Mastering flats

Adding DSLR flat frames to your workflow to improve your deep-sky images

Getting the most out of our deep-sky data is a constant learning process. Not only do we need to master the complexities of capturing the data in the first place, but the art of postprocessing is just as important. Between the data capture and processing stages lie calibration frames. They are integral as they allow us to overcome some of the limitations in our imaging setups, such as sensor noise, dusty lenses and vignetting. Reducing noise and vignette allows us to stretch our data further.

The two most common types of calibration frame are 'dark' and 'flat' frames. While dark frames reduce the noise in an image, flat frames help subtract anomalies in the image train, such as dust bunnies and vignette. Vignette presents as dark edges in the field of view (FOV) or an uneven field illumination, and can be due to lens/mirror design or something interfering with the light path in the image train.

It's good practice to add calibration frames to all astro images. First, we'll explain how to take these using a DSLR and then we'll explore how flat frames improve an image of the Sadr (IC 1318) and Crescent Nebula (NGC 6888) region of Cygnus.

There are three main ways to take flat images: using a flat panel, a white T-shirt or a Word document. The best time to take them depends on the method that you use. Flat frames need to be taken at the exact same focus point as the light frames and with the DSLR set to 'aperture priority' (AV).