next up previous contents
Next: 12.7 High-Dispersion Resampled Image Up: 12 Final Archive Data Previous: 12.5 Vector Displacement FITS

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:

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.


 
 
Table 12.8:  SILO - Basic FITS Keywords
Keyword and value Description
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


next up previous contents
Next: 12.7 High-Dispersion Resampled Image Up: 12 Final Archive Data Previous: 12.5 Vector Displacement FITS
Karen Levay
12/4/1997