Source
High Threshold
Starglow
The pre-process section lets you set a threshold for controlling where the Starglow effect appears. It also lets you use a built-in circular mask to restrict the glow area.
Threshold defines the minimum level for highlights. A low value will give more glow in the image, and a high value will make only the very bright areas glow. The images below show Starglow output using a shot of the sun on water with different Threshold values.
Source
High Threshold
Low Threshold
Threshold Soft:
This controls the softness of the border between lowlights and highlights.
Use Mask:
Check this to enable the internal circular mask.
Mask Radius:
This control sets the size of the internal circular mask in pixels.
Mask Feather:
The Mask Feather value sets the softness of the internal circular mask.
Mask Position:
This control defines the center point of the circular mask.