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12.5 Vector Displacement FITS File (VDLO/VDHI)

The VD defines the final SI coordinate values in the x (wavelength) and y (spatial) directions for every LI pixel. The final coordinates in SI space for any photometrically-corrected pixel in the LILO/LIHI are determined by:

x_{final} = VD(i,j,1) - x\_\,o\!f\!f\!set(cam,disp)\end{displaymath}

y_{final} = VD(i,j,2) - y\_\,o\!f\!f\!set(cam,disp)\end{displaymath}

where i and j range from 1 to 768, and x_offset and y_offset are given in the following table.

  x_offset y_offset
  disp=L H L H
LWP 100   297  
LWR 100   250  
SWP 130   490  

The output displacements between the SI and LI coordinates are recoverable by:

DELTA_x = VD(i,j,1)-i~~~~{\rm and}~~~~DELTA_y = VD(i,j,2)-j,\end{displaymath}

where i and j range from 1 to 768. xfinal and yfinal contain the final x and y coordinates in the SILO/SIHI. The x and y coordinates of the displacement vectors are stored as a 3-D primary array consisting of 768x768x2 elements. The displacements are coded as 32-bit, floating point numbers.

The XC allow the user to recover the calculated displacement vectors, mapping the science image (in raw space) to the ITF. For each of the approximately 500 (140 for low dispersion) points used to obtain the displacement between the science image and the corresponding level of the ITF, the binary table extension will contain the following columns of information: science image x-position (I*2), science image y-position (I*2), ITF x-position at position of best match (R*4), ITF y-position at position of best match (R*4), the cross-correlation coefficient (R*4), number of points used to calculate the coefficient (I*2), and the ITF level used in the correlation (I*2). The x and y positions correspond to the sample and line numbers in the RI. The resulting ITF positions of the best match are pre-filtered positions (before invalid matches have been identified and deleted) and will not necessarily correspond exactly to the photometric registration displacement components utilized to create the final displacement vector.

Basic keywords in the VDLO/VDHI headers and binary table extensions are shown in Table 12.7. Note that the CTYPE1 and CTYPE3 keyword values listed and as stored in the archived VDLO/VDHI are incorrect and should be interchanged. Unfortunately, this error was not discovered until the majority of images were processed and so was left uncorrected for consistency. Note also that the VDLO/VDHI will not be available for images processed at VILSPA, nor for images processed at GSFC after July 31, 1997.

Table 12.7:  VDLO/VDHI - Basic FITS Keywords
Keyword and value Description
SIMPLE = T Standard FITS Format
BITPIX = -32 IEEE single precision floating point
NAXIS = 3 Three-dimensional image
NAXIS1 = 768 Dimension along x-axis
NAXIS2 = 768 Dimension along y-axis
NAXIS3 = 2 Dimension along z-axis
EXTEND = T Extensions are present
CTYPE1 = ' ' Units x-axis
CTYPE2 = 'PIXEL ' Units y-axis
CTYPE3 = 'PIXEL ' Units z-axis
BUNIT = 'PIXEL ' Pixel units
TELESCOP= 'IUE ' International Ultraviolet Explorer
FILENAME= 'AAAnnnnn.VDdd' Filename(camera)(number).VD(disp)
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= 'BINTABLE' Table extension
BITPIX = 8 Binary data
NAXIS = 2 Two-dimensional table array
NAXIS1 = 20 Width of table in bytes
NAXIS2 = nnn Number of entries in table
PCOUNT = 0 Number of bytes following data matrix
GCOUNT = 1 Number of groups
TFIELDS = 7 Number of fields in each row
TFORM1 = '1I ' Count and data type for field 1
TTYPE1 = 'XRAW ' Science image x-position
TUNIT1 = 'PIXEL ' Unit is pixels
TFORM2 = '1I ' Count and data type for field 2
TTYPE2 = 'YRAW ' Science image y-position
TUNIT2 = 'PIXEL ' Unit is pixel
TFORM3 = '1E ' Count and data type for field 3
TTYPE3 = 'XITF ' ITF x-position of best match
TUNIT3 = 'PIXEL ' Unit is pixel
TFORM4 = '1E ' Count and data type for field 4
TTYPE4 = 'YITF ' ITF y-position of best match
TUNIT4 = 'PIXEL ' Unit is pixel
TFORM5 = '1E ' Count and data type for field 5
TTYPE5 = 'XCOEFF ' Cross correlation coefficient
TUNIT5 = ' ' Unitless
TFORM6 = '1I ' Count and data type for field 6
TTYPE6 = 'NPOINTS ' Number of points used
TUNIT6 = ' ' Unitless
TFORM7 = '1I ' Count and data type for field 7
TUNIT7 = ' ' Unitless
FILENAME= 'AAAnnnnn.XCdd' Filename (camera)(number).XC(disp)
EXTNAME = 'XCOEFF ' Cross correlation coefficients

next up previous contents
Next: 12.6 Low-Dispersion Resampled Image Up: 12 Final Archive Data Previous: 12.4 Linearized Image FITS
Karen Levay