The image above was taken from the Curiosity rover (I hope). There appears to be a coin in the image.
Is that really a coin or some kind of button? Is it really present on the rover?
If it is indeed a coin, why is it there?
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Yes, this is a real Lincoln cent and it makes part of the camera calibration target for the Mars Hand Lens Imager (MAHLI) instrument.
A geologist tradition is to place coins (or other object of a known size) besides photographed objects to show the scale of the photograph. This seems to be a homage to this tradition.
For details see:
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1$\begingroup$ I'd like to imagine the procurement procedure for that penny, including a call for the lowest bid. $\endgroup$ – Pavel Nov 29 '18 at 14:19
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When calibrating a camera, there are typically 3 things that are used:
- Some sort of a color pallet
- A line chart of some sort to identify the fine resolution
- A real object, to make sure there isn't something fundamentally wrong.
NASA followed this same suit, choosing the penny to be the real object, claiming that it give homage to the practice of using coins to get a sense of scale in photographs. If you take a look below, you will see color calibration targets, resolution targets, and the penny, for the final touch of realism.
As for why that particular coin, NASA states:
The coin is from 1909. That was the first year Lincoln pennies were minted and the centennial of Abraham Lincoln's birth.
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7$\begingroup$ Having a known circular object in the frame allows the calibration of pixel aspect ratio and/or the angle of view at any relative orientation. $\endgroup$ – dmckee Sep 27 '13 at 22:42
