{"id":23351,"date":"2021-09-16T00:00:00","date_gmt":"2021-09-16T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=23351"},"modified":"2021-10-22T11:43:11","modified_gmt":"2021-10-22T11:43:11","slug":"astronomy-under-light-polluted-skies","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2021\/09\/16\/astronomy-under-light-polluted-skies\/","title":{"rendered":"Astronomy under light-polluted skies"},"content":{"rendered":"\n

Astronomer Will Gater <\/strong>guides you through the possibilities of stargazing from more brightly-lit locations<\/p>\n\n

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Even in light-polluted urban areas, there\u2019s a wide selection of night-sky targets waiting to be discovered <\/figcaption><\/figure><\/div>\n\n

While the world of astronomy regularly trades on visions of dark expanses swarming with stars and the river-like Milky Way arcing over remote landscapes, the reality for most of us is that stargazing occurs under, or close to, the bright lights of towns and cities. From these location contemplating the Universe can be challenging, as wasted artificial light fills the sky with a diffuse, nocturnal glow.<\/span> Although the near-fantastical vistas of Instagram and space documentaries might be lightyears from our \u2018every night\u2019 experience, there\u2019s still an enormous diversity of engaging targets for an astronomer to enjoy under urban and suburban night skies. In this article we highlight a few key examples \u2013 and offer some tips on how to get started \u2013 to show that you don\u2019t need to live under the darkest skies to connect with the wonders of the cosmos above. <\/span><\/p>\n\n

Getting transparent about light pollution<\/h3>\n\n

A key factor that influences how bad the effects of light pollution are is the haziness of the sky<\/p>\n\n

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Atmospheric \u2018transparency\u2019 is essentially a measure of how clear a clear night sky is <\/figcaption><\/figure><\/div>\n\n

If you live in an urban or suburban area, the fact that badly directed and excessive artificial light obscures the stars over our towns and cities isn\u2019t going to be news to you. But if you\u2019re just getting into stargazing, it can be useful to explore exactly how this light pollution affects certain kinds of astronomical observing \u2013 not least because this can help you prepare for nights when the effects are lessened somewhat. Clouds, of course, can spoil an observing session no matter where you\u2019re observing, but there\u2019s another atmospheric phenomenon that all townbased stargazers will know is hugely important to the clarity of their views: astronomers call it the \u2018transparency\u2019.<\/span><\/p>\n\n

Atmospheric transparency is a measure of how clear your clear skies are. Hazes or pollution (suspended in the air) can create milky, murky skies and what can be called \u2018poor\u2019 transparency. Conversely, crystalclear nights free of these intrusions are said to possess \u2018good\u2019 transparency.<\/p>\n\n

The reason why transparency is important to monitor when observing the night sky from urban areas is that hazy skies can elevate the problems associated with light pollution: the particles in the air scatter the glow from below and the result is a brighter night sky, where it\u2019s harder to see fainter stars. Paying attention to transparency levels can help you plan observing sessions. If you can see that the sky is milky and the skyglow is enhanced you can focus your attention on brighter targets like the Moon and planets. When the transparency is good you can go after fainter objects like star clusters.<\/span><\/p>\n\n

Know your limits<\/h3>\n\n

Working out the faintest stars you can see will help you get your bearings in the night sky<\/p>\n\n

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Choose a constellation that\u2019s always visible, like Ursa Minor, to work out the magnitude of the dimmest star you can see<\/figcaption><\/figure><\/div>\n\n

When you begin exploring the night sky from a town or city it\u2019s a great idea to have a star atlas, a planetarium computer program or an app to help you navigate your way around the stars. However, before you open one of these up, it\u2019s well worth doing a quick exercise to establish what you can actually see under your local night sky.<\/span><\/p>\n\n

Essentially, you want to find out what the faintest stars are that you can discern \u2013 what astronomers call the \u2018nakedeye limiting magnitude\u2019 or NELM. To do this you can use a constellation that\u2019s always visible at night in the UK, like Ursa Minor, the Little Bear. It\u2019s a fairly easy pattern to find as the tip of the Little Bear\u2019s tail is the bright star Polaris, the North Star. Using the chart here (right), a star chart or an app, look carefully to see what brightness, or magnitude, the dimmest star you can perceive is \u2013 remember, the lower the number, the brighter the star. For suburban locations under fairly average conditions, this might be around mag. +3.0 to +4.0.<\/p>\n\n

Many planetarium programs and apps have tools that have the ability to limit the magnitude of the stars displayed. So if you set the magnitude at the level of your local NELM, what\u2019s shown on screen will be only the stars you can see from your observing site. In turn, this can make star-hopping, or jumping from star to star to locate fainter objects, much easier.<\/span><\/p>\n\n

