by NGC3314 » Mon Mar 23, 2015 1:30 pm
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Chris Peterson wrote:MarkBour wrote:In today's APOD, why does the dark portion of the moon get darker as one approaches the terminator ?
You need to consider how to make an image like this. The dynamic range of the actual Earthshine lit Moon is much to wide to display on a monitor in a linear fashion. Indeed, it is too wide to effectively capture in a single image with a typical electronic detector. So you need to use images made at several exposures, and composite them using something like an HDR technique. But the images exposed for the dark region will have a large amount of scatter and glare from the massively overexposed lit region, which will largely be near the terminator. So in combining the images, it's necessary to create some sort of gradient mask to reduce that glare. But that mask will also darken the zone just inside the terminator on the dark side. It would be extremely tricky to eliminate this effect entirely.
In other words, what we're seeing is a residual processing artifact.
Indeed, all that. I pretty much got rid of these problems in
this image of the Moon in front of the Pleiades, which includes data from 18 CCD images of various exposure times and center locations. Even so, I had to get some scattering information and fill in some saturated regions near the terminator using an image of the full Moon, warped to compensate for the difference in libration. After all that, it is about 5000x7600 pixels, so it was sort of worth the messing around over a couple of months to get.
[[quote="Chris Peterson"][quote="MarkBour"]In today's APOD, why does the dark portion of the moon get darker as one approaches the terminator ?[/quote]
You need to consider how to make an image like this. The dynamic range of the actual Earthshine lit Moon is much to wide to display on a monitor in a linear fashion. Indeed, it is too wide to effectively capture in a single image with a typical electronic detector. So you need to use images made at several exposures, and composite them using something like an HDR technique. But the images exposed for the dark region will have a large amount of scatter and glare from the massively overexposed lit region, which will largely be near the terminator. So in combining the images, it's necessary to create some sort of gradient mask to reduce that glare. But that mask will also darken the zone just inside the terminator on the dark side. It would be extremely tricky to eliminate this effect entirely.
In other words, what we're seeing is a residual processing artifact.[/quote]
Indeed, all that. I pretty much got rid of these problems in [url=http://astronomy.ua.edu/gifimages/m45.html]this image of the Moon in front of the Pleiades[/url], which includes data from 18 CCD images of various exposure times and center locations. Even so, I had to get some scattering information and fill in some saturated regions near the terminator using an image of the full Moon, warped to compensate for the difference in libration. After all that, it is about 5000x7600 pixels, so it was sort of worth the messing around over a couple of months to get.
[attachment=0]M45Moon.jpg[/attachment]