Light Pollution Reduction

Removing the glow caused by light pollution is a simple way of enhancing your night-sky images

All images: Pete Lawrence

The night sky is best seen in completely dark conditions, an experience that is becoming a rarity for most of us. Street lighting, security lights and floodlights are just some of the sources of artificial illumination that rob the night of darkness in urban areas.

Take a long-exposure photo from most parts of the UK and the chances are that it will have a distinctly orange hue, caused by the light from sodium street lamps reflecting off particles in the atmosphere.

You can deal with light pollution at source: a monochrome CCD camera fitted with a narrowband filter, under certain lighting, can let you image away without interference.

Likewise, a DSLR fitted with a city light suppression (CLS) filter can also help. If you have no filters, then you’ll need to turn to image processing.

STEP BY STEP

Removing the effects of light pollution from night-sky images taken with a DSLR with software such as Photoshop or GIMP

STEP 1

This image shows some of the typical characteristics associated with light pollution – an orange-brown discolouration and an uneven brightness gradient.

STEP 2

If you’re not bothered about the gradient, open the image and, using the levels tool, drag the mid-point slider for the red channel to the right until the sky has a slightly green tinge.

STEP 3

Repeat the process with the green channel, this time dragging the mid-point slider to the right until the sky starts to look black withno significant red or green hue.

STEP 4

Alternatively, if your image has a significant gradient, open it in a layer-based editor and duplicate the main image so that you have two layers containing the same image.

STEP 5

Use the dust & scratches filter on the upper layer, setting the ‘Radius’ value to 100 and the ‘Threshold’ value to 0. Ideally all stars and features should disappear.

STEP 6

Change the blend mode of the layer with stars removed to ‘Difference’. Use the curves tool to brighten or darken as needed, then merge with the main layer to produce a final image.