Raw Image Data and Label Parameters
===================================
Each raw image consists of an array of 8-bit picture elements or
'pixels'. Each vidicon scan line consists of 768 pixels or
'samples' obtained in minor frame units of 96 pixels; 768 such scan lines
compose the entire image. Line 1, sample 1 is at the upper left corner of
the image; line 768, sample 768 is at the lower right corner of the image.
Each raw pixel value lies in the range 0 to 255 (integers only). The units
of raw pixel values are data numbers (DN), which are proportional (up to
the telemetry system limit of 255) to the integrated charge read out from
the SEC Vidicon target in the camera scanning process. Since the telemetry
system saturates at 255, the DN/charge proportionality breaks down at that
level.
 
Associated with each raw image is a set of 20 header, or label, records.
Each record is 360 8-bit bytes long (a concatenation of five 72-byte
logical records). This set of 20 label records is generated by the
Operations Control Center (OCC) software during image acquisition and
contains various identifying parameters and scientific/engineering data
pertinent to the image.
 
Raw images must be corrected for the instrumental effects of the SEC
Vidicon camera system before quantitatively meaningful data can be
extracted from them. The methods of compensation for the radiometric
(photometric) non-linearities and non-uniformities and the geometric
distortion introduced by the vidicon system are described in the NEWSIPS
Manual, Chapters 5 - 11(Garhart et al., 1997 [GARHARTETAL1997]).
In addition, figures 2.1-15 of the same manual illustrate schematically the
spectral formats in both dispersion modes, for both apertures and for all
operational cameras.
 
IUE Final Archive Data Products for Comets
======================================================
 
The output files for the IUE Final Archive are fundamentally different from
those produced by IUESIPS, both in content and format. They are based on
the Flexible Image Transport System (FITS) format (NOST 1995) and
incorporate the FITS binary table extensions (NOST 1995) and FITS image
extensions (Ponz, Thompson, and Munoz 1994). Although some FITS reading
routines may not yet support these new FITS extensions, it was felt that
there was no convenient alternative FITS format available for storing IUE
data. Note that only those features included in the basic binary table
proposal (i.e., excluding the conventions described in the appendices of
the proposal) have been used in the Final Archive file formats. The formats
described below (as originally described in DCG 1995) have been approved by
the IUE Three Agencies as well as the NOST FITS Support Office.
 
 
---------------------------------------------------------------------------
 
   *  IUE Filename Conventions
   *  Resampled Low Image (SILO)
   *  Resampled High Image (SIHI)
   *  Extracted Low-Dispersion Spectra (MXLO)
   *  Extracted High-Dispersion Spectra (MXHI)
---------------------------------------------------------------------------
 
IUE Filename Conventions
========================
 
The basic FITS keywords define the structure and content of the files.
These basic keywords include both the required FITS keywords and, when
appropriate, certain optional reserved FITS keywords.
 
A project-defined keyword that needs to be mentioned is FILENAME. This
keyword describes the camera image number and the type of data contained in
the particular FITS header-and-data unit (HDU) and appears in every HDU
containing data.
 
One purpose of the FILENAME keyword is to provide users with a naming
convention when separating FITS file.   FILENAME is useful for verifying
the contents of the various data sets.
 
The value of the FILENAME keyword is formed by the concatenation of the
following codes:
 
   * Camera: 3 letter code (LWP, LWR, SWP).
   * Image number: 5 digits.
   * File type: 2 letter code as:
     RI
          raw image
     RO
          original RI (low dispersion only, in the case of partial-read
          images)
     VD
          vector displacements
     XC
          binary table extension of the VD file containing the cross
          correlation coefficients
     LI
          linearized image
     LF
          nu flag image extension of the LI file
     SI
          resampled image
     WL
          binary table extension of the high-dispersion SI file containing
          spectral wavelengths and spatial centroid positions of the orders
     SF
          nu flag image extension of the high-dispersion SI file
     CR
          cosmic ray image extension of the high-dispersion SI file
     MX
          merged extracted image (large, small or both apertures)
   * Dispersion: 2 letter code (HI, LO).
 