Make the most of your telescope or binoculars<\/h3>\n\n

Not all deep-sky sights are out of reach from brightly-lit towns and cities<\/p>\n\n

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With a small telescope you can locate targets like the Lagoon Nebula<\/figcaption><\/figure><\/div>\n\n

While many fainter objects will be largely lost or hard to find in the skyglow from suburban locations, a small telescope or a good pair of binoculars will nonetheless show you a respectable number of brighter deep-sky targets including star clusters, bright planetary nebulae, and summer and winter showpieces like the Orion Nebula and the Lagoon Nebula. There are also a few things you can do to reveal these wonders more clearly.<\/span><\/p>\n\n

Firstly, use a light pollution filter tailored to visual observing as these are specifically designed to suppress the glow from artificial light.<\/p>\n\n

Consider your observing position, too. For example, never observe from inside a building with the door or window open \u2013 as the escaping warm air will create air currents and cause the view to shimmer; this is most noticeable when viewing the Moon and planets, but applies to deep-sky observing too. Try not to view targets that are directly over rooftops, or above hot air vents, as that will avoid similar issues.<\/p>\n\n

Finally, just as one would from a darker location, allow your eyes at least 30 minutes to adapt to what darkness there is \u2013 and keep any artificial lights out of your direct line of sight if you can. <\/p>\n\n

Picking out the planets<\/h3>\n\n

These objects can be spied from even heavily light-polluted cities<\/p>\n\n

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These images of Saturn, Jupiter and Mars were taken from central Bristol through a 200mm telescope<\/figcaption><\/figure><\/div>\n\n

The planets are one of the few astronomical target groups that are pretty much completely unaffected by the problems of light pollution. While you\u2019ll probably struggle to glimpse distant Uranus with the naked eye, like you can at dark-sky sites, and telescopic observations of some of the fainter moons of Saturn might be a little harder to come by, for the most part you can get great views of the other worlds in our Solar System with a small telescope even from brightly-lit cities. What\u2019s more, high-resolution planetary imaging can be conducted with great success under lightpolluted skies \u2013 some of the world\u2019s best astrophotographers in this field work from homes in urban and suburban locations.<\/p>\n\n

Jupiter, with its cloud bands, and Saturn with its rings are of course superb targets for even small-aperture instruments. Bright Mars, at opposition, and the crescent Venus also offer captivating views in larger aperture scopes \u2013 the latter needing a careful observing approach, ensuring the Sun is below the horizon before viewing starts. From suburban locations, with a 200\u2013250mm aperture telescope, you can even get nice eyepiece views of the faint discs of Uranus and Neptune.<\/p>\n\n

Subtleties in the stars<\/h3>\n\n

Double and variable stars can provide an interesting observing project<\/p>\n\n

\"\"<\/figure><\/div>\n\n

Among the stars that are able to poke through the light pollution in built-up areas you\u2019ll find some interesting targets for a small to medium aperture telescope. These include some of the brighter double and multiple-star systems, where two or more stars either appear close together or are, in fact, whirling together as a gravitationally-bound group through space. In the summer and autumn months, notable examples such as Albireo (Beta (b) Cygni) in Cygnus, and Epsilon (e) Lyrae in Lyra, are comfortably within reach of a 150\u2013200mm aperture telescope from sites with fairly bright night skies.<\/span><\/p>\n\n

If you\u2019re interested in long-term observing projects with a telescope, you\u2019ll also find that some variable stars \u2013 stars whose brightness fluctuates \u2013 like Algol (Beta (b) Persei) in Perseus, often periodically glow strongly enough to be seen through moderate light pollution. Observing them and recording estimates of their magnitudes can be an interesting and rewarding exercise. If you\u2019re keen, you can even get involved with the scientific work of organisations such as the British Astronomical Association and the American Association of Variable Star Observers by submitting measurements to their variable star monitoring programs.<\/span><\/p>\n\n

Ghostly glows on the horizon<\/h3>\n\n

Keep an eye to the north throughout the summer and you might spot a noctilucent cloud display<\/p>\n\n

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Displays of noctilucent clouds are easily visible from cities in the summer months<\/figcaption><\/figure>\n\n

