This special purpose space mission that has been proposed to NASA Headquarter’s Discovery Program as a practical method for detecting Extrasolar Terrestrial Planets, that is, rocky and Earth-size. Lenox Laser fabricated a custom Starfield Plate for the Kepler Mission.
Quote from NASA’s Kepler Mission Web Site
“The star plate has a large number of holes of various sizes (used to perform time-variant relative photometry) and they are placed in many locations across the field-of-view to support the suite of tests described earlier. The plate is made of 50-micron thick stainless steel and opaque (transparency of less than one part in a million). The hole pattern was drilled with a laser beam by Lenox Laser, with some holes as small as 3 microns diameter (for the mv=19 stars).
There are 84 holes for the 9<14 target stars in the uncrowded region of the plate. These are used to isolate the effects of faint background stars, bright stars, smearing, etc. Some of these have very nearby stars as faint as mv=19 to demonstrate that stars five magnitudes fainter than the target star are not a problem even when spacecraft jitter is simulated. Bias-Smear Graphic of 84 Star plate Array There is a crowded portion of the plate with 1540 stars having the same star field density to mv=19 as the actual Cygnus region to be viewed by the Kepler Mission. This region was used to demonstrate the ability to perform the high-precision relative photometry even in crowded fields."
Dr. David Koch – Deputy Principal Investigator, Kepler Mission
Kepler Website – NASA