Taking pictures of the night sky

I recently moved from a flat in Kingswood to a house in Fishponds, on the very outskirts of Bristol. It overlooks the massive Oldbury Court Estate, which shortly gives way to the countryside. Compared with Kingswood, there is hardly any visible light pollution here – at least to my urban-accustomed eyes. Back in Kingswood the sky was as orange as Fanta. Here it seems a much more natural blue/black.

Since moving, I’ve been waiting for a clear night so I can point my camera at the sky and take in a wide view of billions of stars (as opposed to regular astronomy, where the photographer/observer focuses one one object). Last night presented just that opportunity, so I whipped out my 450D and fitted my 50mm f/1.8 prime lens.

I quickly found out that astronomic photography is a balancing act and a series of compromises. In short, the thing you are pointing the camera at is very dark, so you need to collect a lot of light to make a decent picture. There are three real ways of doing this (and one way of cheating).

  1. Expose for longer. The trouble is, stars move across the sky. Exposing for 30 seconds, you see some blurring that just looks like poor focus. Exposing for 60 or 90 seconds, you can clearly see that the stars are all moving and have blurred tails. So in practice we have to keep the shutter speed faster than at least 30 seconds.
  2. Open the aperture wider. The trouble is, the wider the aperture, the worse the sharpness. When taking photos of fine points of light, f/1.8 just doesn’t cut it (unless you have a fantastic lens). You need to stop down to at least f/8 to see the sharpness of the stars come through.
  3. Increase your camera’s sensitivity (by increasing the ISO of your sensor or film). This effectively means you collect more light, so you can shorten your exposure, or decrease the size of your aperture. The trouble is, higher ISO means higher noise or grain in your picture. ISO1600 on your DSLR sounds great, until you see the results and find out it’s a load of speckle.
  4. Fix it in Photoshop. This has the same fundamental effect as increasing the ISO, although you can be a bit more intelligent about noise reduction. I’m no whizz at Photoshop myself, and in any case I prefer to get it right in the camera.

So, we have to find a suitable compromise. The best combination I came up with was a 20-second exposure at f/6.3, and ISO400. Then I briefly altered the curve in GIMP to keep the noise down in the paler areas of the sky. The result was a pretty mediocre photo. The stars were a bit blurred (motion blur) and a bit blurred (poor sharpness) while the background was also a bit noisy. I somehow don’t think it will feature on the cover of New Scientist any time soon.

These are the two best photos out of about two dozen that I shot. It doesn’t sound much, but bear in mind that each one was exposed for anywhere between 30 and 90 seconds! I’m sure that converting them to JPEG and resizing down to this size has done them no favours at all, but never mind. In the first photo, the shutter speed is too long (90 seconds) and the stars are blurred, while in the second one I shot at ISO400 and played with the result in GIMP, so the noise is bad instead.

Stars flying by
The night sky

Some of these issues can be worked around. I’m using a standard Canon f/1.8 50mm prime lens. If I had some more pocket money, I could upgrade to an L-series lens (for example) that would probably offer better sharpness at wider apertures. I’m also using a decidedly consumerish EOS 450D. I am a consumer, after all, but an EOS 1Ds mkIII with a full-frame sensor would offer lower noise at higher ISOs. I’m not sure what can be done about the movement of the stars across the sky though, regardless of personal wealth. I suppose I could buy a sky-tracking tripod like they use for real observatories.

Next time It’s clear I’m going to try approaching this from a different angle. I’ll keep the ISO low at ISO100, and the aperture small at f/16 perhaps. To compensate I’ll expose for a “while” – perhaps five minutes. This should give me a low-noise, sharp photo of some extremely blurred stars. It’ll be like the world is spinning, and I hope it will look interesting. Watch this space*!

Finally, coming back to the issue of light pollution from sodium street lights – despite it seeming far better here in Fishponds, the images from the camera still seemed horrifically orange. I can get around this by either adjusting the colour balance, or by simply converting the images to black and white. I suppose I could also solve this one with money, by buying a house in the countryside 😉

* Actually, watch my photo blog. I’ve said it before but I’ll say it again – this blog is for nerdy discussions about photography, while my photo blog is for showcasing the pictures that actually turned out OK.

3 thoughts on “Taking pictures of the night sky

  1. Take, say, 100 10-second exposures at high sensitivity, find the 2-d cross-correlation function between each pair of consecutive images (assuming you have nothing else in the FOV, or if so then mask it out), and shift and co-add them? If the noise is additive and normally distributed you can continue ad nauseum–the SNR is proportional to sqrt(N) where N is the number of integrations.


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