Often I see small crosses on space pictures like this.
They are regularly spaced, thin and black. What are them? What's their purpose?
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2 Answers
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These crosshair marks are called registration marks and are produced by the camera optics.
These marks ensure that the exact geometry of the image (and succesive angle/distance measurements) are preserved despite deformation of the negative, or deformations during image processing such as scanning or printing to paper.
If you are taking images on the moon, you'd better preserve the most information intact.
The same techniques are used in aerial images for cartography.
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Expanding on the answer of @Juancho, the marks are also called reticles or fiducials. They are engraved into a thin, optically perfect piece of glass that is placed just in front of the film in the astronauts camera. (You remember film, that long piece of cellophane coated with Jello that was ... But I digress). When a picture is taken, the image of the fiducial is recorded on the film along with the image of the lunar landscape. If the film is distorted during subsequent handling or processing, the distortion can be detected and eliminated by applying a transformation to the image that moves each fiducial back to its the correct position.
The fact that these crosses appear to go behind some bright objects on the lunar surface has been cited by Apollo deniers as evidence of the great conspiracy...
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$\begingroup$ I'm having trouble finding a reference now, but I read a book on the Lunar Orbiter program which said that the registration marks were pre-exposed on the film before it was loaded into the orbiters. I don't doubt that the hand-held cameras used by the astronauts have reticles, but I don't think those are the only source of such alignment marks. $\endgroup$ Commented Oct 16, 2015 at 18:15
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$\begingroup$ history.nasa.gov/printFriendly/apollo_photo.html applies to Apollo photographs: $\endgroup$– DJohnMCommented Oct 16, 2015 at 20:31
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$\begingroup$ But astrogeology.usgs.gov/search/details/Moon/Lunar-Orbiter/… does indeed refer to pre-exposed registration marks $\endgroup$– DJohnMCommented Oct 16, 2015 at 20:33
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1$\begingroup$ Pre-exposed registration marks are light in color. The Apollo marks are dark, so you can tell they came from a glass instead. $\endgroup$– SkylerCommented Sep 26, 2017 at 18:50