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Part 3 Reduce Light Pollution

Part 3 Reduce Light Pollution

 

This is a real obstacle to astro imaging. However there are ways to lessen its effect and take successful images.

 

What is Light Pollution?

 

Light Pollution is basically the effect of other light sources lighting up the atmosphere. This can be the light from the Moon or terrestrial light sources such as street lighting. The effect is to light the atmosphere to a level where it drowns out the light from stars etc. In the daytime you don't see the stars because the Sun is so bright it lights up the Earth and atmosphere to the point where the stars are not bright enough to be seen. The Moon on the other hand is lit up so much by reflected sunlight that it can be seen during the day.

 

You can think of light pollution as a form of noise. Star light is the signal you want to capture. A higher signal to noise ratio means you can capture more star light.

 

The worst form of light pollution is street lighting. That horrible orange glow that lights up the sky. Below is an image taken in London. You can see its almost entirely orange. Not much we can do about this you may think. Well there are two ways to reduce its impact on your images.

 

Light Pollution Auto White Balance


Remember this? This is your camera on standard settings doing its best to faithfully reproduce the cast caused by all those orange street lights. The longer your exposure or higher ISO you use, the worse it gets.

 

White Balance

 

Your camera uses something called White Balance to decide how colours should look. Normally this is set to Auto or Daylight. If you use this setting at night then it gets confused and images have a colour cast. If you shoot JPEG files, you can't do much about this afterwards. However if you shoot in RAW you can change the White Balance in software.

 

Your camera will have a custom white balance option. In this mode you normally point the camera at something white and it decides what the white balance should be. You don't have to use white though. If instead you poin the camera directly up at the sky and set a custom white balance it will compensate for all that sky glow. While you could do this later on in software, I like to be able to review the images I have taken and looking at orange ones is not very helpful!

 

The image below was taken at the same time as the one above but instead the camera used a custom white balance. The effect is quite dramatic.

 


 

Light Pollution Custom White Balance

By pointing the camera at the sky and setting a custom white balance, a much better result can be acheived. The 'bright' nature of the sky is still the cause of light pollution but overall the result is much better and can be improved further in image editing. 6 second exposure, f4, ISO1250

 

Light Pollution Filters

 

Another way to combat light pollution is to use a light pollution filter. A filter basically only allows certain wavelengths of light to pass through it. There are many light pollution filters on the market, some better than others. We believe the Astronomik CLS filter is the best. This filters out the light that Sodium and Mercury streetlights emit, among others. This has a dramatic effect on cleaning up the sky. A light pollution filter like the CLS is worth using always as there is always some form of light pollution around somewhere. Below is the same image but using an Astronomik CLS filter. This has then been further improved by using a custom white balance in the image below this.

 

Light Pollution Filter Auto WB

The image above was taken using a CLS Light Pollution Filter with the camera using normal Auto White Balance. Exposure is the same as the previous images. We now have a blueish cast but notice how star colour is becoming visible. Betelgeuse, the red giant in the upper left corner is now red! There is a slight loss in the number of stars as the filter normally requires increased exposure time. For this example though the exposure has been kept at the same 6 seconds at f4 as before

 

Light Pollution Filter Custom WB

This is with a CLS filter and a Custom White Balance. The exposure has now been increased from 6 to 8 seconds. See how now we have a much more natural look to the sky and that some star detail has returned. CLS filter. Don't leave home without one!

 

So in this section we have learnt how to clean up our image and practice Astrophotography even in City skys. For best results though it is always worth seeking out a dark sky site where you will have a head start gathering star light.

 

Note: The quality of images shown here is not optimum. None of the images displayed are subject to any processing.