The Milky Way

One of the drawbacks of living in Dublin is that if there’s reason to look at the night sky, there’s too much light pollution over the city to be able to make out much but the brightest stars.  No such problems in West Cork, where I was staying last night.  After the annual Perseid meteor shower, which was supposed to peak on Thursday night, I had been looking skyward for the last few evenings in the hope of spotting a “shooting star” but hadn’t spotted one.  When I looked at the sky last night not only did I see a few meteors, but I could also quite clearly see the Milky Way.  And if I could see it, that probably meant I could photograph it.

My first task was to determine if I had a tripod with me and when I found it in the boot of the car I set up the camera with the 24-70mm lens pointing straight up at the sky.  I had two technical issues between me and a decent image.  One was to get the focus right, the other was to get the exposure right.  With these sorted, any possibility of capturing a meteor in a photograph would purely be a matter of luck.

To focus I was assisted by the presence low in the sky of Venus, which was shining brightly enough (you can see it near the bottom of the frame above) to satisfy the camera’s auto-focus.  For the initial photos I didn’t have Venus in the frame, but was still able to use it to focus before switching the camera to manual focus and recomposing.

To get the exposure right I knew from previous experience that I’d need to be thinking wide aperture, high ISO and slow shutter speed.  For a lot of low light photography any two of these will get you to where you want to be, but when photographing space from earth you pretty much always need to go to the limits of all three.  My starting point was f/2.8, ISO 3200, 30 second shutter speed and it proved to be pretty good.  In fact both the image at the top of this post and this image here were taken with those settings.  The difference between the two is that for the top image I switched to my fisheye lens to include a 180 degree sweep of the sky (and Venus) in the frame, with some nearby trees framing the bottom of the image.

f/2.8 @ 30s, ISO 3200, 24-70mm lens at 24mm, Nikon D700

At the top of the previous image you can see what I believe was a meteor that crossed the sky at the edge of the frame during the exposure, although since I posted this on Twitter it suggested that perhaps it was a satellite.  Without knowing the exact part of the sky I was photographing here it’s hard to know, but either way it’s a nice little bonus to capture it.

I did try to use a longer shutter speed to allow me to do two things – add a little bit of light to the exposure and drop the ISO by a stop to reduce noise (though as it turns out even at ISO 3200 I’m impressed with how little noise there is in the final images).  So I dropped the ISO to 1600.  To keep the same exposure would have required a 60s shutter speed, so I decided to add half a stop by opening it for 90 seconds.  This required me to use a remote trigger and to time the shutter speed myself.  My concept of how long one second is must be slightly off, because I ended up with a 96.5 second exposure.

At that shutter speed, the rotation of the earth really comes into play, as you can see from a crop of the frame that resulted:

f/2.8 @ 96.5s, ISO 1600, 24-70mm lens at 66mm, Nikon D700 (crop of image)

You can get pretty impressive images using this idea, but taking it further with exposures of 20 minutes or so – which I could have done with an ISO of 200 and a bit more patience.  Personally I think the effect achieved in the image above is neither here nor there.  Not enough to be particularly impressive, but enough to be a bit distracting.

In hindsight I could have achieved an increase in exposure by switching to a faster lens.  For instance my 50mm f/1.8 lens, wide open, would allowed me to add the stop and a half of exposure without moving beyond the 30 second shutter speed.  Something to try next time the skies are this clear in West Cork.   And perhaps I’ll try an extremely long exposure to get those star trails just right too.