Observing Under Light Polluted Skies
By Steve Smith
Observing under a clear dark sky is of course the wish of every amateur astronomer but unfortunately many of us have no option but to contend with the light pollution of nearby towns and cities or the result of poorly designed or installed streetlights or security lighting.
I personally live on the outskirts of Birmingham UK although thankfully the city centre is to the north so at least my southern sky is not as bad as it otherwise would be. This article is mainly therefore, a summary of my personal experiences of countering light pollution, I will not deal with the issue of light pollution itself or the reduction of this menace - this subject is discussed in detail in our light pollution section. The fact that I concentrate here on coping with light pollution rather than solving it does not mean that I think we should give up and accept what light pollution we have. However I can only deal with so much in this space and reducing light pollution is unfortunately a longer term aim whereas I want to observe now!
If you would like a much more in-depth study of light pollution and dealing with it when observing I recommend Astronomy from Towns and Suburbs by Robin Scagell as a good all round reference source.
1. Choosing your telescope
First a word about binoculars. As binoculars are very portable you can take them with you at a moment's notice to a dark site from which they will be very useful. With a good pair of binoculars with reasonably large objective lenses and low magnification such as 7x50's you will be able to observe a good selection of deep sky objects. However from a light polluted site I find binoculars to be of little use (apart from the fact that being so portable they're easy to take somewhere darker). A telescope however seems to be much better at dealing with a poor sky background, the higher magnification resulting in a greater contrast between the object being observed and the background sky.
If you live in a light polluted location, travelling to a darker site for observation might sometimes be possible. If so, portability might be an important factor in the choice of your telescope unless you are fortunate enough to be able to afford a large fixed telescope at home and a more portable instrument for remote observing. Even if you are so fortunate you might question whether you can justify such expenditure on an instrument that, due to your location, will not be used to its full potential. If you are unable to transport you telescope to a dark site you may be able to rely on the generosity of friends who are blessed with a dark sky and who are only too happy to have company while observing. Joining an astronomical society will introduce you to friends to observe with and furthermore the society may offer an observatory at a dark site. For example, our Society has an observatory at a dark site and our Observing Section members regularly meet at the observatory or a variety of other dark sky sites.
In choosing your telescope, besides portability there are many other things that will influence your decision. Your budget will of course be an important factor and so will the types of object in which you have greatest interest and whether you wish to do astrophotography or CCD imaging. Let these questions determine your choice of telescope rather than the quality of your skies alone. However, be aware that short focal (eg f4 or f5) reflectors tend to be more suited to observing deep sky objects in a wide field of view from a dark site and if you suffer from light pollution you will not be able to take full advantage of their particular benefits.
Even with access to generous friends, a dark site observatory and a portable telescope often you will wish to observe from your home so what else can you do to improve your prospects?
2. Choosing your targets
Your view of brighter objects will be less effected by light pollution. The Moon and planets out to Saturn can be observed from just about any location, will not be significantly affected by the level of light pollution and will provide many hours pleasure. The Moon is often neglected by amateur astronomers because it is such an easy target and isn't thought of as presenting a challenge. However as the angle of illumination changes so does the appearance of the lunar surface from one hour to the next.
Mercury provides something of a challenge in that favourable appearances are quite short-lived and it is only visible shortly after sunset or before sunrise. Being quite bright, the only light pollution you will struggle with is the natural variety from the nearby sun. Venus is much easier and can be viewed a little further from the sun. However, Venus is perhaps most interesting either side of superior conjunction (when it is between us and the Sun) as it then appears as a large and slender crescent. Neither Mercury or Venus move far from the sun in the sky so it is crucial to take great care when observing them to avoid damaging your eyesight.
Mars is best viewed for a few months every two years near opposition (when it is in the opposite direction to the sun from Earth). Jupiter and Saturn are also best near opposition but as they are much more distant this occurs approximately annually. Uranus and Neptune are visible in binoculars although if the light pollution is bad you may need a moderate sized (say 120mm) telescope. Pluto will present a challenge and will require a telescope of at least 200mm diameter from any location.