During summer in the UK, noctilucent \u2013 meaning \u2018night-shining\u2019 \u2013 clouds can appear on the northern horizon about an hour and a half after the Sun has set. Composed of ice crystals, these clouds form around 85km up in our atmosphere and shine because, at those heights, they\u2019re in sunlight. The brightest displays can be easily seen and photographed from light-polluted towns and cities against the twilight. Noctilucent clouds, or NLCs, have a characteristic blueishwhite hue to them and so stand out quite clearly from regular high clouds, like<\/span> cirrus, from urban spots. This is because when cirrus is being lit by light pollution it will usually have a slight orangey or greenish-white hue. Wisps of regular high cloud also tend to look quite dull when scattering skyglow, whereas bright NLCs really do look like they\u2019re glowing.<\/p>\n\n

Snap an asteroid<\/h3>\n\n

Several of the brightest inhabitants of the asteroid belt are easy imaging targets from brightly-lit areas. We show you here how to shoot a large target like Vesta<\/p>\n\n

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TIMING IS KEY <\/strong><\/p>\n\n

To be in with a good chance of capturing Vesta on camera you\u2019ll want to image it when it\u2019s around \u2018opposition\u2019, ie when it\u2019s opposite the Sun in the night sky. Typically, objects are near their brightest at this time. BBC <\/em>Sky <\/em>at <\/em>Night <\/em>Magazine\u2019s <\/em>\u2018Sky Guide\u2019 regularly details asteroid oppositions.<\/p>\n\n

GO TO GREAT (FOCAL) LENGTHS <\/strong><\/p>\n\n

A basic tracking mount to carry your camera and a lens will help you take pin-sharp exposures of 30 seconds to a minute in length. Aim to use a lens of at least 50mm in focal length, and preferably longer, as this should be sufficient to pick out larger asteroids, such as Vesta.<\/span><\/p>\n\n

PINPOINT IMAGING <\/strong><\/p>\n\n

Gather many exposures and stack them in software like DeepSkyStacker or similar to create a smoother image, where it\u2019s easier to discern the pinpoint light of an asteroid. If you don\u2019t have a tracking mount, use a static tripod and capture lots of exposures (a few seconds in duration) at a fairly high ISO setting.<\/span><\/p>\n\n

REPEAT TO SEE VESTA MOVE<\/strong><\/p>\n\n

Once you\u2019ve produced an image of the tiny spot of light that is Vesta among a star field, you can repeat the whole process again a few days later. Load the two images into an image editor and flick between them to watch Vesta moving against the background stars.<\/p>\n\n

<\/div>
\n

Metropolitan Moon imaging<\/h4>\n\n\n\n

The bright Moon cuts through skyglow, making it a great target for urban astrophotography<\/p>\n\n\n\n

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A lens with a long focal length can make the Moon appear large as it rises over a city, like in this view of London\u2019s skyline<\/figcaption><\/figure><\/div>\n\n\n\n
NIGHTSCAPES<\/strong><\/h5>\n\n\n\n

Dramatic city horizons and striking buildings make for great foregrounds in nightscape photos containing the Moon. What\u2019s more, this kind of image \u2013 perhaps showing a Moonrise, lunar halo or total lunar eclipse \u2013 can be achieved with just a smartphone camera.<\/span><\/p>\n\n\n\n

With a basic DSLR or bridge camera and a longer lens you\u2019ll be able to push things further, capturing more detail on the lunar disc. Very long focal-length lenses \u2013 above, say, 300mm \u2013 can allow you to get an eye-catching compressed depth-offield effect, making a full Moon appear huge as it rises up over a distant urban skyline.<\/p>\n\n\n\n

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By using a high frame rate camera combined with a telescope and laptop you can obtain stunning images of the lunar surface<\/figcaption><\/figure><\/div>\n\n\n\n
IMAGING WITH A HIGH FRAME RATE CAMERA<\/strong><\/h5>\n\n\n\n

Lunar imaging with a high frame rate device, like a webcam or a dedicated astronomical camera, can be a hugely rewarding pastime and is one of the few types of astrophotography that can be carried out without any substantial loss in quality due to skyglow. To do it you\u2019ll need a small telescope, a camera and a laptop to receive the data. High frame rate imaging works by the camera taking a video of the view through the telescope. Software like the free RegiStax then selects the best frames to stack into one image that is then sharpened and processed further. The results are far superior to what you can achieve with a single shot, from something like a DSLR.<\/p>\n<\/div><\/section>\n\n

To learn more about capturing the heavens from urban environments, sign up for our Masterclass series on astrophotography at skyatnightmagazine.com\/virtual-events<\/a><\/strong><\/p>\n\n

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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
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Will Gater is an astronomy journalist and science presenter. His book, The Mysteries of the Universe, is published by DK<\/p>\n<\/div>\n<\/div>\n\n

Photos: WILL GATER; OVERSNAP\/ISTOCK\/GETTY IMAGES; STEVE MARSH<\/p>\n","protected":false},"excerpt":{"rendered":"

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