Update on SOT/NFI performance

Information posted by Ted Tarbell (LMSAL) on SolarNews

Data from the Solar Optical Telescope on Hinode is freely available to the community following the mission's open data policy. Many scientists have begun to analyze photospheric and chromospheric observations of unprecedented quality from SOT's Broadband Filter Imager and Spectro-Polarimeter. Recently a larger variety of data has become available from the third instrument channel, the Narrowband Filter Imager (NFI). The NFI has observed in all of its spectral bands and collected filtergrams, longitudinal magnetograms, Dopplergrams and Stokes IQUV images at various wavelengths. Several Fe I lines formed in the photosphere, Na D1 and Mg b2 in the low chromosphere, and H-alpha are all being observed. Na D1 is the default line for longitudinal magnetic and Doppler observations, due to its higher light level and more robust prefilter.

Images from the NFI contain blemishes which degrade or obscure the image over part of the field of view. The artifacts are caused by air bubbles in the index matching fluid inside the tunable filter. They distort and move when the filter is tuned. For this reason, NFI observing is usually done in one spectral line at one or a small number of wavelengths for extended periods of time. Rapid switching between lines is not allowed. Software changes made since launch have given us control over the location of the bubbles; targets can usually be placed in large blemish-free areas of the CCD. Tuning schemes have been developed which permit tuning to different positions in a line profile without disturbing the bubbles. This has enabled collection of the various observations listed above. Flat field correction of NFI images is still a challenge, but progress is being made on this; magnetograms and Dopplergrams are usually self-correcting since they are made from ratios of intensity differences. The Solarsoft routine fg_prep is being updated to handle the additional observation types. Specific questions about NFI images and their processing may be sent to sot_nfi@lmsal.com.