The brighter asteroids will not be too difficult from most locations but, unless you have a particular interest in asteroids, they are unlikely to provide a regular source of interesting targets. It can however be quite fun to positively identify one or two asteroids by comparison with a detailed star chart, drawing the field of view and confirming the identification by returning to the same location the following night (or even several hours later) redrawing the view and comparing it with the first drawing.
For comets there really is no other answer than travelling to a dark site. Occasional comets will be visible in a telescope from a light polluted site. Maybe once each year a comet will visit us that is bright enough to be seen in binoculars. Rarely will a comet be bright enough to be seen with the naked eye. In all cases, you really need to get to a dark site to appreciate the full beauty of any comet. Let's face it, bright comets are such rare events that it really is worth the effort to get to a dark site from which to observe them! The images of Comet Hale Bopp on the right illustrate this by comparing the view from a light polluted site with that from a reasonably dark site. A list of currently visible comets is available at http://cometography.com/current_comets.html.
Meteor showers are also better if observed from a dark site. You will see more meteors because you will be able to see dimmer meteors that, from a light polluted site, will be lost in the sky glow. However if you can't get to a dark sky meteor showers are still worth watching - you will simply be restricted to the brighter meteors. The Perseids peaking around August 12 and the Geminids around December 14 are good showers. For a complete list of meteor showers see Gary W. Kronk's Meteor Observing Calendar.
Moving outside our solar system, the deep-sky provides a wealth of observing targets but unfortunately being relatively dim it is these that are most affected by light pollution. It is therefore with deep sky objects that one has to be most selective in choosing observing targets. It might seem obvious to concentrate on the brighter examples but it's not as simple as that. The measure of brightness of an astronomical object is its magnitude. An object with a magnitude of 5 is 2.5 times brighter than one with a magnitude of 6. Unfortunately with deep sky objects this measure of brightness can be misleading. This is because the magnitude that is quoted will normally be the total magnitude of the object and because deep sky objects vary greatly in apparent size, the total magnitude does not completely provide an indication of how easy the object will be to observe. A much better measure would be surface brightness but you are unlikely to see any measure of surface brightness quoted. The angular size of objects is often quoted and so it is possible to judge the approximate surface brightness of objects, for example of two objects with similar magnitudes the smaller will normally be easier to see. Deep sky objects include a variety of delights: galaxies, star clusters and nebulae.
Galaxies can be quite difficult from a light polluted site and magnitude is not necessarily a good indication of how difficult they will be. Some of the brighter galaxies also span a greater area and so may have a relatively low brightness per square area. For example the 5.7 magnitude of M33 suggests it is a good target but its large diameter of over a degree results in it being rather disappointing. Smaller face-on galaxies, compact barred spirals and edge-on spirals provide a much better view.
Star clusters seem to cope quite well with light pollution. The magnitude quoted will be the combined magnitude of all the member stars but as the light from each is concentrated in a point-like object you will generally not have too much difficulty. Open clusters have a number of irregularly spaced stars in the same area of space and were probably all born in the same cloud of gas and dust and therefore are likely to be relatively young stars.
Globular clusters on the other hand comprise some of the oldest stars in the universe. The chemical composition of such stars measured by their spectrum indicate that they may be 12 billion years old compared to an estimated age of the universe of 15 billion years. Globular clusters are thought to inhabit a spherical halo surrounding our galaxy and tend therefore to be found well away from the milky way (although if you live in a light polluted area the Milky Way is likely to be just a fond memory of your last visit to a dark area).
A typical globular cluster is M13 at a distance of 21,000 light years and containing several hundred thousand stars in a ball 200 light years in diameter (its central core has a diameter of about 100 light years representing an angular diameter of only 8 arcminutes or an eighth of a degree). Some of the dimmer globular clusters can be quite difficult but as globular clusters tend to have a similar angular size, their quoted magnitude provides a good indication of their relative apparent brightness compared to each other.
