Wednesday, 19 August 2015

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.