== Notes on the HMI Noise Mask Used in Vector Field Disambiguation ==
The average vector magnetic field intensity measured by HMI in weak-field regions varies across the disk, with Sun-spacecraft velocity, and with HMI tuning. The noise is important for setting the threshold for disambiguation of the vector field. This is described in the [[http://link.springer.com/article/10.1007/s11207-014-0516-8|Vector Field Pipeline Overview]] paper.

For each tuning of the HMI instrument, a map is made of the average weak-field intensity.  A separate map is made for each 100 m/s interval in Sun-SDO velocity.  The map is projected onto Chebychev polynomials and stored in the data series hmi.lookup_chebycoef_bnoise, as specified in the keyword MASKINFO.

The reconstructed noise mask is interpolated to the specific velocity for each disambiguation of the field. See the [[http://jsoc.stanford.edu/cvs/JSOC/proj/mag/ambig/apps/imagefromchebyshev.c|noise reconstruction code]] for implementation.

=== Notes and Changes ===
For some data calculated between December 2013 and May 2014 MASKINFO will point to a different data series, hmi.lookup_ChebyCoef_TWindow. Those maps are now included in hmi.lookup_chebycoef_bnoise

Additional information can be found at [[http://sun.stanford.edu/~yliu/hmi/noisemask/testmask/Noisemask_summary.html|Noise Mask Summary]]

=== Links ===
 . PipelineCode
 . MagneticField
 . FullDiskDisamb
 . JSOC FrontPage