Recalibration of 720s HMI Data Products Obtained with Two Cameras
Bug discovered in HMI Vector Field Data Processed Since 13 April 2016
On 13 April 2016 HMI changed the vector field observations to use data from two cameras. Despite careful testing, a bug has recently been discovered such that the filtergrams from the two cameras were not properly aligned. The 0.08 degree offset in image rotation results in a displacement of a couple pixels at the solar limb. The offset is most apparent in the Stokes Q and U components relative to Stokes V. The error affects all of the data products that depend on the HMI Stokes data product, HMI.S_720s. In particular the vector magnetic field provided in hmi.B_720s and hmi.sharp_720s deteriorate away from disk center. The error was first noticed in measurements of the radial magnetic field component near the poles derived from the vector data. The HMI 45-second data products are not affected. The bug has been corrected starting with data processed on 1 September, 2017 and we will reprocess the data since 13 April 2016 over the next several months. Data products with keyword CAMERA=3 and keyword DATE_S prior to 1 September 2017 should be replaced. Please contact a member of the HMI team if you have questions.
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