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12.7 High-Dispersion Resampled Image FITS File (SIHI)

The SIHI contains more information than stored in the corresponding low-dispersion file and, as a result, the FITS format is slightly more complex. Overall, the SIHI is comprised of a primary array containing the resampled image, a binary table of wavelengths and both predicted and found line positions, an image extension of $\nu$ flags, and a second image extension of background cosmic ray flags.

The high-dispersion SI data is similar to the low-dispersion SI data except that the high-dispersion wavelength linearization varies with spectral order, and the entire image is stored in the primary array. Each pixel is resampled to the position determined by the summation of the vectors computed for:

The high-dispersion SI is stored in the SIHI as a 2-D (768 samples × 768 lines) primary array. Each pixel represents an FN scaled up by a factor of 32 for storage purposes. The pixels are coded as 16-bit, two's complement integers, with the bits stored in decreasing order of significance. When the image is displayed with the origin in the lower left corner, the short-wavelength, closely-spaced high order numbers appear at the bottom, and the long-wavelength, low order numbers appear at the top. Within each order, the wavelengths increase from left to right.

Because the wavelength linearization varies with spectral order, the starting wavelength and wavelength increment values vary with each order. This information is stored in a binary table extension to the SIHI, which follows the primary array. The entire contents of the binary table extension include:

The associated $\nu$ flags and cosmic ray flags are stored in the SIHI image extensions with the same dimensions and orientation as the high-dispersion SI data contained in the primary array. The pixel quality flags are stored as unscaled 16-bit integers, and the cosmic ray flags are unscaled 8-bit integers. Table 12.9 shows the basic FITS keywords for the main and extension headers for the SIHI.


 
 
Table 12.9:  SIHI - Basic FITS Keywords
Keyword and value Description
SIMPLE = T Standard FITS Format
BITPIX = 16 16-bit 2's complement pixels
NAXIS = 2 Two-dimensional image
NAXIS1 = 768 Dimension along x-axis
NAXIS2 = 768 Dimension along y-axis
EXTEND = T Extensions are present
CTYPE1 = 'SAMPLE ' x-axis
CTYPE2 = 'LINE ' y-axis
BUNIT = 'FN ' Flux Numbers
BSCALE = 3.1250E-02 real=tape*bscale+bzero
BZERO = 0. offset
TELESCOP= 'IUE ' International Ultraviolet Explorer
FILENAME= 'AAAnnnnn.SIHI' Filename (camera)(number).SIHI
DATE = 'dd/mm/yy' Date file was written
ORIGIN = 'VILSPA ' Institution generating the file
DATAMIN = nnnnn.n Minimum pixel value
DATAMAX = nnnnn.n Maximum pixel value
XTENSION= 'BINTABLE' Binary table extension
BITPIX = 8 Binary data
NAXIS = 2 Two-dimensional table array
NAXIS1 = 25 Width of table in bytes
NAXIS2 = nn Number of entries in table
PCOUNT = 0 Number of bytes following data matrix
GCOUNT = 1 Only one group
TFIELDS = 5 Number of fields in each row
TFORM1 = '1B ' 8-bit byte
TTYPE1 = 'ORDER ' Order number
TUNIT1 = ' ' Unitless
TFORM2 = '1D ' Double precision floating point
TTYPE2 = 'WAVELENGTH' Starting wavelength
TUNIT2 = 'ANGSTROM' Unit is angstroms
TFORM3 = '1D ' Double precision floating point
TTYPE3 = 'DELTAW ' 3rd field is wavelength increment
TUNIT3 = 'ANGSTROM' Unit is angstrom
TFORM4 = '1E ' Single precision floating point
TTYPE4 = 'LINE_PREDICTED' Predicted line position of order centroid
TUNIT4 = 'PIXEL ' Unit is pixel
TFORM5 = '1E ' Single precision floating point
TTYPE5 = 'LINE_FOUND' Line number where spectral centroid is found
TUNIT5 = 'PIXEL ' Unit is pixel
FILENAME= 'AAAnnnnn.WLHI' Filename (camera)(number).WLHI
EXTNAME = 'SIHIW ' Name of table
XTENSION= 'IMAGE ' Image extension
BITPIX = 16 16-bit 2's complement pixels
NAXIS = 2 Two-dimensional image
NAXIS1 = 768 Dimension of x-axis
NAXIS2 = 768 Dimension of y-axis
PCOUNT = 0 Number of bytes following data matrix
GCOUNT = 1 Number of groups
CTYPE1 = 'SAMPLE ' X-axis
CTYPE2 = 'LINE ' Y-axis
BUNIT = ' ' Unitless
FILENAME= 'AAAnnnnn.SFHI' Filename (camera)(number).SF(disp)
EXTNAME = 'SIHIF ' SIHI pixel quality flags
XTENSION= 'IMAGE ' Image extension
BITPIX = 8 8-bit integer pixels
NAXIS = 2 Two-dimensional image
NAXIS1 = 768 Dimension of x-axis
NAXIS2 = 768 Dimension of y-axis
PCOUNT = 0 Number of bytes following data matrix
GCOUNT = 1 Number of groups
CTYPE1 = 'SAMPLE ' X-axis
CTYPE2 = 'LINE ' Y-axis
BUNIT = ' ' Unitless
FILENAME= 'AAAnnnnn.CRHI' Filename (camera)(number).CR(disp)
EXTNAME = 'SIHIC ' SIHI cosmic ray background flags


next up previous contents
Next: 12.8 Low-Dispersion Merged Extracted Up: 12 Final Archive Data Previous: 12.6 Low-Dispersion Resampled Image
Karen Levay
12/4/1997