ABE Function Degree in PixInsight

As an astrophotographer, one of the biggest challenges I face is dealing with uneven background in my deep sky images. This can come in the form of gradients caused by light pollution, moonlight, flat field errors, even dust donuts. To tackle these issues, I use PixInsight's Automatic Background Extraction (ABE) tool, which helps me generate mathematical models, known as synthetic background models or pseudo-flats, that represent the sky background in my images.

One of the parameters I adjust in ABE is the Function Degree, which can be set to a number greater than 0. In my experience, I use mainly a function degree of 1 or 2, as they are best suited to address the specific problems I face in my images.

Function Degree 1

When I set the Function Degree to 1, I am specifying that ABE should generate a linear model to represent the background in my image. This is ideal when I am dealing with linear gradients in my image, such as those caused by light pollution. By subtracting the generated model from my original image, I am able to correct for these linear gradients and obtain a cleaner, more accurate representation of the sky.

Function Degree 2

In contrast, when I set the Function Degree to 2, ABE generates a quadratic model to represent the sky background. This is useful when I am dealing with more complex gradients, such as those caused by flat field errors, vignetting, or moonlight. By subtracting the generated model, I can correct for these more complex gradients and obtain a more even and accurate representation of the sky.

Analyzing the Background Model

The background model, generated by ABE process, provides an illustration of the optical problem being addressed. For optimal results in correcting the image, it is important to ensure that the background model accurately represents the original image. To achieve a smooth gradient in the model, it is recommended to use 24-bit look-up tables for the Screen Transfer Function (STF). Using this method will prevent the image from appearing quantized and confusing to the user.


In conclusion, the Function Degree parameter in ABE allows me to specify the type of mathematical model I want ABE to generate, which in turn affects the type of correction I can apply to my images. Understanding the function degree parameter, and knowing when to set it to 1 or 2, is a key aspect of using ABE effectively in PixInsight.

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  • #processing
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