Image Processing
Catch the colours of the winter stars.
Ask a casual observer of the night sky what colour the stars are and they’ll probably say “white”. On closer inspection, however, the stars display a range of colours, from white to blue and orange to red. A close look at many of the
constellations visible during the winter months in the northern hemisphere will reveal stars ranging from blue-white to deep orange in colour. A typical image of these stars will show only the most obvious colour differences, but we can bring out the colours of the brightest ones with some unusual astrophotography techniques.
To bring out the colour of each star we’re going to adjust the focus of the lens over the course of a long exposure. A typical in-focus, short exposure shot of a star will show it as a white point, with perhaps some colour around the edge. This is because the star is a point source of light and this tends to oversaturate the pixels
on the camera sensor, resulting in a white image, with some colour around the edge as surrounding pixels pick up some of the light but not enough to become saturated. By capturing the star out of focus its light is spread over a larger area of the sensor, which avoids oversaturation of each pixel and preserves the colour information.
To capture a large enough image of a star, use a DSLR camera with a lens that has a focal length of at least 200mm. Use a low ISO setting to capture a greater tonal range; 100-400 should produce good results. A tracking mount will keep the star in the centre of the frame as you take the shot and a remote shutter release cable will avoid camera shake.
Ensure that your chosen star is as close to the centre of the frame as possible. If the star is off-centre then you’ll see a deformed final image. Use the ‘bulb’ setting on your camera to control how long the shutter is open for. To get yourself and your kit ready for the shot use one hand to operate the remote shutter release cable and the other to turn the focus ring of the lens.
Slow and gentle
As you’ll be moving it yourself, ensure that the focus setting of the lens is set to ‘manual’. Press the button to open the shutter and start taking the picture. Leave the camera to capture an image of the star for a few seconds. This will give a good exposure of the star to form the centre of the image.
Next, very gently turn the focus ring a little over the course of two to three seconds. It’s important to do this with as little vibration as possible – any movement of the camera will result in an off-centre image, which won’t look as good. Leave the focus at this new point for around 10 seconds. Repeat the process of turning the focus ring and waiting for 10 seconds for around one minute.
After the minute is up release the shutter button to close the shutter. You can then review the image on your camera’s screen and try again if you’re not happy with the result. A good shot should look like the picture to the left.
Now import the images into a graphics program for processing. The first thing to do is remove any hot pixels, dust marks and other blemishes from the star image. Use your program’s healing brush tool to remove these.
Next, use the circular selection tool to select the star image and the boldest three or four of the surrounding colour discs. Add a substantial feather to the selection to ensure that the edge blends well with the background of the final composite image. A setting of around 50 is good here. The aim with this processing is to get a perfect circular image with faded edges and no blemishes within the coloured discs. Do this for each image you’ve captured.
Next, create a blank image with a black background. This should be large enough to contain all of your final star images. Copy and paste each image into a new layer. You can then move each one around to line them up.
Finally, flatten the image for the end result, which in this case, was shortlisted in the Stars and Nebulae category of the Insight Astronomy Photographer of the Year awards 2017.