An easy and effective way to reduce star size using Photoshop.
Star reduction is a technique commonly used by deep-sky imagers, but it holds great value for improving wide-field astrophotography as well. Due to conventional post-processing techniques like brightening mid tones and increasing contrast, stars often end up being over exposed or can appear over processed. While these adjustments are necessary to allow faint light and structures within the Milky Way to “pop,” it doesn’t mean that your stars need to suffer.
Star reduction allows you to enhance the faint details without compromising image quality or producing bloated stars! It also ensures that bigger stars and noteworthy structures stand out more, rather than getting lost amongst a rich star field.
The Photoshop star reduction process
- Have your image layer selected in Photoshop.
- Use the eyedropper tool to select the colour of a regular, average-color, white, star.
- Choose Select > Colour Range. (The default “select” dropdown should be “Sampled Colours”.)
- Adjust the “Fuzziness” slider to select less or more from the image in the selected colour range.
- Use the eyedropper+ and eyedropper- tools to remove or add to the colour selection. Once you’re happy with the range of stars selected press OK. A selection boundary will be loaded around the stars.
- Choose: Select > Modify > Feather. Set the feather radius to 1 pixel. Experiment with a 2 pixel radius at longer than 50mm focal lengths.
- Go to Filter > Other > Minimum. Set the pixel radius to 0.4 and ensure that Preserve: Roundness is selected, and hit OK. Minimum and Maximum filters are great, as they allow you to expand or contract in decimal values, unlike the Select > Modify menu that only works in whole numbers.
NOTE: Generally a pixel radius of 0.5 is sufficient for wide-field astro photos, but you can undo this and try higher or lower values that suit your star field best — 0.3 – 0.7 are good ranges to try.
As easy as that!