Dealing with light pollution is a constant challenge especially for urban astronomers, but it is not something that should put you off having a go at Astronomy. Even in some of the most light polluted cities in the UK it is still possible to see some of the brighter night sky objects with a modest telescope. Of course a dark sky is best and will allow you to see much more, but for those of us not lucky enough to be living in a Dark Sky Park a ltlle help is needed. This is where light pollution filters come in. There is a large range of options available but in simple terms they all do the same thing in that they attempt to block certain wavelengths of light. Astronomy filters are mainly available in either 1.25in or 2in sizes and are designed to thread into the bottom of your telescope eyepiece, but we also stock Astronomic Clip filters that fit into a Canon EOS camera instead.
You will see filters sometimes described as visual, photographic or CCD, but despite these classifications some filters can be used for more than one application. The key point to understand is how much light they will let through. A Digital camera or CCD chip is much more sensitive than the human eye and in addition can be programed to take long exposures. This means they can work with much less light than the Human Eye so typically a photographic or CCD filter will be more aggressive in cutting out light. The most aggressive filters are referred to as narrowband filters and this type literally blocks all light except a specific pre-determined wavelength. You could argue that these are the ultimate light pollution filter! but unfortunately many of them will not allow enough light through for you to see anything with your eye through a small telescope. Because narrowband filters only let through such a small bandwidth of light, your chosen filter must also match the light that is being emitted by your target object. This is why you will see many different options available such as OIII, Hydrogen Alpha, Hydrogen Beta, UHC and more. If you are using a larger telescope of 8 inches or more you can sometimes use a 'photographic' filter for visual use because of the amount of light you can gather, but this is very dependant on how tight the bandwidth is.
Chosing the right light pollution filter is therefore all about the balance of cutting out as much light pollution as possible, but letting enough light through for your eye or camera to process, through your chosen equipment, looking at your chosen target. We have listed below details of some of the most common options to help you understand further.
Basic light pollution filters such as the Sky-Watcher ones listed below attempt to block out the wavelengths of light emitted by mercury-vapour light and other common causes of light pollution, or if you like the typical orange glow from traditional street lights. Recommended for use on smaller telescopes up to 114mm diameter, where conventional filters swallow up too much light.
Astronomik CLS Filters are another general light pollution filter suitable for use on telescopes of all sizes. The CCD version is virtually the same as the visual version but has an added IR filter built in. This is only needed for CCD cameras or modified DSLRs that have had their IR filters removed. Recommended for deep sky and general purpose use and one of our favourites.
Baader Moon and SkyGlow Filter is a selective contrast enhancing filter, especially suitable for reflector type telescopes and true Apochromates. It aims to reduce the effects of skyglow and is good for improving surface details on Mars, Jupiter and Moon. I can also enhance many Deep Sky galaxies and nebulae against the background sky.
O-III filters work ideally with larger aperture telescopes in the 200mm and larger range. With larger aperture telescopes, observers will have a better opportunity to identify nebulae objects in the deep sky. It can produce near-photographic views of the Veil, Ring, Dumbell, Orion, plus many other nebulae under dark skies. For smaller telescopes chose the UHC filter instead as it lets more light through.
UHC filters (Ultra High Contrast) permit superb views of objects like the Orion, Lagoon, Swan and other extended nebulae. They performs well in smaller aperture instruments owing to its greater light transmission than the O-III, yet still suppresses light pollution well. A UHC filter is the best all-around dark-sky nebula filter.
Hydrogen Alpha, Hydrogen Beta and other narrowband filters are best used in photographic applications and due to their more specialised nature should be considered individually on a filter by filter basis.