Astronomy Under City Skies or Combatting The Curse of Light Pollution

The question I get asked more than anything else when the subject turns to astronomy is “How can you possibly do astronomy or astrophotography from London when there is so much light pollution?” Here I’ll attempt to answer that question as well as hopefully inspire others who live in or near a city (which is most of us) to give the wonderful hobby of astronomy a go.

As you can see from the map below London is one of the most challenging places in Europe from which to view and image the night sky, so if I can do it anyone can!


Make the most of your location

Whether because of work commitments, family ties or just financial reality most of us don’t have the option of moving to a dark sky site in order to pursue astronomy or astrophotography as a hobby, so what can you do to make the most of your light polluted urban location? Here are a few tips that might help, some are pretty obvious, but I think worth stating anyway.

  • Make sure you are in shadow. Streetlights and neighbour’s bright lights are a universal problem in cities and towns, I’m constantly plagued by a kitchen light in neighbouring apartments that is so bright it illuminates the whole of my garden, I swear they must have to wear sunglasses to cook! I’ve planted some trees to obscure it but it will be a few years before they are going to be totally effective, so the best thing to do in these situations is try to set up in the shadow of a wall, tree or large bush, taking care that you can still see your intended target. If you can’t find that perfect spot then you could try channelling a Victorian photographer and cover your head with a dark cloth or coat. This will enable your eyes to become dark adapted (as long as it doesn’t slip off) and will help you tease out fainter details at the eyepiece.
  • Shade your optics. If you can’t get yourself into shadow, or don’t fancy putting a hood over your head, try to shield your optics from stray light. Dew shields are a good option, you can either buy one or make one yourself, they serve the dual purpose of cutting out light and increasing the length of time your scope can sit outside before water condenses on your optics, extending the time you have for observation or photography.
  • Image/observe when conditions are at their best. Pollution, dust and water vapour in the atmosphere all help to make light pollution worse as more light will be reflected back from all those streetlights etc. Poor conditions are generally found when humidity is high or it has been dry for a long period resulting in increased dust in the air. When you hear the term transparency this is usually what is being referred to, it’s also one of the reasons why observatories tend to be located in dry, lofty locations. For astronomy we also need stable conditions with low wind speeds, both at ground level and at the level of the jet stream. Astronomers call the variations in atmospheric conditions ‘seeing’ the better the ‘seeing’ the better the view you will have. A simple way of judging this is to gauge how much the stars are twinkling, shimmering stars may look pretty but they signify poor conditions for astronomy as they are the result of a turbulent atmosphere. Seeing is especially important for high resolution photography regardless of your location. I’ve included links to some useful websites at the end of this post which will help give you an idea of when conditions out under the stars will be at their best.
  • Plan your observing in advance. Planning is key, from any location you will get the best views of a target when it is highest in the sky, this is even more important when observing or imaging from a city or town. By waiting until your target is at its highest you will minimise the amount of atmosphere you are looking through and lessen the impact of light pollution. There is also no point trying to view dim objects under a full Moon, make sure you know your Moon phases and plan accordingly. When the Moon is big and bright image and observe it rather than try to battle against it.
  • Choose targets least affected by Light Pollution. A simple rule is that the brighter the object the less affected it will be by light pollution, pretty obvious really. Light pollution will have zero impact on lunar and solar observation and planetary observing and imaging is also pretty much unaffected, I’ve been able to view and photograph Neptune from my Wimbledon back garden. When observing and imaging planets atmospheric conditions are the limiting factor not stray light. Globular and open clusters are also a great option when observing under compromised skies but visual observation of galaxies and nebulae will be challenging, having said that big bright nebula like M42 the Orion nebula will still be visible under light polluted skies as will M31 the Andromeda Galaxy.
  • Foster neighbourly relations. Security lights are great generators of unwanted light - it is after all their prime purpose! If you have them then make sure they are switched off when you are observing, if your neighbours have them get them over and introduce them to the wonders of the universe….then ask them if they could turn off their lights while you are indulging in your nocturnal hobby!
  • Become a night owl. As the night wears on the amount of stray light decreases as people go to bed and switch off lights, also some local authorities switch off street lighting after midnight to save cash. The later you can do your observing the better.
  • Speak to your local authority. If you have a street light that shines into your garden it may be worth while bringing this up with your local authority, if it shines into a bedroom even better as this will help you build a case for them to install shielding to reduce the amount of light falling on your property. A polite enquiry can work wonders sometimes.
  • Escape to the country. I know these are supposed to be tips about making the most of your location but sometimes it’s nice to pack up and go somewhere truly dark – just to re-charge your astro batteries and remind yourself of what a truly dark sky can deliver.

