by Chris Peterson » Sun Oct 10, 2010 3:31 pm
Ann wrote:There is no shadow of a straight line on Buzz Aldrin's leg. That's a grid mark. You can see that those grid marks extend all over the picture to the right, evenly spaced. There is a second row of grid marks below the top one. There is an upper row of grid marks too, but you can't see them against the blackness of space. One is visible against the Moon lander, though.
Grid marks were common in early astronomy imagery...
In a much later image of "the Face on Mars", revealing it not to be a face, there are no grid marks, since they are hardly ever seen in modern astrophotography...
I think he is talking about the shadow cast by the adjacent solar panel. Of course, he's wrong that you can't get a natural shadow that looks straight on a curved object.
The subject of the grid marks is very interesting, though. Those of us old enough to remember early space photography, or who enjoy historical images, recall seeing them all the time. They are called
fiducials and are produced by
reticles, lines etched on glass plates called
reseau plates. They were used with film, because film is dimensionally unstable- it can stretch a bit. By pressing the film against the reseau plate for reference, much more accurate dimensional data could be taken from the images.
You don't see these used anymore because we don't use film anymore. CCDs and most other electronic sensors are inherently stable dimensionally, and are pixel based (an exception is scanned tube images, and cameras with those sensors- now obsolete- normally included reticles). When you work with a CCD image, you know exactly where each pixel is, so you don't need an external reference.
Film has many problems that CCD detectors largely avoid. Besides the mechanical instability, it has a very non-linear response, its resolution varies with contrast, and light scatters in the emulsion layer. Because of these effects, the fiducials can look very strange sometimes- I've seen them (or parts of them) inverted, invisible, or appearing to be behind objects in the image. All of these things are pretty easy to understand, but provide material for conspiracy nuts who claim the images are faked (although why somebody faking images would make such obvious "mistakes" is never explained).
[quote="Ann"]There is no shadow of a straight line on Buzz Aldrin's leg. That's a grid mark. You can see that those grid marks extend all over the picture to the right, evenly spaced. There is a second row of grid marks below the top one. There is an upper row of grid marks too, but you can't see them against the blackness of space. One is visible against the Moon lander, though.
Grid marks were common in early astronomy imagery...
In a much later image of "the Face on Mars", revealing it not to be a face, there are no grid marks, since they are hardly ever seen in modern astrophotography...[/quote]
I think he is talking about the shadow cast by the adjacent solar panel. Of course, he's wrong that you can't get a natural shadow that looks straight on a curved object.
The subject of the grid marks is very interesting, though. Those of us old enough to remember early space photography, or who enjoy historical images, recall seeing them all the time. They are called [i]fiducials[/i] and are produced by [i]reticles[/i], lines etched on glass plates called [i]reseau plates[/i]. They were used with film, because film is dimensionally unstable- it can stretch a bit. By pressing the film against the reseau plate for reference, much more accurate dimensional data could be taken from the images.
You don't see these used anymore because we don't use film anymore. CCDs and most other electronic sensors are inherently stable dimensionally, and are pixel based (an exception is scanned tube images, and cameras with those sensors- now obsolete- normally included reticles). When you work with a CCD image, you know exactly where each pixel is, so you don't need an external reference.
Film has many problems that CCD detectors largely avoid. Besides the mechanical instability, it has a very non-linear response, its resolution varies with contrast, and light scatters in the emulsion layer. Because of these effects, the fiducials can look very strange sometimes- I've seen them (or parts of them) inverted, invisible, or appearing to be behind objects in the image. All of these things are pretty easy to understand, but provide material for conspiracy nuts who claim the images are faked (although why somebody faking images would make such obvious "mistakes" is never explained).