 
Resampled Image (SILO)
======================
The resampled low-dispersion image is an array produced by resampling the
photometrically corrected portion of the LILO/LIHI image 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 needed for:
 
  1.  shift to photometric correction (ITF) raw space,
  2.  shift from ITF space to geometrically-rectified space,
  3.  rotation such that orders are horizontal,
  4.  wavelength linearization,
  5.  detilting of large-aperture spectra for low-dispersion extended
      sources only,
  6.  alignment of the low-dispersion apertures for constant wavelength in
      the line direction,
  7.  adjustment so that both LW cameras provide coverage of the same
      spectral range,
  8.  adjustment to maintain the spectrum at approximately the same
      location in the file in the spatial direction (low dispersion only),
  9.  adjustment to LWP data to put the large-aperture data at the top of
      the file,
 10.  corrections for the spatial deviations (cross-dispersion wiggles) for
      the LWP and LWR low-dispersion data,
 
The low-dispersion SI is stored in the SILO as a 2-D (640 samples x 80
lines) primary array, with the y coordinate in pixels and the x coordinate
in in Angstroms.   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 pixel quality flags
are stored as an image extension which has the same dimensions as the
primary array. Table 12.8 in the NEWSIPS Manual (Garhart et al., 1997
[GARHARTETAL1997]) shows the basic FITS keywords for the main header and
the image extension header.
 
High-Dispersion Resampled Image FITS File (SIHI)
================================================
 
The SIHI contains more information than stored in the corresponding
low-dispersion file and, as a result, the FITS format is slightly more
complex. Overall, the SIHI is comprised of a primary array containing the
resampled image, a binary table of wavelengths and both predicted and found
line positions, an image extension of nu flags, and a second image
extension of background cosmic ray flags.
 
The high-dispersion SI data is similar to the low-dispersion SI data except
that the high-dispersion wavelength linearization varies with spectral
order, and the entire image is stored in the primary array. 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,
   * adjustment to maintain the echelle orders at approximately the same
     locations in the file in the spatial direction,
   * corrections for the spatial deviations (cross-dispersion wiggles) for
     LWP, LWR, and SWP data,
   * heliocentric velocity correction, and
   * de-splaying correction.
 
The high-dispersion SI is stored in the SIHI as a 2-D (768 samples  768
lines) primary array. Each pixel represents an 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
short-wavelength, closely-spaced high order numbers appear at the bottom,
and the long-wavelength, low order numbers appear at the top. Within each
order, the wavelengths increase from left to right.
 
Because the wavelength linearization varies with spectral order, the
starting wavelength and wavelength increment values vary with each order.
This information is stored in a binary table extension to the SIHI, which
follows the primary array. The entire contents of the binary table
extension include:
 
   * Order Number, one 8-bit integer.
   * Starting wavelength, one double-precision floating point number.
     Heliocentric velocity correction has been applied.
   * Wavelength increment, one double-precision floating point number.
   * predicted line position of order centroid, one single-precision
     floating point number.
   * line position where spectral centroid is found, one single-precision
     floating point number. (This is determined by the high-dispersion
     spectral flux extraction module and written back into the SIHI file
     retroactively.)
 
The associated nu flags and cosmic ray flags are stored in the SIHI
image extensions with the same dimensions and orientation as the
high-dispersion SI data contained in the primary array. The pixel quality
flags are stored as unscaled 16-bit integers, and the cosmic ray flags are
unscaled 8-bit integers. Table 12.9 from Garhart et al. (1997) shows the
basic FITS keywords for the main and extension headers for the SIHI.
 
 
 
Extracted Low-Dispersion Spectra (MXLO)
=======================================
The extracted low-dispersion file uses the binary 3-D table extension with
fixed-length floating point vectors to contain the extracted fluxes and
associated data quality flags. Since no primary data are included, the
extension header immediately follows the primary FITS header. Each row of
the binary table includes the following columns:
 
  1.  Aperture designation as 'LARGE' or 'SMALL', stored in 5 ASCII
      characters.
  2.  Number of extracted points, one 16-bit integer. The number of
      extracted points is 640.
  3.  Starting wavelength, one single precision floating point value.
  4.  Wavelength increment, one single precision floating point value.
  5.  Net flux spectrum, array with 640 single precision floating point
      values.
  6.  Background flux spectrum, array with 640 single precision floating
      point values.
  7.  Sigma vector, array with 640 single precision floating point values.
  8.  Data quality flags, array of 640 16-bit integers.
  9.  Absolutely calibrated net flux spectrum, array with 640 single
      precision floating point values.
 
Wavelengths are linearly sampled, and referenced to vacuum. Double aperture
low-dispersion spectra will contain two rows in the above format, with one
row for each aperture. Table 12.10 in the NEWSIPS Manual (Garhart et al.,
1997 [GARHARTETAL1997]) shows the basic FITS keywords for the MXLO file.
 
