Space photo cross hairs?

Question

What are the cross hairs in space photos used for? I'm guessing they're used as points of reference.

Answer 1

The marks are called "fiducials."  In film photography they are applied by a device called a Reseau plate, which sits at the film plane.  In digital photography they are applied by hard-wiring certain sensor elements to fixed values.  In each case the fiducial grid is aligned very precisely with the optical axis of the camera, and the fiducials themselves are located with extreme precision.  The Apollo 70 mm Hasselblad 500/EL Data camera positions the fiducials to an accuracy of 0.02 millimeter.

The first role of the fiducial grid is to detect and correct any distortion that might occur during any duplication process.  Because the grid is precisely etched into the original, any deviation that is detected in a copy will indicate that the spatial arrangements in the copy cannot be trusted.  There are digital tools to rectify duplication errors.

The second role is as a reference to the original optical axis and other angular deviations.  That becomes important when you want to measure objects in the photo and determine their size and position in real life.

Answer 2

Yes, they are used as points of reference, primarily as Measurement.

With a phot you can enlarge or diminish it, thereby making it smaller or larger, points of reference maintain the comparative size for scientists.