Being an city-dwelling astronomer, light pollution is a huge problem for me. Sure, I love the urban convenience of Tesco Extra for emergency overnight bacon but the orange glow of sodium street lights really ruins a night of stargazing. I always drive to my dark site in rural Somerset for “proper” stargazing nights and there it is pretty good – but you can still see the glow of Bristol on the horizon, and it is difficult to photograph objects too low in the sky.
I decided to buy an Astronomik CLS-CCD filter which blocks the sodium and mercury wavelengths associated with the most common types of light pollution, while still allowing the important emission lines to reach the camera’s sensor. This diagram shows the transmission of the filter, allowing everything under the red line to pass while blocking the wavelengths between.
The discontinuous transmission causes objects seen with the naked eye through the filter to appear purply-green so it isn’t really appropriate for daylight photography. However, I decided to march up Trooper’s Hill in Bristol, which overlooks most of the city along with its light pollution, and see if the filter helped.
For this highly scientific experiment, I used two Canon EOS 600D cameras, one fitted with a CLS-CCD filter. I used a Canon EF-S 17-85mm lens for the non-modified camera, and a Canon EF 28-80mm lens for the modified camera. The “clip” filter obstructs the opening of the SLR’s mirror box and it is not possible to use an EF-S lens with this filter, as they protrude too far into the body.
I set both cameras to ISO 400. Using aperture priority mode set to f/5.6, I photographed three scenes with varying levels of light pollution, letting the camera decide the shutter speed. Then using the modified camera, I used manual exposure to set the same exposure values to ensure a like-for-like test. All images were taken at (roughly) the same focal length. The unmodified camera used auto white balance, so the colours are funny. The modified camera used a custom white balance to sort of compensate for the filter, so the colours are also funny 😉
- A – Bristol cityscape photographed at 35mm, 4″, f/5.6
- B – houses photographed at 80mm, 6″, f/5.6
- C – St Aidan’s church photographed at 80mm, 6″, f/5.6
As you can see, the filter is pretty good at blocking a lot of light pollution. Of course it’s not 100% effective and there is still a lot of white light pollution from incandescent lights that can’t be selectively filtered in this way. However, it’s a huge help and will really boost the contrast and chase away that last little bit of light pollution at my dark site.
Jonathan Gazeley 