Removing Light pollution Astrophotography Urban Astronomy

The moon and the stars no longer come to the farm. The farmer has exchanged his birthright in them for the wattage of his all-night sun. His children will never know the blessed dark of night.

 — Leslie Peltier in his autobiography

Comparison of original image and final one after removing of the light pollution – night photo of Golden Gate Bridge, San Francisco, California.

Light pollution Astrophotography Urban Astronomy

Photoshop instruction:

Step1: Select the Layers panel and then  duplicate the layer
Step 2: Filter -> Noise -> Median… (about 50 px)
Step 3: Filter-> Blue -> Gaussian Blue… (about 30px)
Step 4: Select original layer
Step 5: Image -> Apply image (Blending: Subtract; Offset: 40)
Step 6: Delete the duplicate layer

See for example video in my other post Comet 103P/Hartley 2 from New York City

Note, that Astronomy Tools has more sophisticated methods to remove light pollution, or you can do some variations of such methods (it depends from size of you original image). You also can apply it only for part of original image.

The other way to remove light pollution is using multiple images, see for example my other post M42 Orion Nebula using iOptron SkyTracker in NYC