Data Set Overview
  =================
    This data set contains stellar occultation data from the 1989
    occultation of 28 Sagittarii (28Sgr) by Saturn obtained by the
    NASA InfraRed Telescope Facility (IRTF) at the Mauna Kea
    observatory. Data include occultation profiles for both ingress
    and egress. Observations began well after ring ingress had
    commenced capturing part of the C Ring only. However they span the
    entire ring egress under excellent conditions.
 
    Ring profiles are archived as resampled normal optical depth
    profiles. Geometry solutions and noise models are also provided.
 
 
  Parameters
  ==========
    The raw data consists of more than 44,000 individual frames
    taken with the University of Rochester 62 x 58 pixel InSb array
    camera. The units are DN/pixel. Each frame was recorded with an
    effective integration time of 0.25 sec. A special-purpose 0.2
    micron wide filter centered at 3.225 microns was used.
    This filter was designed to occupy the overlap between methane
    and water-ice absorption bands.
 
 
  Processing
  ==========
 
    Detailed descriptions of the data processing can be found in
    NICHOLSONETAL2000 and are summarized here. Preliminary reduction
    of the images consisted of sky-frame subtraction, flat-fielding,
    and simple aperture photometry on the flattened images
    (HARRINGTONETAL1993).
 
    The original observations were combined to provide a time
    sequence of signal intensities. The raw data are sampled every
    0.25 sec which corresponds to approximately every 5 km in the
    ring plane at Saturn. However, the raw data sampling is non-
    uniform in radius.  The apparent diameter of the star was
    approximately 18 km, so the raw data are significantly over
    sampled.
 
    The fully processed and resampled ring profiles have been
    generated as follows:
 
      (1) Observations of the 28 Sgr occultation made from several
      observatories were combined with Voyager PPS and RSS data sets
      to determine precisely the ring radial scale and the Saturn pole
      orientation (FRENCHETAL1993, HUBBARDETAL1993,
      NICHOLSONETAL2000).
 
      (2) Using the improved ring radial scale, the time-series of raw
      samples was converted to a uniformly-spaced series of radial
      samples. The spatial sampling interval used was 10 km,
      corresponding to Nyquist-sampled data based on the apparent star
      diameter of approximately 18 km.
 
      (3) Raw data DN levels were converted to normalized flux based
      on the viewing geometry, and subsequently to normal optical
      depth.
 
 
  Data
  ====
 
    Derived profiles (DATA/IRTF sub-directory)
    ------------------------------------------
 
      The resampled data are presented in seven column ASCII tables.
      The columns are (1) radius(km), (2) normalized flux, (3) sigma
      flux, (4) normal optical depth(tau), (5) tau lower, (6) tau
      upper, (7) quality flag.
 
      The quality flag has five possible values. They are: 0=ok, 1=tau
      greater then tau_max, 2=non-photometric, 3=corrupted data,
      4=missing data. For detailed discussion of the model used to
      determine noise and reasonable values for optical depth see
      MODEL.TXT in the DOCUMENTS sub-directory.
 
      In general, for corrupted or missing data, dummy values of
      9.9990 are used in columns 3-6.
 
 
 
  Ancillary Data
  ==============
 
    Browse data (BROWSE/IRTF sub-directory)
    --------------------------------------
      This subdirectory contains two types of plot for each
      observation run.
 
      Plots of radial profiles of stellar flux normal optical depth of
      the rings as derived from the observations of the 28 Sgr
      occultation by the rings of Saturn. Each plot is based on the
      corresponding profile data file in the corresponding
      subdirectory of the DATA directory.
 
      Plots of ancillary geometry information. Each plot is based on
      the corresponding profile data file in the corresponding
      subdirectory of the GEOMETRY directory.
 
    Geometry information (GEOMETRY/IRTF sub-directory)
    -------------------------------------
      The geometry information is provided in separate files for
      ingress and egress. Data are arranged in four columns:
 
        a) Equatorial Radius (Km),
        b) Observatory received time (UTC),
        c) Ring Plane inertial longitude of the intercept point (deg),
        d) delta time (s). The latter is a correction factor used when
           determining the line-of-sight distance between the ring
           intercept point and the observer. The data are presented
           with uniformly varying radius (1000 km increments) between
           73,000 km and 142,000 km.
 
      The data are arranged in the order of increasing radius
      regardless of whether the observation is ingress or egress.
      Geometry data were obtained using a polynomial fit. The
      polynomial coefficients can be found in DOCUMENTS/POLY_CO.TXT
 
 
  Coordinate System
  =================
    All geometric quantities appearing in the labels are in J2000
    coordinates.  In this coordinate frame, the z-axis points
    northward along the Earth's J2000 rotation axis and the x-axis
    points toward the First Point of Aries.  In some cases, B1950
    coordinate values are also included.
 
  Media/Format
  ============
    This data set is archived on compact disc (CDROM) media.
    Organization and formats are according to PDS and ISO 9660
    level 2 standards.
 
    Most binary data files are in least-significant-byte first, which
    is the native format for PCs and Digital workstations.  Users of
    Suns and other workstations may need to swap bytes in some data
    files before use. Note that the software tools provided on this
    volume swap the bytes automatically if this is necessary.

Confidence Level Overview
  =========================
    Overall the confidence level is high. Flawed data are clearly
    identified by non-zero values for the quality flag.
 
    An oblong synthetic aperture 5.9 X 5.0'' in diameter was used for
    aperture photometry of most frames, surrounded by a reference
    ?sky? annulus, but this was reduced to 4.2'' in diameter for the B
    Ring. No correction was made for the variable aperture size in
    this data set. The full stellar flux levels were linearly
    interpolated between measurements made interior to the C Ring and
    exterior to the A Ring. The noise level in the unocculted signal
    averaged 1.4%, while the maximum detectable optical depth for the
    IRTF data is estimated at Tau max greater than or equal to 3.0,
    comparable to that of the Palomar data.
 
  Peer Review
  ===========
    Peer review of this data set was completed in February 2005. The
    peer reviewers were Shawn Brooks (JPL), Richard G. French
    (Wellesley College), Colleen A. McGhee (Wellesley College), Cathy
    Olkin (South West Research Institute), and Leslie Young (South
    West Research Institute). The data set was also reviewed at the
    PDS Engineering Node by Steven L. Adams.
 
 
  Data Coverage and Quality
  =========================
    Observations began well after ring ingress had commenced
    capturing part of the C Ring only. However they span the entire
    ring egress under excellent conditions.