For most of the Spitzer mission, a derived third order polynomial distortion correction with a residual error of ~100mas, or < 0.1 pixel, exceeded the needs for most science observations. The recent discoveries of study brown dwarfs within the solar neighborhood has been a main driver to increase the correction.
The new distortion correction increased the order of the polynomial correction from a third order to a fifth order. Using cluster observations and relative astromerty, the new correction produces an reduction in the astrometric error floor down to under 30mas. From there the number of iterations of the observations could easily approach the 5-10mas needed for pushing the parallax measurable out to possibly 50pc.
The new distoriton correction has been verified and agrees with two other independent derivations using spearate data sets.
Reference: Lowrance et al 2016 SPIE 9904-230