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{ Practical astronomy | Astronomy | The Moon | Albedo }


images of and brightness cuts through a lump of coal and the Moon
The Moon and a lump of coal, 2010-10-30.

After the Sun, the Moon is the brightest object in the sky. Astronomical observatories divide their observing programmes into those that can tolerate a moonlit night sky and those that require a moon-less sky. But is the Moon really that bright? Its "albedo" – the fraction of incident light that it reflects – is in fact similar to that of coal, roughly 10%.

The montage of images shows on the left a lump of coal perched on a wall and in silhouette against the blue sky. On the right is the Moon at the same time in the daytime sky. Both images have the same exposure, and are raw data with dark and bias signals subtracted. In the intensity plots below the images, zero then means no light. We can see that the lump of coal reflects at a brightness of 3500 to 3800 intensity units. For the Moon, we have to subtract the daytime sky, as it is located between the camera and the Moon and adds to the lunar light. We measure about 1000 in the dark maria and 3000 in the bright highlands near the limb. If anything, the Moon appears to be even darker than coal. But closer to Full Moon it would probably be similar to coal.

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With these images, we carry out a crude form of photometry, measuring the brightness of things. It is important to use an image format that stores numbers for each pixel that are proportional to the amount of light received by the pixel. The raw format of dSLR cameras usually does this. The JPG format from any camera most definitely does not do this. In the case here, it is also important to have the correct zero level. While the shutter is open and the detector collects light, it also collects "dark current" – electrons that are collected whether there is light or not. Even before the exposure begins, it already has a "bias" charge of electrons. In addition to the image of the object itself, we must take a dark/bias image and subtract it from the image proper. A dark/bias image can be taken simply by repeating the exposure with the lid on the lens so that the detector is in darkness. When working with raw images, it is best to convert these to floating-point FITS format (separately for the RGB colour channels or only for the G channel when not interested in colour).

Image parameters: