Orion’s Sword has a wide brightness range that cannot be captured in a single exposure. To produce my image I had to take three exposures of 19, 44 and 117 seconds. These images were saved as Tiff files and named Orion 1, Orion 2 and Orion 3.
When I reviewed them it was immediately apparent that detail in the brightest areas had been lost and the outer areas of the Orion Nebula did not contain much colour or structure; I had to use Photoshop to bring out these finer details.
The Shadows/Highlights tool is a simple and easy way to make adjustments to the light and dark areas of an image.
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For the shortest exposure image, select Image > Adjustments > Shadows/Highlights and set the shadows to 0% and highlights to 100%.
The 19-second exposure shows the core clearly but none of the extended nebulosity around it. Credit: David Tolliday
This darkens the burnt out areas but leaves the stars bright. Save this image as Orion 0. Next use the longest exposure image (Orion 3), and repeat the process but set shadows to 50% and highlights to 0% (this brings out more detail in the underexposed areas), then save as Orion 4.
The five images have different colour balance and exposure levels, which need to be corrected, but you have to take care not to create a totally black, unnatural sky. This is done by adjusting the image’s black and white points.
Conversely in the 117-second image the nebula is majestic but the core awfully overexposed. Credit: David Tolliday
Select the eyedropper tool and in the menu at the top of the screen set the sample size box to ‘3 by 3 average’. Hold the shift key and click on a dark point in the image to create colour sample point #1.
Now zoom in to the brightest part of the image and repeat to create colour sample point #2.
Click Image > Adjustments > Levels and double click on the ‘set black point’ eyedropper icon.
In the RGB boxes type 20, 20, 20 and click OK; now use the eye dropper tool to click on colour sample point #1.
Balance colour by setting new black and white sample points (circled) in each image. Credit: David Tolliday
Repeat for the ‘set white point’ eyedropper icon, setting the RGB values to 245, 245 and 245, and clicking in colour sample point #2.
This process needs to be repeated for all of the five images, and you need to ensure that the two sample points are in the same position in each one.
Save the individual images at the end of each stage. Using these settings will not give a burnt out or too black an appearance to the sky.
Each image should now have the same exposure range and colour balance but differing amounts of detail.
Combine the best detail from each layer using layer masks, but leave the blend mode unaltered. Credit: David Tolliday
Now we need to blend the five images together while keeping the appropriate detail from each. Open Orion 0 (the darkest image) first, as your background image, then open Orion 1-4 sequentially as new layers above it.
Make sure the alignment of each layer is correct. You will be left with a stack, with the darkest image at the bottom and the brightest image at the top.
You can change the title of each layer by double clicking on it, but this is not really necessary.
Select the Background Layer and Layer 1, with Layer 1 being the active layer. Select Layer > Layer Mask > Reveal All to create a layer mask.
Set the paintbrush tool to colour black, size 20, hardness 0% and opacity 50%, then click in the layer mask (the white box) to make it active.
Paint over the burnt out area of Layer 1 to reveal the darker area of Layer 0 below. Any errors can be painted over by using a white paintbrush.
Repeat this process for the remaining three layers, with the layer blending mode set to Normal. Once you are satisfied that the best selections have been made for all layers, save your file in the PSD format.
Finally click Layer > Flatten Image and crop as required. Make any other minor adjustments – perhaps the dodge and burn tools, or adjust colour saturation –and save the final image with a different file name.
The final processed image, showcasing the huge brightness range within the Orion Nebula. Credit: David Tolliday
David Tolliday won the Astronomy Photographer of the Year 2015 Sir Patrick Moore Award for Best Newcomer with this image. This guide originally appeared in the October 2016 issue of BBC Sky at Night Magazine.