DOT database description

The basic DOT data product consists of the fits files in the FITS directories of the DOT database.  For many runs processed data are supplied as well in Cubes directories.  The directories are ordered per observing run (usually the date).

The spatial sampling in the x and y directions is 0.071 arcsec/pixel for the blue cameras and the older red cameras; the newer red cameras have 0.110 arcsec/px.  The x direction is East-West, parallel to the geocentric celestial equator and corresponding to the horizontal direction in the Mees active region maps accessible per observing session from the database search engine and the chronological database index.  The heliographic coordinate X used by SOHO and TRACE is tilted Eastward from this direction over the solar pole position angle P0.

The diagnostic wavelength abbreviations are defined in the table in section DOT tomography.

The FITS directories are split between DOT cameras as specified by the diagnostic wavelengths.  Each fits file usually contains three images as 16-bit integer arrays: the speckle reconstruction, the best frame of the speckle burst, and the temporal average of the whole speckle burst.  Each fits file is usually about 7 MByte.  A fast way to display them is to enter them in John Bradley's xv.  Linux/Unix command xv *fits will show all reconstructions in the directory; step forward and backward in time with space and backspace (pagedown and pageup sample the three images per file).

The fits headers specify the array size, the time of observation (also as Julian Day), the wavelength, the angular scale, and the seeing-quality alpha determined in the speckle reconstruction of each burst (alpha = Strehl percentage; for DOT data alpha times 0.45 = Fried parameter in cm).  The FITS directories contain files alphalist and alphalist.sort enabling you to quickly select the best images (largest value of alpha).  Graphs of the burst-by-burst variation of the Fried parameter r_0 are available per observing session in the database search engine and the chronological database index.

Unfortunately, we can not yet supply DOT pointing coordinates more precisely than the specifications given in the database search engine and the chronological database index. A good recipe is to align the scene with space data, for example Ca II H with TRACE 1600 Å.

For Halpha line-profile sampling (filenames containing "scan") the FITS files contain N+1 frames per file, where N is the number of wavelength samplings (usually 3 or 5).  The first is the red continuum image, then follow the N narrow-band Halpha filtergrams.  They are reconstructed together with the first through two-channel speckle processing following Keller & von der Lühe (1992). Unfortunately the FITS header does not yet specify the narrow-band wavelengths, but they are inserted in the file names in the Cubes directories.  Such profile-sampling cubes are perfectly aligned with each other spatially, but their temporal sampling differs slightly within the specified cadence intervals.

The files in the Cubes directories of the DOT database result from processing of all FITS files per pointing into co-aligned image sequences.  From these we also produce our DOT movies.  They are three-dimensional 16-bit integer arrays readable with IDL, most easily with Tim van Werkhoven's procedure readdotcube.pro, as for example in
            HalphaRed=readdotcube('QS-ha+0350-20051019-cube-1112x818x71-dt60').
The cube file names specify the diagnostic, the x, y, and time dimensions, and the time step in seconds.  For example, the cube above is a three-dimensional integer array containing a quiet-sun (QS) image sequence ("data cube") sampling Balmer Halpha (ha) at +350 mA from line center (+350), taken on October 19, 2005 ( 20051019), having array size nx=1112, ny=818, nt=71 ( 1112x818x71 = 130 MByte), taken at regular 60s cadence ( dt60), and thus having 71 minute duration.  Cubes with the same file name except for the diagnostic are exactly synchronous.  They are also aligned to be co-spatial.  A zero value for the time step means that the cadence is irregular and that the precise times of each exposure should be distilled from the FITS headers.  Cubes of which the file name contains -lo7 have been 3D-cone Fourier-filtered to remove modulations travelling horizontally at speeds larger than the 7 km/s sound speed ("subsonic").

Cubes such as  Limb-ca-0000-20051013-cube-1046x744x86-dt50-even   and  Limb-ca-1000-20051013-cube-1046x744x86-dt50-odd   are alternating sequences where the wavelength of the Ca II H filter was switched sequentially between line center (0000) and the blue wing at -1000 mÅ.  The designation even implies that these images correspond to the even frame numbers (starting IDL-wise at 0) of the other cubes which have twice the cadence.

Other directories in the DOT database furnish image mosaics, movies, and some of the IDL bookkeeping and programs used in our processing.  README files give information in some directories.

For further DOT descriptions and DOT data analyses see DOT publications.  The Open DOT program, the telescope specifications, and DOT multi-wavelength tomography are described in Rutten et al., A&A, 413, 1183, 2004.

Rob Rutten 2017-12-12