Note: The keyword NAXIS1 in the table extension defines the number of bytes
per row in the table.
 
 
High-Dispersion Merged Extracted Image FITS File (MXHI)
=======================================================
 
The wavelengths, nu flags, and fluxes extracted from the SIHI are
stored in the MXHI as a binary table extension using fixed-length floating
point vectors. No primary data or additional extensions are included.
 
The binary table contains 17 fields of various data types. All vectors are
padded with zeroes (both before and after the extracted data) to maintain a
fixed length of 768 points. Wavelengths are uniformly sampled for each
order, are measured in vacuum, and have had the heliocentric velocity
correction applied. The width of each row (i.e., 65 + 22 * 768 = 16961)
bytes, and the number of rows (i.e., NAXIS2) is equal to the number of
extracted orders. In this manner, all the information pertaining to one
spectral order is contained in one row of the binary table. The fields are
defined in the order shown below:
 
   * Order number, one 8-bit byte.
   * Number of extracted points n, one 16-bit integer.
   * Starting wavelength, one double-precision floating point value.
   * Starting pixel at starting wavelength, one 16-bit integer.
   * Wavelength increment, one double-precision floating point value.
   * Slit height in pixels, one single-precision floating point number.
   * Line number for found centroid of spectrum, one single-precision
     floating point number.
   * Net flux spectrum, 768 single-precision floating point numbers with n
     extracted data points.
   * Background flux spectrum, 768 single-precision floating point numbers
     with n extracted data points.
   * Noise vector, 768 single-precision floating point numbers with n
     extracted data points.
   * nu flags as n 16-bit integers stored in two's complement form.
   * Ripple-corrected net flux spectrum, 768 single-precision floating with
     n extracted data points.
   * Absolutely-calibrated, ripple-corrected net flux spectrum, 768
     single-precision floating point numbers. with n extracted data points.
   * Start pixel for background fit, one 16-bit integer number. *
   * End pixel for background fit, one 16-bit integer number. *
   * Chebyshev scale factor, one single-precision floating point number. *
   * Chebyshev polynomial coefficients for global background correction, 7
     single-precision floating point numbers. *
 
Note that unlike the MXLO, SILO, and SIHI, the starting wavelengths listed
in the MXHI table do not refer to the first data point in the flux vectors,
but rather the starting pixel listed in field four. In this manner, the
768-point flux vector can be mapped directly to the 768-pixel wide
high-dispersion SI array.
 
As in low dispersion, since the absolute calibration covers the range of
1150-1980 for short-wavelength spectra and 1850-3350 for long-wavelength
spectra, data points outside this wavelength range are set to 0 in the
absolutely-calibrated flux vector. The net, background, and noise vectors
are not affected. (Note that unlike the sigma vector in the MXLO file, the
MXHI noise vector is uncalibrated.) Uncalibrated data points are also
flagged in the nu flag vector with a value of -2.
 
*IMPORTANT NOTE: Several adjustments must be made to the last
four parameters (fields 14-17) if the user wishes to evaluate the Chebyshev
coefficients in order to reproduce the background fluxes as stored in the
ninth field of the MXHI extension header. First, the parameters have
inadvertently been stored in the reverse order (i.e., the parameters
written in the first row of the table should have been stored in the last
row, the parameters for the second row in the second to last row, etc.).
So, for example, in the case of the LWR camera, the starting and ending
pixels, Chebyshev scale factor, and Chebyshev coefficients found in row 1
(echelle order 127) actually pertain to row 61 (echelle order 67). Second,
the true starting pixel is 768 minus the stored ending pixel and the true
ending pixel is 768 minus the stored starting pixel. These true pixel
values must be used to correctly evaluate the Chebyshev coefficients.
Third, once the Chebyshev coefficients have been evaluated, the resultant
background ``fluxes'' must be scaled in the following manner: multiply each
background value by both the Chebyshev scale factor and the corresponding
extraction slit height then divide this result by 32. Finally, the
resultant array of background fluxes which are produced upon evaluation of
the Chebyshev coefficients must be reversed (i.e., the computed background
flux for pixel 1 becomes the background flux for pixel 768 and vice versa).
We emphasize that these reversals and scalings are needed only when using
the Chebyshev parameters in fields 14-17 to reproduce the background
fluxes-the background fluxes themselves as contained in the ninth field are
correct.

N/A