Under a Starry Sky

St Martha Under a Starry Sky (0257)Last weekend, as you may already be aware, was one of considerable interest to stargazers. This year’s Perseids meteor shower coincided with some reasonable weather and almost-moonless nights. So I knew I had to plan to spend some time under the stars. With some luck I would get to see a few meteors and try my hand at long exposure photography.

So as the sun was close to setting, Andy and I walked up to St Martha-on-the-Hill from Newlands Corner and set up cameras. I knew Perseus would rise roughly East/North-East, so I found a composition facing that general direction. Then the experimentation started. I was very surprised how much light would still show up in the images, when my eyes were simply seeing darkness. I mean, I knew things would show up – after all it’s only a matter of sufficiently increasing the time the shutter is open to allow enough light to be captured. What I did not expect was what I thought was a beautiful soft quality of light.

Anyway, the first image I wanted to share, above, was taken with dusk approaching. There was still enough light for the sky to show up blue, while dark enough for the stars to peek through. The image was taken as a single exposure with the shutter open for two minutes. With this duration, the stars move enough for their trail to start showing, which is why they look like short lines in the image. The long diagonal line is the trail of a flight. Processing involved the usual selective curves to adjust contrast, and some extra sharpening in the sky to accentuate the stars. I also took the liberty of editing out a few low-level flight trails which I thought were really distracting.

It is usually good practice with long exposure work to enable the in-camera dark-frame subtraction. What this does, after a long exposure, is to take another exposure of the same duration with the shutter closed (the so-called dark frame). This determines the dark current in the various pixel locations, and does wonders to remove hot pixels and fixed pattern noise. However, this time I was planning to take some longer exposures for stacking, and since I did not want any gaps between frames, I had to leave the dark-frame subtraction disabled. Unfortunately this meant that my images had quite a few hot pixels (more for longer exposures of course). Rather than editing these out manually, I put my knowledge to good use: at home, I manually took an exposure with the body cap on (no lens) at the same ISO and duration as this image. Thresholding this at a sufficiently low level revealed the hot pixels (not too low or you just pick up stochastic noise). I then selected white pixels in this image, expanded the selection by a couple pixels’ radius, and applied a median filter across this selection. Result: hot pixels gone.

Click on the image for a larger version – this is really necessary to see the detail, as the rescaling done by the web server softens everything. As usual, comments welcome.

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3 comments

    • Thanks Mark, glad you liked it! Hope you find the dark frame subtraction useful too – probably only really needed in longer exposures, and even then it depends on the specific camera.

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