Tuesday, May 27, 2008

High Dynamic Range Moon

The eye and brain has an amazing ability to be able to perceive things that are intrinsically very bright and also very faint at the same time - and in astronomy this is most evident as one looks at the crescent Moon.

Once the sky darkens when the Moon is just a few days old and sinking fast after the setting sun, one can easily glimpse the phenomena that is Earth Shine. This is where the light from our star, the Sun, is reflected off the still sunlit parts of the Earth's surface, and is reflected back out towards the Moon. It then reflects for a second time back to the Earth whereby we can juts perceive it as a ghostly image of the lunar surface. The effect is most pronounced early and late in the Moons cycle as the bright Sun lit crescent is small compared to the faint Earth lit globe.

Note, you must have your monitor correctly calibrated to see this properly, otherwise it might all look a little bit dark. Try this website to get a rough setup.


Digital cameras are not as good as the eye in this respect. Take a picture of the Moon and you'll find that you cannot correctly expose both the darker Earth lit surface and the bright Sun lit crescent at the same time. Why is this? Well, a digital camera has what we call a fixed dynamic range. That is, it has a finite range of brightnesses that it can capture in one go, and it's generally not as good the the Good 'Ole Eye!

To get around this problem, one can take multiple images at different exposures and use a photo package such as Photoshop to blend them together to create what has been coined a "High Dynamic Range" (or HDR) image. I won't go into detail here about how I did it, just do a Google for HDR blending. In my case, I took exposures from 0.25 to 30 seconds through my ED80 'scope and blended them together. The result was to capture both the brightest part of the Mo
on (magnitude -7.2 according to calsky) to the faintest star just to the top left of the Moon at magnitude 10.1 (according to Sky Charts). Quite some range!

Here's a tighter crop of the Moon showing more of the details of the "shadowed" Maria - note that it looks just like a filtered down full moon as the light source is, for all intents and purposes, coming from the same angle as that for a full moon - albeit with somewhat less illumination. Easily visible are the major Maria, Tycho with its outspread rays, Copernicus, and the "bright" Aristarchus near the far left limb.

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