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12.9 High-Dispersion Merged Extracted Image FITS File (MXHI)

The wavelengths, $\nu$ flags, and fluxes extracted from the SIHI are stored in the MXHI as a binary table extension using fixed-length floating point vectors. No primary data or additional extensions are included.

The binary table contains 17 fields of various data types. All vectors are padded with zeroes (both before and after the extracted data) to maintain a fixed length of 768 points. Wavelengths are uniformly sampled for each order, are measured in vacuum, and have had the heliocentric velocity correction applied. The width of each row (i.e., 65 + 22 × 768 = 16961) bytes, and the number of rows (i.e., NAXIS2) is equal to the number of extracted orders. In this manner, all the information pertaining to one spectral order is contained in one row of the binary table. The fields are defined in the order shown below:

Note that unlike the MXLO, SILO, and SIHI, the starting wavelengths listed in the MXHI table do not refer to the first data point in the flux vectors, but rather the starting pixel listed in field four. In this manner, the 768-point flux vector can be mapped directly to the 768-pixel wide high-dispersion SI array.

As in low dispersion, since the absolute calibration covers the range of 1150-1980Å for short-wavelength spectra and 1850-3350Å for long-wavelength spectra, data points outside this wavelength range are set to 0 in the absolutely-calibrated flux vector. The net, background, and noise vectors are not affected. (Note that unlike the sigma vector in the MXLO file, the MXHI noise vector is uncalibrated.) Uncalibrated data points are also flagged in the $\nu$ flag vector with a value of -2. Table 12.11 shows the basic FITS Keywords for the MXHI.

$^\ast$   IMPORTANT NOTE: Several adjustments must be made to the last four parameters (fields 14-17) if the user wishes to evaluate the Chebyshev coefficients in order to reproduce the background fluxes as stored in the ninth field of the MXHI extension header. First, the parameters have inadvertently been stored in the reverse order (i.e., the parameters written in the first row of the table should have been stored in the last row, the parameters for the second row in the second to last row, etc.). So, for example, in the case of the LWR camera, the starting and ending pixels, Chebyshev scale factor, and Chebyshev coefficients found in row 1 (echelle order 127) actually pertain to row 61 (echelle order 67). Second, the true starting pixel is 768 minus the stored ending pixel and the true ending pixel is 768 minus the stored starting pixel. These true pixel values must be used to correctly evaluate the Chebyshev coefficients. Third, once the Chebyshev coefficients have been evaluated, the resultant background ``fluxes'' must be scaled in the following manner: multiply each background value by both the Chebyshev scale factor and the corresponding extraction slit height then divide this result by 32. Finally, the resultant array of background fluxes which are produced upon evaluation of the Chebyshev coefficients must be reversed (i.e., the computed background flux for pixel 1 becomes the background flux for pixel 768 and vice versa). We emphasize that these reversals and scalings are needed only when using the Chebyshev parameters in fields 14-17 to reproduce the background fluxes-the background fluxes themselves as contained in the ninth field are correct.

Table 12.11:  MXHI - Basic FITS Keywords
Keyword and value Description
SIMPLE = T Standard FITS Format
BITPIX = 8 Binary data
NAXIS = 0 No image data
EXTEND = T Extensions are present
TELESCOP= 'IUE ' International Ultraviolet Explorer
DATE = 'dd/mm/yy' Date file was written
ORIGIN = 'VILSPA ' Institution generating the file
XTENSION= 'BINTABLE' Binary table extension
BITPIX = 8 Binary data
NAXIS = 2 Two-dimensional table array
NAXIS1 = 16961 Width of row in bytes
NAXIS2 = nn Number of orders
PCOUNT = 0 Number of bytes following data matrix
GCOUNT = 1 Only one group
TFIELDS = 17 Number of columns in the table
TFORM1 = '1B ' 8-bit byte
TTYPE1 = 'ORDER ' Order number
TUNIT1 = ' ' Unitless
TFORM2 = '1I ' 16-bit integer
TTYPE2 = 'NPOINTS ' Number of non-zero points
TUNIT2 = ' ' Unitless
TFORM3 = '1D ' Double precision
TTYPE3 = 'WAVELENGTH' Starting wavelength
TUNIT3 = 'ANGSTROM' Unit is Angstrom
TFORM4 = '1I ' 16-bit integer
TTYPE4 = 'STARTPIX' Starting pixel at starting wavelength
TUNIT4 = 'PIXEL ' Unit is pixel
TFORM5 = '1D ' Double precision value
TTYPE5 = 'DELTAW ' Wavelength increment
TUNIT5 = 'ANGSTROM' Unit is Angstrom
TFORM6 = '1E ' Single precision
TTYPE6 = 'SLIT HEIGHT' Height of extraction slit
TUNIT6 = 'PIXEL ' Unit is pixel
TFORM7 = '1E ' Single precision
TTYPE7 = 'LINE_FOUND' Line number where spectral centroid is found
TUNIT7 = 'PIXEL ' Unit is pixel
TFORM8 = '768E ' Single precision array
TTYPE8 = 'NET ' Net flux array
TUNIT8 = 'FN ' Unit is IUE Flux Number (FN)
TFORM9 = '768E ' Single precision array
TTYPE9 = 'BACKGROUND' Background flux array
TUNIT9 = 'FN ' Unit is IUE Flux Number(FN)
TFORM10 = '768E ' Single precision array
TTYPE10 = 'NOISE ' Noise spectrum
TUNIT10 = 'FN ' Unit is IUE Flux Number (FN)
TFORM11 = '768I ' 16-bit integer array
TTYPE11 = 'QUALITY ' Data quality flag
TUNIT11 = ' ' Unitless
TFORM12 = '768E ' Single precision array
TTYPE12 = 'RIPPLE ' Ripple-corrected net flux array
TUNIT12 = 'FN ' Unit is IUE Flux Number (FN)
TFORM13 = '768E ' Single precision array
TTYPE13 = 'ABS_CAL ' Absolutely-calibrated net flux
TUNIT13 = 'ERGS/CM2/S/A' Unit is ergs/cm2/sec/Angstrom
TFORM14 = '1I ' 16-bit integer
TTYPE14 = 'START-BKG' Beginning pixel of background fit
TUNIT14 = 'PIXEL ' X-axis in SIHI image
TFORM15 = '1I ' 16-bit integer
TTYPE15 = 'END-BKG ' End pixel of background fit
TUNIT15 = 'PIXEL ' X-axis in SIHI image
TFORM16 = '1E ' Single precision
TTYPE16 = 'SCALE_BKG' Chebychev scale factor
TUNIT16 = ' ' Unitless
TFORM17 = '7E ' Single precision array
TTYPE17 = 'COEFF ' Chebychev coefficients of background fit
TUNIT17 = ' ' Unitless
FILENAME= 'AAAnnnnn.MXHI' Filename (camera) (number) .MXHI
EXTNAME = 'MEHI ' Name of table

next up previous contents
Next: 13 Assessing NEWSIPS Data Up: 12 Final Archive Data Previous: 12.8 Low-Dispersion Merged Extracted
Karen Levay