12.6 Low-Dispersion Resampled Image FITS File (SILO)

The low-dispersion SI is produced by resampling the photometrically-corrected portion of the low-dispersion LI using the modified Shepard algorithm taken from the Numerical Algorithms Group (NAG) software package. 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,
• detilting of large-aperture spectra for extended sources only,
• alignment of the apertures for constant wavelength in the line direction,
• adjustment so that both long wavelength cameras provide coverage of the same spectral range,
• adjustment to maintain the spectrum at approximately the same location in the file in the spatial direction,
• adjustment to LWP data to put the large-aperture data at the top of the file, and
• corrections for the spatial deviations (cross-dispersion wiggles) for LWP and LWR data.

The low-dispersion SI is stored in the SILO as a 2-D (640 samples × 80 lines) primary array, with the y coordinate in pixels and the x coordinate in Ångstroms. Each pixel represents a flux number (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 large-aperture data appears at the top of the file and the wavelengths increase from left to right. The associated $\nu$ flags are stored as a SILO image extension, which has the same dimensions as the primary array. Table 12.8 shows the basic FITS keywords for the main header and the image extension header. The starting wavelength and wavelength increment are stored as keywords in the primary header.

 

 

SIMPLE = T Standard FITS Format

BITPIX = 16 16-bits 2's complement pixels

NAXIS = 2 Two-dimensional image

NAXIS1 = 640 Dimension along x-axis

NAXIS2 = 80 Dimension along y-axis

EXTEND = T Extensions are present

CRPIX1 = 1. x reference pixel

CRPIX2 = 1. y reference pixel

CRVAL1 = nnnn.nn Wavelength at reference pixel

CRVAL2 = 1. Coordinate of CRPIX2

CDELT1 = nn.nnnn Increment in wavelengths

CDELT2 = 1. Increment unit along y-axis

CTYPE1 = 'WAVELENGTH' Units along x-axis

CTYPE2 = 'SCAN ' Units along y-axis

BUNIT = 'FN ' Flux Numbers

BSCALE = 3.1250E-02 real=tape*bscale+bzero

BZERO = 0. Pixel offset

TELESCOP= 'IUE ' International Ultraviolet Explorer

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

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= 'IMAGE ' Image extension

BITPIX = 16 16-bit 2's complement pixels

NAXIS = 2 Two_dimensional image

NAXIS1 = 640 Dimension along the x-axis

NAXIS2 = 80 Dimension along the y-axis

PCOUNT = 0 Number of bytes following data matrix

GCOUNT = 1 Number of groups

CRPIX1 = 1. x reference pixel

CRPIX2 = 1. y reference pixel

CRVAL1 = nnnn.nn Coordinate of CRPIX1

CRVAL2 = 1. Coordinate of CRPIX2

CDELT1 = nn.nnnn Increment unit along the x-axis

CDELT2 = 1. Increment unit along the y-axis

CTYPE1 = 'WAVELENGTH' x-axis units

CTYPE2 = 'SCAN ' y-axis units

BUNIT = ' ' Unitless

FILENAME= 'AAAnnnnn.SFLO' Filename (camera)(number).SF(disp)

EXTNAME = 'SILOF ' SILO pixel quality flags