SIMBAD references

The Wide-field Infrared Survey Explorer and its follow-up Near-Earth Object (NEO) mission scan the mid-infrared sky twice a year. The spatial and temporal coverage of the resulting database is of utmost importance for variability studies, in particular of young stellar objects which have red W1 - W2 colors. During such an effort, I noticed subarcsecond position offsets between subsequent visits. The offsets do not appear for targets with small W1 - W2 colors, which points to a chromatic origin in the optics, caused by the spacecraft pointing alternating “forward” and “backward” from one visit to another. It amounts to 0.″1 for targets with W1 - W2 ≈ 2. Consideration of this chromatic offset will improve astrometry. This is of particular importance for NEOs that are generally red.