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August 29, 2007, HiRISE release
dvandorn
post Sep 19 2007, 05:51 AM
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You obviously observe vertical and near-vertical surfaces from a slant angle. Now, as good planetary imaging afficienados, we tend to think of getting the greatest resolution, and the greatest resolution (and most useful angle for projecting our images onto DEMs or using them for mapping) is from directly above.

But especially in the game of reconaissance spacecraft, what you want to look at might not pass directly underneath the resource(s) you have in place. So, you end up working a series of compromises; do you want 15-cm resolution of the ground 112 miles away from what you really need to look at, or do you want 23-cm resolution of what you need to see, at a slant angle of, say, 17 degrees?

And, of course, there are times in the recon biz when you *want* to see things from a slant angle, when you need to analyze an object or a construct in three dimensions. There is only so much you can do to reconstruct something's shape from a direct overhead shot and a short shadow...

-the other Doug


--------------------
“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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edstrick
post Sep 19 2007, 08:06 AM
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Recon sats can look at targets at way off verticle slant angles. An infamous pair <near vertical and 45'ish deg slant angle> of recon sat images leaked about or a little before 1990 of a Soviet shipyard (the leaker was imprisoned for it) show good slant angle capability.

The basic fact is that recon sats have had hubble size mirrors, basically. An Aries-5 could or Saturn 5 could have launched much bigger optics. You can play with clean, oversampled high signal to noise ratio diffraction-limited images to extract about 2 times the apparent raw resolution out of the data, but that's about it.
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Hugh P
post Sep 9 2009, 05:12 PM
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Moved to correct thread - Mod

No one seems to have noticed details in this pit -
Attached Image


I tweaked the levels in Photoshop and applied a slight blur to reduce the scan line artifacts, and tweaked the levels again.
It seems that there an extra shelf below the eastern wall, which may be an overhang or even a tunnel, or more likely rubble that's fallen off the eastern wall.
There's a shadow gradient from top left to bottom right which cannot be a camera artifact, and is either caused by the slope and/or from the light reflecting off the opposite side wall.

There also appears to be chunks of rubble at the base of the pit, although this may well be in part noise from the HiRISE camera. See this image to compare how light plays off rubble in the bottom of a pit, and it seems fairly similar. However the gradient is in the opposite direction possibly because this hole is shallower relative to the width.

I'm very surprised no one has spotted this before as it had quite widespread media coverage when the image was captured in 2007 and I have been unable to find any other reference to this. Here is the link to the HiRISE source - the details are only visible on the non-map projected image.
Here are the original musings about it on my blog and I would be very interested to know what the experts think smile.gif
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