Nebulae fall into two categories. Diffuse nebulae are caused by clouds of gas and dust whose light is provided by reflecting the light of nearby stars or by the emission of light by excited atoms within the nebula probably also as a result of nearby stars. Planetary nebulae are caused by the emission of light from atoms in the shell of gas and dust blown off from a star in the later stages of it's life. With the exception of a small number of bright diffuse nebulae such as the Orion Nebula, the best targets from a light polluted site will be planetary nebulae. This is mainly because being small in angular size their brightness is concentrated in a small area but also because they respond well to light pollution reduction filters, of which more later.
Many amateur astronomers begin working through the Messier catalogue using this as a useful list of targets. Indeed Charles Messier did provide us with many excellent targets visible with a variety of instruments and in a few cases with the naked eye. However it is often mistakenly assumed that this catalogue contains the brighter objects and that it is not worth moving on to other catalogues until one has exhausted Messier's recommendations. This is quite wrong as while many of the brighter objects were catalogued by Messier, many were not.
Further suggestions of objects to observe including some visible from the Southern Hemisphere are provided in Astronomy from Towns and Suburbs by Robin Scagell.
3. Telescope accessories
Light pollution reduction (LPR) filters work by only allowing through certain wavelengths of light. Other wavelengths are rejected so it would be ideal if the wavelengths you wanted to observe were different from those within the light pollution. Unfortunately, reality is not quite so simple. The easiest sort of light pollution to deal with is that produced by high pressure sodium street lights as this has a very specific wavelength. This can be cut out by a broadband filter which allows through a wide range of wavelengths but not that of high pressure sodium. Light pollution however normally results from a wide variety of lighting sources and as many of these are designed to mimic sunlight they emit light with the same wavelengths of stars and therefore of many of the objects we wish to observe.
Narrowband filters work by cutting out all wavelengths except those in a very narrow range which are emitted by certain types of object. These filters are therefore much more effective in cutting out light pollution but they also cut out light from more "normal" objects. Narrowband filters are excellent for observing emission nebulae (such as planetary nebulae) but are not particularly useful for galaxies or star-clusters. Narrowband filters are not only used as light pollution filters, even from a relatively dark site they may be useful in increasing the contrast of the object you wish to observe compared to the background sky. Some narrowband filters are particularly specialist in that they are designed to allow through the light of very specific objects such as the Horsehead nebula eg the Lumicon H-beta filter. Don't expect to be able to observe the Horsehead nebula from a light polluted site even with one of these filters - besides the filter you will need a very dark sky, dark adapted eyes, a reasonable size telescope and excellent observing conditions.
A good example of a broadband filter is the Lumicon Deep-Sky Filter. Good examples of narrowband filters are the Lumicon Ultra High Contrast (UHC) Filter and the Lumicon O III (Oxygen-3) filter.
I find the Lumicon UHC filter particularly useful for planetary nebulae. When trying to locate a planetary nebula I often hold the filter by its edge between my fingers and repeatedly pass it in front of a low power eyepiece. The nebula, if it is in the field of view, will appear to blink on and off. Once I've located the nebula I sometimes then fix the filter to the rear of the eyepiece for observing or often I will simply replace the low power eyepiece with a higher power.
Note: Whilst writing this article I was concerned to discover that Lumicon appear to have ceased trading. I hope that this is only a temporary situation and that it will not impinge upon the supply of these filters - your astronomy dealer should be able to advise.
Once you have located an object at low power, it is often possible to observe it better using a higher power eyepiece as this increases the contrast of the object. The higher power reduces the brightness of the object but it reduces the brightness of the background sky even more. Sometimes when observing planetary nebulae, I find replacing my 30mm eyepiece with a 12.5mm eyepiece has a similar effect as adding the Lumicon UHC filter to the 30mm eyepiece. The best thing to do is experiment - try different configurations and see which works best for you.
