Data Set Overview
  =================
    This data set contains stellar occultation data from the 1989
    occultation of 28 Sagittarii (28Sgr) by Saturn obtained by the
    Hale 5 meter (200 inch) telescope (PAL200) at the Mt. Palomar
    observatory. Data include occultation profiles for both ingress
    and egress which each cover a radial slice from the outer edge of
    Saturn's atmosphere to beyond the orbit of the F-ring.
 
    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 64,000 individual frames
    taken with the Cassegrain IR camera using a 58 x 62 pixel InSb
    detector array with a circular variable filter (CVF). The units
    are DN/pixel. Each frame was recorded with an integration time of
    0.15 sec. Both ingress and egress ring occultations were
    observed, but with 1.5 to 3.5 minute data gaps every 20 minutes
    associated with opening new disk files.
 
 
  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.
 
    The original observations were combined to provide a time
    sequence of signal intensities. The raw data are sampled every
    0.15 sec which corresponds to approximately every 3 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/PAL200 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.
 
      Saturn?s air mass varied from 2.15 at the start of observations,
      through a minimum of 1.77 at 7:48 UT, to 2.12 at the end. Sky
      conditions were excellent during the ingress period, but thin
      clouds affected the ring egress, especially the data for the A
      Ring. Throughout the period of ring occultations the telescope
      pointing was controlled by an autoguider set on a nearby star,
      so that 28 Sgr remained at an almost fixed location on the
      detector array while Saturn drifted slowly across the field.
      Absolute timing, tied to WWVB radio signals, was provided by
      periodic offsets applied to the telescope?s chopping secondary
      mirror. Further details on the timing calibration are provided
      in FRENCHETAL1993.
 
 
  Ancillary Data
  ==============
    Browse data (BROWSE/PAL200 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/PAL200 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.
 
    Photometric uncertainties in the raw ingress lightcurve are due to
    background noise (125 DN), shot noise in the stellar signal (36 DN
    unocculted), and array read noise (89 DN), for a total of sigma(F)
    of 158 DN. As the average unocculted stellar flux is 114,000 DN,
    this implies a formal 1 sigma measurement error at low optical
    depths of ~ 0.0006. In reality, the rms noise in the unocculted
    stellar signal due to seeing fluctuations is 1100?1200 DN, leading
    to a more realistic 1 sigma random measurement uncertainty of ~
    0.004. See NICHOLSONETAL2000 for the complete analysis.
 
    The flux normalization may also introduce systematic errors of as
    much as 4%, based on comparisons of different polynomial fits and
    observed slow flux variations prior to ingress, at least in
    regions far from the inner or outer edge of the ring system. This
    may lead to low-frequency errors in the optical depths of up to
    0.02, independent of optical depth.
 
  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
  =========================
    For the approximately last half of the egress observations, cloud
    cover significantly affected the observations. Ring features can
    still be detected, but the observing team judged that absolute
    flux values are not valid. These data are flagged as non-
    photometric in the resampled data files.