12.8 Low-Dispersion Merged Extracted Image FITS File (MXLO)

The data extracted from the low-dispersion SI are stored in the MXLO using a binary table extension with fixed-length floating point vectors to contain the extracted fluxes and associated $\nu$ flags. Since no primary data are included, the extension header immediately follows the primary header. Each row of the binary table includes the following columns:

• Aperture designation as `LARGE' or `SMALL', stored in 5 ASCII characters.
• Number of extracted points, one 16-bit integer. The number of extracted points is always 640.
• Starting wavelength, one single-precision floating point value.
• Wavelength increment, one single-precision floating point value.
• Net flux spectrum, array with 640 single-precision floating point values.
• Background flux spectrum, array with 640 single-precision floating point values.
• Sigma vector, array with 640 single-precision floating point values.
• $\nu$ flags, array of 640 16-bit integers stored in two's complement form.
• Absolutely-calibrated net flux spectrum, array with 640 single-precision floating point values.

Wavelengths are linearly sampled to a uniform step size and measured in vacuum. Double aperture low-dispersion spectra will contain two rows in the above format, with one row for each aperture (LARGE first, then SMALL). Note the NAXIS1 keyword in the Binary table extension defines the number of bytes per row in the table and is equal to 15+18*640, or 11,535 bytes.

The absolute calibration covers the range of 1150-1980Å for short-wavelength spectra and 1850-3350Å for long-wavelength spectra. Since the NEWSIPS software extracts data over a slightly larger wavelength range, data points outside this wavelength range are set to 0 in the absolutely-calibrated flux vector and -1 in the sigma vector. The net and background vectors are not affected. The uncalibrated data points are also flagged in the $\nu$ flag vector with the value of -2. Table 12.10 shows the basic FITS Keywords for the MXLO.

 

 

SIMPLE = T Standard FITS Format

BITPIX = 8 8 bits ASCII

NAXIS = 0 No image data

EXTEND = T Extensions are present

TELESCOP= 'IUE ' International Ultraviolet Explorer

DATE = 'dd/mm/yy' Date file is written

ORIGIN = 'VILSPA ' Institution generating the file

XTENSION= 'BINTABLE' Table extension

BITPIX = 8 Binary data

NAXIS = 2 Two-dimensional table array

NAXIS1 = 11535 Bytes per row (15+18*NPOINTS)

NAXIS2 = n Number of apertures (1-single, 2-both)

PCOUNT = 0 Number of bytes following data matrix

GCOUNT = 1 Only one group

TFIELDS = 9 Number of columns in the table

TFORM1 = '5A ' Count and data type of field 1

TTYPE1 = 'APERTURE' Aperture type (large or small)

TUNIT1 = ' ' Unitless

TFORM2 = '1I ' Field 2 has one 2-byte integer

TTYPE2 = 'NPOINTS ' Number of points

TUNIT2 = ' ' Unitless

TFORM3 = '1E ' Count and data type of field 3

TTYPE3 = 'WAVELENGTH' 3rd field is starting wavelength

TUNIT3 = 'ANGSTROM' Unit is angstrom

TFORM4 = '1E ' Count and data type of field 4

TTYPE4 = 'DELTAW ' 4th field is wavelength increment

TUNIT4 = 'ANGSTROM' Unit is Angstrom

TFORM5 = '640E ' Count and data type of field 5

TTYPE5 = 'NET ' 5th field is net flux array

TUNIT5 = 'FN ' Unit is IUE FN

TFORM6 = '640E ' Count and data type of field 6

TTYPE6 = 'BACKGROUND' 6th field is background flux array

TUNIT6 = 'FN ' Unit is IUE FN

TFORM7 = '640E ' Count and data type of field 7

TTYPE7 = 'SIGMA ' 7th field is the sigma

TUNIT7 = 'ERG/CM2/S/A' Unit is erg/cm2/sec/Angstrom

TFORM8 = '640I ' Count and data type of field 8

TTYPE8 = 'QUALITY ' 8th field is the data quality flag

TUNIT8 = ' ' Unitless

TFORM9 = '640E ' Count and data type of field 9

TTYPE9 = 'FLUX ' 9th field is the calibrated flux

TUNIT9 = 'ERG/CM2/S/A' Unit is erg/cm2/sec/Angstrom

FILENAME= 'AAAnnnnn.MXLO' Filename (camera)(number).MXLO

EXTNAME = 'MXLO ' Name of table