ABSTRACT
The title for this chapter is a reminder that certain image processing tools are better suited to image files before they are stretched. Indeed, the typical workflow shown in fig.1 can equally be called “basic processing”. As mentioned earlier, stretching causes a linear image to become non-linear and certain specialist image manipulation tools are optimized for linear images. These generally fix problems in the original files such as background gradients, color balance and convolution (blurring). If these issues are not fixed early on, subsequent image stretching exaggerates these problems and they become increasingly difficult to remove. General image processing programs like Photoshop work implicitly on mildly stretched (non-linear) images. An in-camera JPEG file is already stretched and similarly a RAW file converted and tagged with Adobe RGB assumes a gamma stretch of 2.2. A linear image in Photoshop would have a gamma setting of 1. For that reason I prefer dedicated astronomy imaging programs for the linear processing leading up to image stretching.
