Measuring Polar-Angle Errors in Imaged Helioseismology

A sixteen-day sequence of GONG full-disk Dopplergrams was computer-rotated by plus and minus 1° with respect to the midpoint of the equator to produce two additional data sets simulating errors in the instrumental alignment with the rotation axis. The unrotated and two rotated data sets were then reduced to produce tables of mode linewidths and line-peak powers. The line characteristics of the two rotated sets were compared to the unrotated set for 20 l 120. It was found that the linewidths increased as much as 55% and the line-peak powers decreased as much as 17%, with increasing l. These results are in good agreement with an earlier model (Kennedy, 1997). Differential linewidths studies indicated that the GONG instrument-network rotation-axis alignment was within 0.048° ± 0.040° during the period of the observation.