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:

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

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

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:

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

where i and j range from 1 to 768. x_final and y_final 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.

 

 

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

TTYPE7 = 'ITFLEVEL' ITF level

TUNIT7 = ' ' Unitless

FILENAME= 'AAAnnnnn.XCdd' Filename (camera)(number).XC(disp)

EXTNAME = 'XCOEFF ' Cross correlation coefficients