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:

• Order number, one 8-bit byte.
• Number of extracted points n, one 16-bit integer.
• Starting wavelength, one double-precision floating point value.
• Starting pixel at starting wavelength, one 16-bit integer.
• Wavelength increment, one double-precision floating point value.
• Slit height in pixels, one single-precision floating point number.
• Line number for found centroid of spectrum, one single-precision floating point number.
• Net flux spectrum, 768 single-precision floating point numbers with n extracted data points.
• Background flux spectrum, 768 single-precision floating point numbers with n extracted data points.
• Noise vector, 768 single-precision floating point numbers with n extracted data points.
• $\nu$ flags as n 16-bit integers stored in two's complement form.
• Ripple-corrected net flux spectrum, 768 single-precision floating with n extracted data points.
• Absolutely-calibrated, ripple-corrected net flux spectrum, 768 single-precision floating point numbers. with n extracted data points.
• Start pixel for background fit, one 16-bit integer number. ^*
• End pixel for background fit, one 16-bit integer number. ^*
• Chebyshev scale factor, one single-precision floating point number. ^*
• Chebyshev polynomial coefficients for global background correction, 7 single-precision floating point numbers. ^*

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.

 

 

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