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 | Here are some exposures I made in 2005 of M42. The equipment was a Canon EOS 20Da on a Televue 85 operating at f/5.6. Camera settings were at ISO 1600, fixed white balance at 6500K. The exposures range from 30 to 300 seconds, with the longest ones clearly showing the "fog level" of the background sky. Even though these were taken on a dark night in remote Arizona, the sky is not black, and it shows up in these exposures.
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| JPEG versions of the images
IMG_0981, 30 sec
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IMG_0980, 60 sec
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IMG_0979, 120 sec
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IMG_0978, 300 sec
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| These were converted via the Canon EOS viewer utility to 16-bit linear luminance tiff files. Converting to linear makes the images less interesting visually, but they can now be properly treated mathematically. They were then aligned to each other using RegiStar and cropped so that only image data remained (the intersection of the set, no zero padding).
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| Linear 16-bit, registered, and cropped images
 IMG_0981_RT16L_reg
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IMG_0980_RT16L_reg_cp
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IMG_0979_RT16L_reg_cp
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IMG_0978_RT16L_reg_cp
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| These four frames were then added together, giving weights inversely proportional to their exposure times. Saturated regions and zero regions were rejected. The actual weighting includes terms that minimize noise, attempting to maintain a target signal to noise ratio. The result of the sum is an image of floating point pixel values with a dynamic range larger than any of the contributing frames (in this case 10X).
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