Stars in the city: an introduction to urban astrophotography

As a twelve year old astronomy nut, I was lucky enough to receive a small refracting telescope. Almost immediately, I utilised scrap timber to build an observatory in my back garden, just about large enough for two children (plus star charts, a moon map and at least as important in my opinion, a flask of hot chocolate). I recall it even had a sliding roof, thanks to a pair of dismantled wardrobe doors.

Although the imaging wasn't too bad - I lived in a small town, so light pollution was relatively low - I soon discovered that good optics are only part of the story: without a proper mount, a telescope can be next to useless. In this particular case, I obviously hadn't read the brief introduction to mounts in my trusty The Observer's Book of Astronomy by Patrick Moore. At any rate, I clearly didn't understand the difference between proper equatorial or alt-azimuth mounts and the piece of junk that allowed my refractor to sit on a table top. Therefore, except for getting to know the lunar landscape, I saw little that I couldn't more easily view with my 20x50 binoculars.

Jump forward thirty or so years and courtesy of a large tax refund I found myself in possession of a small reflector, complete with equatorial mount and right ascension motor. After some months getting to know it I started buying accessories, aiming to learn the ins and outs of astrophotography. Thanks to numerous websites I picked up some useful techniques and excellent free software - and as importantly, how to use the assemblage - and now feel it's about time I offered a one-stop-shop guide to getting the best images on a low budget in your own backyard. Of course there are plenty of books available, but most are at least one to two hundred pages long and often specify expensive kit, so this post is an attempt to cover the gap for those wanting an astrophotography 101 with the absolute minimum of basic equipment. Of course, it's entirely my approach, so there are no doubt plenty of other tutorials out there. But at least mine's short!

1. Equipment

I have to admit that I order all my kit from overseas, since New Zealand has few astronomy retailers and those there are appear to have a fairly limited range, often at uncompetitive prices. However, it is possible to accumulate a decent beginner's assortment for around a NZ$1000 / £500. I would always recommend a reflector as a first telescope, being far cheaper than a refractor with similar capability. The Newtonian is the most common, least expensive and easiest to maintain type of reflector, mine being a Sky-Watcher 130. As per the name, the primary mirror is 130mm (about five and a half inches in old money), which is really the minimum useful size for a reflector.

The telescope came with a red dot finder scope, several okay-ish eyepieces, a right ascension motor drive, a poor 2x Barlows and a reasonably stable equatorial mount. Since then I've bought a planetary camera, a good quality 2.5x Barlows, a compact camera adaptor, an adjustable polarising filter and a collimating eyepiece*. I've also made my own Bahtinov mask, courtesy of a website that supplies patterns for various diameter/focal length combinations. Although 'go-to' mounts are available, I agree with the general consensus that the best way to learn the night sky is by manually pointing the telescope, not just programming a target and letting the telescope slew into position for you.

*For complete newbies, a Barlows is a cheap method for increasing magnification with only a limited number of eyepieces, fitting into the eyepiece holder below the eyepiece. A collimator is used to check and correct misalignment between the primary and secondary mirrors, whilst a Bahtinov mask is a simple focussing aid.

I'm lucky to live in the 'winterless north' of New Zealand, but for those in colder climates it's probably wise to make or purchase a dew cap, or rather one for the main tube and another for the finder scope. A rubber eyecup for the eyepiece might also be a good idea; there's not much point in trying to observe anything if water is condensing on the mirrors or lenses.

I would recommend a CCD or CMOS telescope camera or modified webcam, since they are a lot cheaper than a digital SLR and far lighter. The EQ2 mount supplied with the Sky-Watcher needs adjusting on both axis depending on the combination of items in the eyepiece holder, otherwise at high angles it has a tendency to droop. The EQ2 counterweight can just about handle the long tube: experiments with a compact digital camera in a purpose-built mount have confirmed that additional off-centre mass requires regular fine-tuning to retain balance. Incidentally, I use a colour planetary camera since I tend to have short sessions - around two hours - and so only want to film each pass once rather than repeating in triplicate for colour filters, even if mono cameras achieve better resolution.

