Astrophotography - Star size reduction

[TonyG55]

Kudos to Affinity Photo creators for adding a couple of dedicated Astrophotography features! Here's a routine I use frequently in 'That other program' but am unable to replicate in Affinity and which would I think, be a welcome addition for many astrophotographers.
It's a star size reduction routine that is quick to use and reduces the apparent size of stars, which due to factors such as atmospherics, may appear a bit bloated. In PS it goes like this:
Select/ click in a typical star;
Select/similar (most stars in the image will then be selected)
Select/Modify/Expand, 3px or as suitable
Select/Modify/Feather, 3px or as suitable.
Filter/Other/Minimum, adjust to suit, 1-4px.
Suggested values are based on an APS-C sensor of 6mp (QHY8L) and would obviously vary with the size and resolution of the sensor.
This maybe a niche requirement but it would add greatly to the Astrophotography friendly aspect of Affinity.

[James Ritson]

Hi @TonyG55, you can do this in Affinity Photo using a very similar approach—better still, you can use a live Minimum Blur filter so the effect is non-destructive.

Try this:

• Make sure your pixel layer is selected (don't forget you can use Layer>Merge Visible to create a temporary layer if you're working with several greyscale layers and tone stretching them)
• Select>Select Sampled Colour
• Single click on a bright star to sample it
• For the colour model, try Intensity—alternatively, if you're dealing with rich colour stars, you may want to try RGB or CIELAB.
• Adjust tolerance as required, click Apply
• Select>Grow/Shrink, grow selection as required
• Optionally, Select>Feather to feather the selection
• Now go to Layer>New Live Filter Layer>Blur>Minimum Blur and adjust Radius to suit
• At any point, you can change the Minimum Blur layer's Radius parameter, or even change its Opacity if the effect is too strong.

Just one caveat: by default, live filters child layer into whichever layer you have selected—which might not be ideal if you're working with several composite layers. Just drag the live filter layer out onto the parent stack if it's in the wrong place, or alternatively click the suit icon on the top toolbar (Assistant Options) and you can change 'Adding filter layer to selection' to 'Add filter as new layer'.

Hope that helps!

[TonyG55]

Wow! Thanks James! That does indeed yield a very similar result! There is no way I'd have figured it out for myself though (and I have tried) so thanks again. That's been stored away in my stash of 'very useful techniques'.
Just when you think you're pretty good at this stuff along comes someone who makes you feel like a novice! 🤣