  Geometric optical distortion is a significant contributor to the astrometric
error budget in large telescopes using adaptive optics. To increase astrometric
precision, optical distortion calibration is necessary. We investigate using
smartphone OLED screens as astrometric calibrators. Smartphones are low cost,
have stable illumination, and can be quickly reconfigured to probe different
spatial frequencies of an optical system's geometric distortion. In this work,
we characterize the astrometric accuracy of a Samsung S20 smartphone, with a
view towards providing large format, flexible astrometric calibrators for the
next generation of astronomical instruments. We find the placement error of the
pixels to be 189 nm +/- 15 nm RMS. At this level of error, milliarcsecond
astrometric accuracy can be obtained on modern astronomical instruments.
