Shooting for the Stars: Astrotrac TT320X-AG / by Conor Cunningham

I'm a bit of geek, no two ways about it. Science is cool and I reckon I can prove it, too. The proof is the night sky and all it takes to perform the experiment is to lie flat on your back after dark on a clear night (the further you are from city lights, the better). How you end up on your back is up to you but I can attest to  walking out of a pub in Oslo on an icy winter's evening can have one vertical in less than a second. After peeking up for a bit you'll notice satellites streaking across the sky, meoteors before they evaporate entirely or become meteorites (meteoroids in space, meteors in our atmospehere, meotorites once they hit the deck), plenty of stars and if you're lucky some planets, too. Depending on how you look at it, you may be lucky enough to see the moon, too.

Moonie. As seen from Oslo, Norway

But I digress. The point I'm trying to make is that if you take a picture of the stars for less than a few seconds, you probably won't see them in your picture. Take a picture longer than a few seconds and you'll see them, but they probably won't be round. The reason is because the earth spins and the stars (for the all intents and purposes) don't move. That's why we see them going across our sky.

Stopping the Stars

Well, we can't, but we can track them. That's where the Astrotrac TT320X-AG (boring name, great device) comes in. I picked it up when I was in London a few weeks ago at European Astrofest. In short, the Astrotrac allows us to follow the stars in the night sky whilst our shutters are open. By aligning the central axis of the Astrotrac with the Earth's axis so that it can rotate the camera at the same rate as the Earth's rotation, albeit it in the opposite direction (there is a setting for both northern and southern hemispheres).

Last night was the first clear night since the night before, but the night before was the first clear night since around the new year. With that in mind and the forecast gloomy, I loaded up the car and headed into a carpark on the outer edge of Oslo's forests. Unfortunately there were a few too many skiers with headlamps in the area, then some learner drivers arrived with their instructors to use the area for lessons on night driving. Not a good start.

Well, it's only a test. I setup the tripod, mounted the three way head, then the astrotrac, polar aligned and put the camera on its ballhead and started to shoot. The alignment wasn't too bad, but not fantastic either. My biggest problem however was by far and away the light pollution from Oslo which effectively ruled out shooting south or west. North was my best bet so I pointed it north, framed the Plough/Big Dipper. I chose 30 seconds, ISO 800 and f/4 with a 24 - 105 @24mm.

Best viewed in full screen.

The big dipper is on the right just to the right of the streak (a satellite most likely) in the sky

Tips for Shooting the Night Sky

A tripod. No ifs or buts. The sturdier the better. A fast lense will of course help, the more light the better. ISO of between 800 - 1600. The higher you go the more noise you'll get, but the shorter the exposure time the less star trails you'll see (unless of course you use an Astrotrac!). Finally, you will need a remote trigger so that you don't shake the camera when you press the trigger. If you don't have a remote trigger, use a 2 or 10 second delay on the shutter. It's also worth considering mirror lockup (live view works just as well), too, if you really want to keep the vibrations to an absolute minimum.

Finally, for focus, I turn on live view (if you don't have it, don't fret) find the brighest star, point my camera at it, turn on manual focus and then turn on zoom in 10x and focus as accurately as I can. If you don't have live view (which I didn't until recently) you can use a dioptrix or simply your eyes.

Good luck!