-focus shadows that appear as dark donuts or blobs. These are especially visible in narrowband images.
Uneven illumination — slight gradients caused by the light path, filter tilt, or optical train flexure.

When you calibrate your light frames with matching flats during stacking, the stacking software divides each light frame by the flat frame. This mathematically cancels out all the imperfections, producing a clean, evenly-illuminated result.

Why Flat Frames Matter for Astrophotography Stacking

Without flat frames, your stacked image will have:

Dark corners that are difficult or impossible to remove in post-processing without cropping significant portions of the image.
Dust donuts that shift position between sessions (because you rotated the camera or cleaned the sensor), creating ghosting artifacts in the stack.
Uneven background that makes gradient removal and background extraction in tools like PixInsight or Siril much harder.

With proper flat frames, these problems disappear completely. The flat calibration step normalizes the entire field of view, so every pixel in your final image has received the same effective exposure. Whether you're stacking in Siril, PixInsight, or through an automated stacking tool like AstroMate, flats are what separate a clean result from one that needs hours of manual correction.

A common mistake: many beginners skip flats because their individual light frames "look fine." The issues only become apparent after stacking dozens or hundreds of frames, when the vignetting and dust patterns compound.

How to Capture Flat Frames on AsiAir

AsiAir makes shooting flats straightforward. There are two common approaches:

Method 1: Light Panel or Tablet Screen

Place a flat-field light panel (or a tablet displaying a plain white screen at medium brightness) over the front of your telescope.
In the AsiAir app, go to Autorun and set up a flat frame sequence.
Select the same filter you used for your lights. This is critical — each filter needs its own set of flats because dust positions shift relative to the sensor with each filter.
Set the exposure mode to Auto — AsiAir will calculate the correct exposure to hit the target ADU (typically around 25,000-35,000 ADU for a 16-bit sensor, or about 40-60% of the histogram).
Shoot 20-50 flat frames per filter. More is better for noise reduction, but 30 is a good minimum.
Repeat for every filter you used during your imaging session.

Method 2: Twilight Flats

Point the telescope at a clear patch of sky during dawn or dusk twilight.
Use the same AsiAir Autorun setup, with Auto exposure.
This method produces very even illumination but requires careful timing — you have a narrow window where the sky brightness is suitable.

Key Rules for AsiAir Flat Frames

Same optical train: Do not rotate the camera, change the focuser position, or remove/replace filters between your lights and flats. Any change alters the dust and vignetting pattern.
Same filter: Each filter must have its own matching flat set.
Same camera orientation: If you rotate the camera for framing, take flats before rotating.
Exposure doesn't need to match lights: Flat exposures are typically very short (milliseconds to a few seconds). What matters is hitting the right ADU level, not matching your light frame exposure time.
Temperature doesn't matter for flats: Unlike darks, flat frames are not affected by sensor temperature.

Where AsiAir Stores Your Flat Frames

AsiAir stores all captured frames on its internal storage, organized under the /Autorun/ directory on the SMB share. The file structure typically looks like this:

ASIAIR/
  Autorun/
    Light/
      M42_Light_300s_Ha_0001.fits
      M42_Light_300s_Ha_0002.fits
      ...
    Flat/
      Flat_5.2s_Ha_0001.fits
      Flat_5.2s_Ha_0002.fits
      ...
    Dark/
      ...
    Bias/
      ...

Each flat frame filename includes the exposure time and filter, making it possible to identify which flats go with which lights.

Matching Flats to Light Frames for Stacking

For your flats to correctly calibrate your lights, they must match on:

Filter: Ha flats for Ha lights, OIII flats for OIII lights, etc.
Optical configuration: Same telescope, same camera orientation, same focuser position.

In practice, this means you need to:

Keep a log of which flats you shot and when.
Know which filter was installed when each set of flats was captured.
Organize your files into directories that keep lights and their matching flats together.

For a typical narrowband session shooting Ha, OIII, and SII, you need three separate sets of flats. If you're imaging over multiple nights, you may need new flats each night (if you changed anything in the optical train).

This organizational overhead is where most beginners make mistakes — using the wrong flats, forgetting to shoot flats for one filter, or mixing flats from different sessions with different camera orientations. It's also the part of the astrophotography workflow that benefits most from automation, since the matching rules are mechanical and predictable.

What's Next?

Now that you understand what flat frames are and how to capture them, the next challenge is getting them off your AsiAir and into your stacking workflow. In our follow-up post, Automating AsiAir Flats With AstroMate, we cover every approach — from USB-C and network shares to scripted automation and fully automated astrophotography stacking pipelines.