Fight Light Pollution with Filters

Of course as well as the ‘no cost’ options described above you can also throw science and money at the problem by employing filters. Filters specifically designed to reduce the effect of light pollution go by a number of different names, City Light Suppression (CLS), Anti-Light Pollution (ALP), Light Pollution Suppression (LPS), Ultra High Contrast (UHC) etc. They all work by tuning out the wavelengths emitted by common street lights and can make a big difference to both the eyepiece view and that obtained via imaging. There is however some bad news on the horizon in that the move to LED lighting by many local authorities will make light pollution filters less effective as they emit over a much broader spectrum.

My Canon 6D with Astronomik CLS filter fitted

My Canon 6D with Astronomik CLS filter fitted

It is safe to say that there is a fair bit of debate about the effectiveness of these types of filters for visual astronomy but in my experience CLS filters are very effective when imaging from my London location making wide-field, long exposure DSLR shots viable. Have a look at the images below. Both were taken with the same exposure and while balance settings, the only difference being that the image on the right was taken with an Astronomik CLS filter in place. I haven't done any processing on either of the images so that a direct comparison can be made. The difference in the sky background in the image on the left and that on the right is obvious with the sky in the unfiltered image getting close to swamping out any detail while the filtered image on the right would benefit from a longer exposure. Both images could be improved significantly with processing, not least corrections in colour balance etc.

Both Images were taken with a Canon 6D, 20 second exposure at ISO1600. The image on the left was taken with no filter and white balance set to daylight. The image on the right was taken with the same settings but with a CLS filter in place. The bight blob is globular cluster M2

Both Images were taken with a Canon 6D, 20 second exposure at ISO1600. The image on the left was taken with no filter and white balance set to daylight. The image on the right was taken with the same settings but with a CLS filter in place. The bight blob is globular cluster M2

A few points regarding light pollution filters are (I think) uncontroversial and worth listing.

  • Filters work by subtracting (filtering out) particular wavelengths of light. This will make objects dimmer, but will, if the correct filter is used, improve contrast which will make faint nebula easier to see. By blocking out the wavelengths of light emitted by certain common artificial lights but allowing through the emission lines of the nebula you are viewing the sky will appear darker and the nebula brighter as contract will be improved. This also has the advantage of allowing longer exposure times for astrophotography as the sky background will take much longer to swamp the image of the object you are photographing.
  • The effectiveness of light pollution filters varies depending on the object you are viewing. If you are viewing a source that emits across a broad range of wavelengths such as a galaxy or reflection nebula then the filter will not be as effective as when viewing an object that emits at a narrow wavelength, such as H-Alpha or Oiii for emission nebula. An urban location is definitely much more of a barrier to achieving good results for galaxies and reflection nebula, it may be that only a huge number of stacked un-filtered ‘short’ exposures will give you the result you crave!
  • Light pollution filters will not block out all artificial light sources. They are not a substitute for truly dark skies.
  • Filters will increase the time required to image an object. This may not be strictly true in all cases but in my experience it is certainly desirable to increase image times if possible.

It is also worth noting that when imaging using light pollution suppression filters some colour shift will occur, some filters producing a more noticeable cast than others. This can however be easily remedied by creating a custom white balance for your camera with the filter in place. As with most things you get what you pay for, the limitations described above are less marked the higher up the price scale you go. Ultimately these filters let you make the most of your location, at least until you can afford that mountain top bolt hole.