4. Astrophotography and CCD Imaging
Astrophotography using film whilst creating a useful record of observations (and some nice pictures) doesn't exactly aid observation due to the processing delay involved. However, I've included it here as many observers will wish to have a go and I include myself amongst their number. For wide field astrophotography (eg using a standard lens on a 35mm camera) you will be severely limited in what you can achieve from a light polluted site as the film will become 'fogged' after a fairly short exposure. However with greater focal lengths using prime focus, eyepiece projection or afocal methods you will be less troubled by light pollution. As a result, photographing the Moon, planets and compact deep sky objects becomes possible. The longest exposure I have attempted from my light polluted home site is 25 minutes using 400ISO film at f10 sufficient to successfully record M27 the Dumbbell Nebula. For the Moon and planets you need not be troubled by such long exposures. With deep sky objects where long exposures do become necessary, try to pick a time when your target is near the zenith and when the transparency of the atmosphere is good.
In contrast to film, CCD's yield fairly instant results and thus have greater potential as an observing tool. In order to bring out the best of CCD images a great deal of processing work is required after the image has been acquired but the raw image will usually reveal more detail than can be observed through the eyepiece and in some cases can reveal objects that cannot be seen in the eyepiece. This is achieved partly through the great sensitivity of the CCD, partly though the ability to combine a series of shorter exposures and most importantly through the ability to reduce unwanted background light by image processing techniques.
This comes at a price because CCD equipment can be very expensive, possibly as expensive or even more than the telescope itself. However recent innovations in the modification of certain CCD based webcams provides a source of economical entry level CCD's. For imaging the Moon and major planets the only modification required is to remove the webcam lens and replace it with a means of fixing the webcam to the telescope focuser. This can be done at no cost using a 35mm film canister or more professionally using a purpose built adapter from Steve Mogg.
For deep sky objects more complex modifications are required involving altering the circuitry of the camera in order to disable the camera's exposure control and enable hardware control via a separate computer port. However for someone with patience, modest skill with circuit boards and the right soldering equipment, it is possible to make a camera that rivals the cheaper conventional CCD's but at a fraction of the price. The long exposure modification of webcams was first developed by Steve Chambers and full details can be found on his web site at http://home.clara.net/smunch/wintro.htm along with links to sites of others who have modified various different models. A good summary of webcam imaging is available from The QuickCam and Unconventional Imaging Astronomy Group.
This beautiful image of the Trifid Nebula M20 was captured from a balcony of a block of flats in Kosice, a Slovakian city with 300,000 inhabitants, by Peter Katreniak. The naked eye limiting magnitude was only 4.2, there was a 9 day old moon in the sky and the M20 was only 17° above horizon. Peter combined eleven 25 second images captured using a modified Philips Vesta 675 and a 200mm Newtonian telescope. More examples of Peter's images, full details of his modified webcam and a download of the excellent K3 CCD Tools (software developed by Peter for capturing and processing astronomy webcam images) can be found on his website http://www.pk3.host.sk/Astro.
Even if, like me, you live in a light polluted location I would encourage you not to give up on observing and resort only to armchair astronomy - that would be a great shame. I personally enjoy astronomy either in my armchair or at the eyepiece and would not be content with only one or the other. Some people may consider me to be slightly weird but I get great excitement from a night out under the stars, the extra adrenalin presumably preventing the onset of tiredness. When we hold star parties for the public or at schools the whoops of delight from people who are enjoying their first view of Saturn, for example, through a telescope are both rewarding and suggest that maybe I'm not so weird after all.
Astronomy has much to offer from towns and cities and I encourage fellow city dwellers to take what steps they can to enjoy astronomy from home in addition to seeking out darker skies whenever they can. For an indication of your observing prospects tonight see our observing prospects section.