2. Where to observe?

Of course this is the least flexible part of astrophotography, since you are restricted by the buildings and trees in your garden - or any other convenient location. Not only is your view of the night sky limited by physical obstructions but pollution can severely impact viewing. As I have discussed previously, light pollution is the most obvious form, with street lighting often worse than that of buildings. I've found that even as low as ten percent cloud cover can degrade astrophotography, due to the artificial light reflecting off the clouds.

Heat pollution may be less obvious but can also severely reduce image quality. Therefore, try to avoid pointing the telescope directly above nearby rooftops or you will be looking through a rising column of hot air, either the radiating heat from earlier that day or leaking from poorly-insulated buildings that are heated at night. Also, never set the telescope up indoors and point it through an open window: the thermal variations will generate shimmering galore. Wind above the lightest of breezes cannot be recommended either, not just for 'scope instability but also because dust and particulates can deteriorate the viewing. High water vapour content is bad for the same reason; here in humid Auckland I'm frustrated by the hours before and after rain, meaning the best seeing I've ever had has been in high summer after a rain-free week.

Before using a reflecting telescope, it needs to be set up outdoors well in advance of the viewing session in order to allow the mirror to cool down to the ambient temperature. The cooling time is directly proportional to the primary mirror diameter, which for my 130mm is usually about one hour.

3. What to photograph?

For urban astrophotography I've found the moon and planets to be by far the best targets. By planets I mean just Mars, Jupiter and Saturn. Venus may be both large and bright but due to its cloud cover will never present anything other than a featureless crescent or globe.

The moon is endlessly fascinating, best observed between new moon and first or last quarter (i.e. half full). During these periods, the low-angle sunlight generates shadows that model the features without being overly bright. When observing closer to full moon I always use a polarising filter to reduce the incredibly intense light, but since sunlight is then perpendicular there is little modelling to give relief to the geology.

Jupiter is by far the best planetary target for small telescopes; in addition to the cloud patterns you can see some or all of its four largest moons (Ganymede, Callisto, Europa and Io), their number and position changing on a nightly basis. Saturn is an excellent target too, the angle of the rings varying widely. I've also found Mars to be surprisingly worthwhile even when not at its closest to Earth, with the major features clearly visible in reasonable seeing conditions.

The problem with deep sky objects in urban astronomy is that they are both difficult to locate and their light is easily degraded by light pollution and particulates. I've attempted to get images of more familiar DSOs such as the Orion Nebula with several cameras, but the results are hopeless.

Once you have some experience under your belt, you may want to attempt photographing the International Space Station. Various websites list details for near-future visible passes over any location, when it is easy to spot due to being obviously brighter than any other man-made orbiting object. However, since the ISS will only be visible for around four minutes each pass you have to quickly manoeuver the telescope whilst keeping it in an area that is only about thirty arc seconds in diameter. If I manage to get any image at all, it is usually a few dozen frames resembling an out of focus capital 'H', so it's definitely a target for those with a lot of patience - and good hand-eye co-ordination.

4. Locating targets

Although I'm against beginners using go-to mounts, there are various planetarium programs and mobile apps that are extremely convenient for locating target objects. I use Stellarium, excellent freeware that can be set to any location on Earth and has a night time (i.e. red on black) mode to help keep your eyes sensitive to the dark.

Northern Hemisphere observers are at an advantage compared to their counterparts south of the equator due to the ease with which the North Celestial Pole can be found. Not only is Sigma Octantis slightly further from the SCP than Polaris is from the NCP, it is considerably dimmer. Therefore I've always had great difficulty in lining up the telescope to the South Celestial Pole for setting circles with the polar axis motor drive. There are telescope-camera combinations that allow use of auto guiding software but I prefer the manual approach to finding your way around the night sky. Besides which, spotting the closer planets is pretty easy, the most common potential mix-up being Mars with the red star Antares (whose name after all means 'equal to Mars')! All in all, manually slewing the telescope using a printed or online star chart as a guide is the best way to learn.

