Apply final color and density adjustments to the composite.Tweak the color of the core to match the rest of the image.Adjust the density and contrast of the mask with curves.Open a new view of the image to see effects of manipulating the mask.Paste the short exposure on top of the long exposure in a layer.Copy the short exposure to the clipboard.The secret to getting the blend perfect is that we will make the mask from the overexposed portion of the long exposure itself, and we will be able watch the changes in the composite as we adjust the mask. Any gray transitions from black to white mask the image in a gradation depending on the density of the mask. Think of the white in the mask as a hole that lets the image on top through. Any light or white areas are unmasked and all of the image on top gets through. Where the mask is totally black, no portion of the image on top gets through. The dark portion of the mask protects the image under it. The mask will block out everything on the base image except for the overexposed area in the long exposure and allow the correctly exposed core of the Trapezium from the short exposure to blend in perfectly. In this example, the long exposure will become the base image and the short exposure will become the "layer" on top of it. Like just about everything else in Photoshop, there is more than one way to composite two images together to combine shadow and highlight detail. Short exposures can record detail in the brightest areas without overexposure, but obviously will record nothing in the faint areas that require an exposure to the sky fog limit. In a long exposure that is made to capture the faintest details, the bright core will be so overexposed it will have no detail as in the 50 minute exposure. At right is a composite of the two exposures. At center is a single 50 minute exposure with my Astro-Physics 130 EDT at f/8 with gas-hypersensitized Kodak PJM, which, at my site, is the sky-fog limit. Yet it also contains beautiful nebulosity in the faint outlying areas of the loop to the south of the Trapezium, seen here at the top of the long exposure.Īt left above is a single 5 minute exposure which records detail in the bright core of the Trapezium region. Its core is extremely bright, which is why we can see so much detail, and even some color visually. This problem is commonly encountered on objects like M42, the Orion Nebula.
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