Go Narrowband

Why not throw even more money and science at the problem. A great solution for the astrophotographer is to image using narrowband filters, the most common being those that transmit the emission lines of H-Alpha, Oiii and Sii. With these three filters you can try to re-create the famous ‘Hubble palette’, utilised by the Hubble Space Telescope for many of its amazing images.

These filters work really well on emission nebula, enabling you to image even very faint targets from an urban environment. They work by capturing very specific wavelengths of light, typically those dominant in star forming regions (Orion Nebula, Lagoon Nebula), and the gas emitted by exploding stars such as planetary nebula (Dumbbell nebula, Ring Nebula) and supernova remnants (Veil nebula, Crab Nebula). The bad news is that if you want to image the beautiful blue nebulosity around the Pleiades then narrowband filters do not work well, this nebulosity is lit by reflection from the nearby stars and so is a broadband source.

Here’s a brief summary of the emission lines passed by the most common filters:

Hydrogen-Alpha, H-Alpha (656.3nm) – This is most dominant in star forming regions such as the Orion Nebula. It is in the red part of the spectrum which is why on standard RGB colour images these types of nebula are red in colour.

Oxygen-III, Oiii (500.7nm) – This emission line is in the blue green part of the spectrum and is dominant in planetary nebula, the Dumbbell nebula shows up incredibly well when this type of filter is used.

Sulphur-II, Sii (672.4nm) – This emission line is well into the red part of the spectrum, it is generally weak and requires a lot of exposure time to generate a decent result. In bi-colour imaging this filter is typically left out but is included as you will want to use it if your desire is to faithfully re-create the Hubble palette.

The positions of these emission lines is shown in the graph below.

Image Credit

Image Credit

The Horse head nebula, imaged from London using a H-Alpha filter

The Horse head nebula, imaged from London using a H-Alpha filter

Embrace the Power of Stacking

If you want to take pictures under light polluted skies but can’t afford, or don’t want to use filters there is another solution, stacking.

The key drawback to imaging under light pollution is the fact that longer exposures cause the sky background to wash out or overwhelm the dimmer or more subtle objects and features that would be visible under less polluted skies. The solution is to keep your exposure times within the limit dictated by your sky brightness, you can work out what this is by taking a number of exposures of varying length and checking the image histogram or by visual assessment. When using the histogram you basically don’t want the peak of the graph to be right over to the right and you definitely don’t want to be touching the right hand edge of the graph as this indicates overexposure and total loss of data for that part of the image.

Once you’ve established how long an exposure you can get away with take lots of short exposures that you can then stack (you’ll need a tracking mount and intervalometer). This increases the signal to noise ratio and will enable you to tease out faint detail in post processing, even under light polluted skies.

The subject of image stacking and processing would require a book to cover properly and as this post isn’t about image processing I’ll leave the details of theory and work-flow to another time. If you do want to investigate this further there are plenty of resources out there on the web along with free software such as Deep Sky Stacker (DSS) which will do a lot of the hard work for you.

Give up and Go Robotic

All is not lost if you still want to image and process your own images of the night sky but don’t have the heart for the fight against light pollution. Robotic imaging offers you the chance to use amazing telescopes sited under some of the best skies on the planet, all from the comfort of your home. I wrote and article about it a few years ago which can be found here (it says written by Ralph, but it was actually me!).

Useful Websites and Apps

Weather and seeing

Clear Outside – Great app from first light optics

Planning your targets

There are loads of astronomy applications out there but if you want something to get started with I would recommend Stellarium. I’ve found for planning it is best to use something on your desktop or laptop rather than phone or tablet, having said that there are lots of great apps available for your smartphone. I’ve found ‘Starwalk’ and‘Moon Globe’ particularly useful.

There is quite a comprehensive list at Astronomy Online which is worth a scroll through