5. Harvesting ancient light

I tend to take 20-60 seconds of video or still sequences when imaging the moon and planets, depending on various factors such as target brightness and seeing conditions. Planetary cameras allow some manual adjustments such as exposure length and gain, with shorter exposure lengths usually better so as to minimise degradation within a single image. When the seeing is reasonable I stack the planetary camera on top of the 2.5x Barlows, which gives a decent angular size for the planets. I've also used a compact CCD camera with an eyepiece and Barlows combination, but the camera adaptor is fiddly to align on three axis with the eyepiece and the extra weight can mean regular adjustments to the mount, depending on telescope angle.

6. Image processing

Once you have the raw video or sequence of stills there is a lot that can be done to improve the image quality, initially by aligning and stacking the best individual frames and discarding the rest. Again, there is a lot of freeware available to help with this. I use RegiStax, often creating 3 or 4 permutations from each sequence and then loading the best one in Photoshop for final tweaks. (If you cannot afford the latter, then GIMP - GNU Image Manipulation Program - is a great freeware alternative.) It can take a while to understand how to use the likes of RegiStax, but there are YouTube tutorials covering various processes and I always consider a trial and error approach to be a good way to learn!

So what sort of results can you expect from all this effort? The biggest factor in quality is undoubtedly the seeing conditions, which are outside of your control. However, just occasionally you get a perfect night. I find that it can take a few sessions to generate a half-decent image, so it definitely takes perseverance.  Since a picture is worth a thousand words, you can judge the results for yourself here.

Dark skies vs. light pollution: trying to keep in touch with the cosmos

A few minutes after witnessing the recent solar eclipse - reaching an 87% maximum here in Auckland, New Zealand - I was astonished to overhear the account director of an international advertising agency disparagingly state that all the people on the streets he had just seen staring at the sky would probably have been eaten by dinosaurs a few thousand years ago. I was so shocked by his lack of wonder (and this from the representative of an agency that claims to appeal to the heart as much as to head) that I couldn't even bring myself to ask if he was a creationist, considering his evolutionary timescale differed by approximately sixty-five million years from the scientifically accepted one. As much as the impression he gave of being a follower of the Sarah Palin school of history, what really got to me was his lack of wonder: have many first-worlders become so surrounded by electronic gizmos that they are immune to the marvels of nature?

One of the great natural sights anyone can enjoy is the night sky, but with more than 50% of the human race now living in conurbations we are rapidly cutting ourselves off from view that helped inspire our earliest mythologies. Could an argument be made that as our ability to observe the rest of creation declines, so does our ability to awe? Although my home city of Auckland is less light polluted than my last place of residence, London, a brief visit to rural Queensland, Australia earlier in the year reminded me just how much us city dwellers are missing: for example, Mars really is an angry 'Bringer of War' red whilst the Milky Way does seem like a river cutting through the sky. I also recall that once during a holiday in Cyprus I saw an extremely bright, eerie glow radiating from behind hills near our rural villa, only for the full moon to eventually rise as the source of the light.

Although Auckland isn't bad by the standards of some cities - it's dark enough in my back garden for even a half full (First Quarter) moon to cast strong shadows - the view directly west, currently home to interesting sights such as Mars, Mercury and Venus, is pretty much ruined by the stadium lights used in the local shopping centre car park, which remain on until very late. In addition, there is enough general light pollution from buildings and to a lesser extent street lamps that even a modicum of cloud is enough to reflect a diffuse glow and severely impact astronomical 'seeing'.

The crater Copernicus, as seen from Auckland via a Skywatcher 130P reflector using a QHY5V camera.

With numerous forms of contamination now known to be causing environmental degradation it's hard to see where support can be garnered for this most poetic of forms of man-made pollution. After all, astronomers are hardly an endangered species and with professionals able to use the likes of the Very Large Telescope in Chile and plans afoot for the European Extremely Large Telescope to be operational by 2022 things are looking up for the discipline (an old astronomical pun, if you weren't aware). But as for us city-bound amateurs, we're stuck with poor viewing conditions thanks to all the artificial lighting, never mind the turbulence caused by heat radiating from asphalt and the like.

