Camera lenses are increasingly used in wide-field astronomical surveys due to
their high performance, wide field-of-view (FOV) unreachable from traditional
telescope optics, and modest cost. The machining and assembly tolerances for
commercially available optical systems cause a slight misalignment (tilt)
between the lens and CCD, resulting in PSF degradation. We have built an
automated alignment system (Robotilters) to solve this challenge, optimizing 4
degrees of freedom - 2 tilt axes, a separation axis (the distance between the
CCD and lens), and the lens focus (the built-in focus of the lens by turning
the lens focusing ring which moves the optical elements relative to one
another) in a compact and low-cost package. The Robotilters remove tilt and
optimize focus at the sub 10 micron level, are completely automated, take 2
hours to run, and remain stable for multiple years once aligned. The
Robotilters were built for the Evryscope telescope (a 780 MPix 22-camera array
with an 8150 sq.deg. field of view and continuous 2-minute cadence) designed to
detect short timescale events across extremely large sky areas simultaneously.
Variance in quality across the image field, especially the corners and edges
compared to the center, is a significant challenge in wide-field astronomical
surveys like the Evryscope. The individual star PSFs (which typically extend
only a few pixels) are highly susceptible to slight increases in optical
aberrations in this situation. The Robotilter solution resulted in a limiting
magnitude improvement of .5 mag in the center of the image and 1.0 mag in the
corners for typical Evryscope cameras, with less distorted and smaller PSFs
(half the extent in the corners and edges in many cases). In this paper we
describe the Robotilter mechanical and software design, camera alignment
results, long term stability, and image improvement.Comment: Accepted to JATIS, January 202
