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:

• shift to photometric correction (ITF) raw space,
• shift from ITF space to geometrically-rectified space,
• rotation such that orders are horizontal,
• wavelength linearization,
• adjustment to maintain the echelle orders at approximately the same locations in the file in the spatial direction,
• corrections for the spatial deviations (cross-dispersion wiggles) for LWP, LWR, and SWP data,
• heliocentric velocity correction, and
• de-splaying correction.

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:

• Order Number, one 8-bit integer.
• Starting wavelength, one double-precision floating point number. Heliocentric velocity correction has been applied.
• Wavelength increment, one double-precision floating point number.
• predicted line position of order centroid, one single-precision floating point number.
• line position where spectral centroid is found, one single-precision floating point number. (This is determined by the high-dispersion spectral flux extraction module and written back into the SIHI file retroactively.)

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.

 

 

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