Research suggests that the USA alone loses billions of dollars per year on night lighting commercial and corporate premises. So why are shops lit up outside of opening hours: to advertise the company logo and wares for any passing punter, just on the off-chance it triggers a bell in the consumer's head? And what about office buildings? Since movement detectors have been installed in most office tower blocks I've worked in, why do companies still feel the need to have dozens of floors lit up like a Christmas tree? The USA currently imports over 20% of its energy so wouldn't make sense to for the largest consumers to cut down on usage rather than become increasingly beholden to other nations? The lifespan of most fossil fuels deposits is now understood and makes for grim reading, especially with regards to oil. European energy reserves for example are known to be extremely low, so how can non-practical nocturnal lighting be anything other than ridiculous?

And then there are street lights, which in most designs seem to radiate light in all directions. There are models that aim their light where it's needed, i.e. downwards, but the vast majority just aim their beam everywhere. I'm assuming that any street light that channels its light downwards in a tighter beam can utilise lower power bulbs than other styles but even with the obvious eventual power savings I can't see much chance of upgrades en masse; there are an estimated 35 million street lamps in the USA alone, so conversion wouldn't be an overnight process. What about tasking local authorities with switching to more efficient models as and when individual lamps require replacements? You would have thought any opportunity to save energy would be a basic tenet of legislation by now. Or is there a naïve belief that science will come up with a miracle solution in our darkest hour? Personally I'm not sure that nuclear fusion is going to be ready any time soon!

So apart from annoying amateur astronomers there are several strong arguments in favour of reduced nocturnal light pollution. A biological rather than economical one has been suggested by several studies investigating the effects strong nocturnal light levels may have on human health, such as reduced melatonin levels. In addition, various types of wildlife from hatching amphibians to migrating birds are affected by artificial night lighting, and as we are becoming increasingly aware, one small change in the ecosystem can rapidly cause a chain reaction up the food pyramid. As if these problems weren't enough, another issue that seems to have garnered minimal media attention is that artificial lighting at night may break down the nitrate radical NO3, which would otherwise help to neutralise other, smog-contributing, nitrogen oxides. All in all, there seems to be very few areas of concern to humanity that are not affected by nocturnal light pollution. By comparison, the inconvenience to us amateur astronomers seems like small fry!

However, it is not entirely doom and gloom. The International Dark-sky Association (IDA) was formed in 1988 to fight light pollution at a grass roots level and has put together information packs as well as organising the International Dark Sky Places programme. There are to date nearly twenty parks and reserves around the world that have qualified for this status, the majority to be found in Canada and the USA. The largest however is the Aoraki Mackenzie International Dark Sky Reserve on South Island, New Zealand, so I intend to get down there at some point in the next few years...

Another campaign that relies upon public participation is the GLOBE at Night programme, which has collated nocturnal light pollution levels using data supplied by volunteers from 115 countries. It has a family-friendly website with items in up to 14 languages, so for any parents looking to involve their children in an important global experiment, this is the place to go. It even includes instructions on making that essential tool for all night-time observations, a red light, so that you can view documentation without ruining your night vision sensitivity. Incidentally, I know the problems of ruined night vision all too well, since although the superb planetarium freeware I use has a night mode, my telescope camera software does not; I suppose I'll just have to find somewhere that sells sheets of red gel to tape over the laptop screen.

It would appear that the ever-increasing difficulty of viewing at first hand the stars, planets, nebulae and everything else that makes the observable universe is just the tip of the iceberg when it comes to the problems caused by too much artificial lighting at night. But thanks to the likes of the IDA and GLOBE at Night programmes there is now an opportunity for anyone to get involved, both to promote conservation of energy and our fragile ecosystem whilst preserving something of the wonders that previous generations took for granted. As the physicist Brian Greene, author of The Elegant Universe (that's the book with all the tricky stuff about Calabi-Yau spaces) puts it: "I have long thought that anyone who does not...gaze up and see the wonder and glory of a dark night sky filled with countless stars loses a sense of their fundamental connectedness to the universe. And as the astounding vastness of the universe becomes obscured, there is a throwback to a vision of a universe that essentially amounts to earth, or one's country, or state or city. Perspective becomes myopic. But a clear night sky...allows anyone to soar in mind and imagination to the farthest reaches of an enormous universe in which we are but a speck. And there is nothing more exhilarating and humbling than that."

If that's not a call to action, I don’t know what is: come on Brian Cox, please get the ball rolling!