If there’s one step that I personally am guilty of skimping on, it’s this one. I generally have an idea of what I’d like to capture on a given night, but often get distracted by all the other lovely possibilities in the sky, and star simply snapping 20 or so frames of random objects. Although this has led to a few happy successes, for the most part I end up with mediocre images. Now there’s nothing intrinsically wrong with this; if nothing else, it’s good practice at imaging a variety of objects, and helps me to hone my craft. But if you want your images on the covers of magazines, you need to take the time and effort to get as much of the best quality data you can manage. And that requires careful planning.
You see, each object is different, and requires a different configuration. Planets are small and bright, so you might want to boost the focal length of your telescope with a barlow lens and snap hundreds or thousands of frames at very short exposure lengths. Emission nebulae can be quite large and faint, so you might choose to replace the barlow lens with a focal reducer and shoot very long exposures. For any combination of equipment configuration and viewing conditions, you’re going to need to capture a full set of calibration images (darks and flats), and that takes time to do properly. If you decide to swap targets, you’re either going to have to go with your existing configuration and shoot with the wrong settings, or change the configuration and have to start calibration all over. One option gives you less than perfect images, the other wastes a large slice of your evening. So lesson one from this section has to be “Chose which single object you’re going to capture, and stick with that decision”.
When to observe
Having chosen an object, you need to choose a date. As any astronomer knows, all the stars, galaxies and other objects on the celestial sphere rotate around the Earth once every 23 hours and 56 minutes. This means that every objects rises four minutes earlier each night. Or to put it another way, if you look for that object every single night at the exact same time, you’ll find it has moved East by four minutes of Right Ascension every night. Over the course of a full year, those minutes add up to one full rotation around the sky, which is why some constellations are visible in Summer, and others only in Winter. So if you want to image, say, the great nebula in Orion, but it’s the middle of June, then you’re fresh out of luck. It won’t be visible in the evening skies for many months. Therefore: choose a date.
When I studied observational astronomy at the University of South Africa, we were taught to calculate the Sidereal time, and compare that to an object’s Right Ascension to determine whether it would be above the horizon or not. I was never particularly good at this, so I prefer to simply fire up my copy of Stellarium, search for the object, set the time to somewhere around 9pm (when I’m most likely to observe), and then advance the date till my object is nice and high above the horizon. Specifically the North or Western horizons, because my observing site has bad sky glow to the South and East, from neighbouring towns. However you do this, remember to think about rising and setting times, and learn to modify these to suit your local environment. If you’re intending to spend 6 hours imaging (not unusual for the serious guys), that means you have to start more than six hours before the object sets. I’ve made this mistake before, and it’s extremely frustrating to realise, half way through an imaging session, that you’re going to have to pack up because your target is about to vanish behind a distant hill top. Or the roof of your own house.
Stalking your target
If you have a nice computerised mount, with a good database of objects, and if the electronics haven’t failed, then you can skip this section. Just enter the name of the object into the hand controller and enjoy the electronic grinding noises while your telescope robotically heaves itself into position. But if you favour a more hands-on approach, and are imaging anything that you can’t easily see with the naked eye, you’ll need some star charts. If you’re shooting a widefield view, with just standard camera lenses, then the sorts of finder charts one finds in magazines will be fine since you dont have to aim too precisely. But to find a faint object through a telescopes narrow field of view, without electronic assistance, the only way to get around is by using a technique called Star Hopping.
Star hopping is simply a matter of pointing the telescope at some bright, easily found object, and then hopping from star to star till you reach your destination. It takes a little practice, though, and a lot of preparation. You’ll need to know the limiting magnitude of your finderscope (that’s the faintest stars that it can see), and the size of its field of view. Start with a chart that shows stars down to the limiting magnitude of your finderscope, and find the target object. Then, find some bright object nearby that you can use as a starting point. This bright object has to be something that you can find with your naked eye, and get into view through the finder without too much effort, otherwise you’re going to have trouble getting started. Then you simply plot a course from star to star, leading to your target. Remember that no hop should be further than the field of view of your finderscope, else you might get lost trying to make those big leaps. And similarly, the stars between hops need to be bright enough that you can see them without straining. If you want to go old school, use a piece of tracing paper over a printed starchart to draw your own finder chart, showing the route. Oh and… remember that telescope views are usually upside down, or mirrored left-to-right, or both, so you’ll have to turn the completed chart appropriately to match your view. Alternatively, use a software package like Cartes du Ciel (recently renamed to Sky Map, after the French author realised how many english speakers use his excellent software) to generate these charts on a computer.
Once you’ve got all this done, and you’re out in the dark with the target acquired, you’ll still need to take a bit more time to take test shots and ensure that it is nicely centered on the camera sensor. Once you’re satisfied that it’s exactly where you want it in your test shots, and you’ve double-checked that everything is still sharply focussed, then you can mark the target as “Acquired”, and move on to actually taking some pictures.
There are always outside factors that you can’t do much about, such as that the weather, or light pollution, or an unsympathetic spouse. But you can usually find ways to plan around these obstacles. Many individual sources of light pollution are turned off late at night, and if this is the case in your neighbourhood, you can try imaging in the early morning instead of late evening. Or you could plan a weekend away in a nice dark spot away from the city lights. You can deal with weather by keeping a careful eye on weather reports and getting to know your local climate. Where I live, we have consistently dry and cloudless winter evenings, making for excellent viewing, but Summer is humid and rainy and cloudy. I know it’s not worth planning more than a day or two ahead in Summer because I cannot rely on the weather, so I worry less and play more. Winter, by contrast, is for serious viewing campaigns, because I know that I can image through the night and then leave my gear set up outside to continue the following evening without worrying about rain or dew. And if your spouse is unhappy at losing you to the stars yet again, try to get him or her involved so they can join you outside, or perhaps find the time to give them extra undivided attention inbetween your viewing sessions.
Your equipment can and will find interesting and totally original ways to interrupt your imaging sessions. Whether it’s as embarassing as a simple flat battery that you forgot to replace, a forgotten finderscope that you forgot to pack before a weekend excursion, a data cable that didn’t have enough slack and is now being dragged from its socket by your drive motors, equipment failure is frustrating and embarassing and usually fatal to your plans. I’ve experienced every one of these examples at least once, and have learned from them, but youe best bet here is to draw up checklists. If you know that your camera battery will last for 300 shots, make sure that you have a spare, and that they’re both fully charged before heading out to capture 400 frames, for example. The more time you spend thinking about ways things can fail, and the more actual failures you’ve lived through, the longer your list will get and the safer you’ll be.
But it’s not just the machinery you need to look after. Are you dressed warmly enough? Are you going to be out late enough to need some coffee to keep you awake? Are you going to get hungry? Do you have a table for your laptop, and something to sit on, or will you need to squat on the dew-soaked ground for hours, balancing a laptop on your knees? These considerations all seem minor when the Sun is busy setting and you’re impatient to get started. They don’t seem minor at all when it’s past midnight and you’re looking at a few more hours before you can head back to shelter. These little details are not luxuries!
Of course, you can get by without following any of the advice on this page. Many forms of astrophotography can be done quite quickly, with no preparation at all. Eyepiece projection especially is a spur-of-the-moment thing, whipping out a cameraphone and pressing it to the eyepiece of a telescope. But if you want to start taking serious images that will impress other photographers, if you’re spending money on serious equipment and want your results to be worth it, you need to take the time to plan each shot carefully.