An Efficient Focal Plane Alignment Methodology with Application to the ASTERIA Nanosatellite Space Telescope

The process of aligning an instrument focal plane to precursor optics such as a telescope can be a difficult and time-consuming process. The final alignment must achieve sufficient performance at multiple locations across the field of view. In the case of instruments with infinite conjugates, such as a space telescope, there are practical challenges in illuminating the system at multiple off-axis field points simultaneously. Iteration during the alignment process can cause a lengthy search for local minima within the performance cost function. This paper presents a focal plane alignment methodology that avoids the above difficulties and obtains the desired level of optical performance in an efficient manner. The approach is based on systematically measuring spot size across a set of field points and defocus positions, and then using a least squares fit to determine the optimal location for the focal plane. This approach has the additional benefit of revealing the amount of field curvature present in the instrument. It also provides the analyst with a direct calculation of the shims needed to place the focal plane at the computed best-fit location. This approach was successfully applied to the Arcsecond Space Telescope Enabling Research In Astrophysics (ASTERIA) payload. ASTERIA is a cubesat mission that was deployed into low-Earth orbit in November 2017 and operated until December 2019. In addition to presenting the theoretical basis for the methodology, this paper will present laboratory measurements obtained during the payload alignment campaign. This approach is applicable to future space-borne optical instruments that require an efficient methodology for focal plane alignment with limited cost or schedule resources.