Pre-Process
The pre-process section lets you set a threshold for controlling where the Starglow effect appears, and lets you use a built-in circular mask to restrict the glow area. This image shows the Pre-Process interface in Adobe After Effects.
Threshold
Threshold defines the minimum level for the 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 Postion
Define the center point of the circular mask with this control.