Reseau marks on space pictures serve as calibration crosses to measure optical distortion
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Reference documentation indicates that reseau plates feature fiducial grid marks that cast shadows on photographic film, allowing uneven spacing to reveal optical distortion.
Components for an orbiting camera payload system (OCPS) include the large format camera (LFC), a gas supply assembly, and ground test, handling, and calibration hardware. The LFC, a high resolution large format photogrammetric camera for use in the cargo bay of the space transport system, is also adaptable to use on an RB-57 aircraft or on a free flyer satellite. Carrying 4000 feet of film, the LFC is usable over the visible to near IR, at V/h rates of from 11 to 41 milliradians per second, overlap of 10, 60, 70 or 80 percent and exposure times of from 4 to 32 milliseconds. With a 12 inch focal length it produces a 9 by 18 inch format (long dimension in line of flight) with full format low contrast resolution of 88 lines per millimeter (AWAR), full format distortion of less than 14 microns and a complement of 45 Reseau marks and 12 fiducial marks. Weight of the OCPS as supplied, fully loaded is 944 pounds and power dissipation is 273 watts average when in operation, 95 watts in standby. The LFC contains an internal exposure sensor, or will respond to external command. It is able to photograph starfields for inflight calibration upon command.
Réseau plate
An accidental exposure from the Hasselblad lunar surface data camera showing the Réseau crosses.
A Réseau plate is a transparent sheet of glass or plastic engraved with a grid of crosshatches called fiducial markers. It was commonly used in film cameras(before the advent of digital cameras) for scientific and technical photography. The plate is placed at the focal plane of the camera just in front of the film. When the frame is exposed the fiducial markers scatter incoming light, creating a permanent shadow on the negative.
The fiducial markers on a Réseau plate are sometimes called reticles. The Latin root of fiducial means trust, confidence; fiducial markers are trustworthy reference points. The Latin root of reticle means small meshwork bag, small net; a grid of reticles has a net-like appearance.
## Purpose and function
If the film negative is properly printed the fiducial markers will be evenly spaced on the positive image. Uneven spacing reveals that the image has been distorted. Identical spacing on adjacent images (e.g. from a series of aerial photos) ensures that printing magnification is consistent. Irregularities in the grid can also result from incorre
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