Data Set Overview
  =================
    Details of the observations and the data processing used are
    contained in DONES_ETAL_UNPUBLISHED-2004.PDF (DONESETAL2004) which
    can be found in the DOCUMENT subdirectory.
 
    This data set contains images of the Saturn system from the
    Wisconsin-Indiana-Yale- NOAO(WIYN) using the WIYN Imager in late
    November 1995. Saturn system was observed on five half-nights, 19-
    23 November 1995 (The second half of each night, after Saturn set,
    was used to search a selected sample of X-ray binary stars,
    Sterzik et al 1997). The Sun crossed the ring plane during 17-21
    November 1995. The very low solar elevation angle, combined with
    the use of coronagraphic masks, helped to reduce scattered light
    from the main rings.
 
    Every image showing Saturn, its rings or the region of the inner
    satellites has been included in this data set, regardless of the
    original intended purpose.
 
    The WIYN observing program whose results are included in this
    dataset is was headed by:
 
          PI: Richard H. Durisen (Indiana University)
 
    The bulk of the following information has been extracted from
    DONES_ETAL_UNPUBLISHED-2004.PDF (DONESETAL2004) which can be found
    in the DOCUMENT subdirectory.
 
  Objectives
  ==========
    The original goals of the observations were to determine the
    radial extent and color of Saturn's faint E and, possibly, G,
    Rings, and to investigate the main rings and determine the orbits
    of ring moons such as Prometheus and Pandora. However, the high
    signal-to-noise obtained for the small moons Helene, Telesto, and
    Calypso made it clear that the images were well- suited to
    detecting previously unknown small moons.
 
    Observation summary and conditions
    ==================================
 
    The Instrument used to study the Saturn Ring Plane Crossing event
    was a special application of the WIYN Imager(WI). The WI consists
    of a Filter/Shutter Assembly (FSA) that mounts on the Instrument
    Adapter Subassembly (IAS) at the Nasmyth focus of the WIYN
    telescope, plus the CCD detector. The IAS provides for target
    acquisition, autoguiding, and for the wavefront sensing that is
    used to adjust the active optics of the WIYN telescope.  The CCD
    used in this work was a backside-illuminated STIS 2048 x 2048
    pixel detector with 21-micron square pixels.
 
    As noted above, the team observed the Saturn system during five
    nights, 19-23 November, 1995. This was around the time when the
    Sun crossed Saturn's ring plane, so the rings were especially
    dark. The rings were edge-on to the Sun, and were open to Earth by
    2.7 degrees. The phase angle was 5.5 degrees. Saturn was only up
    until around midnight local time.
 
    A total of 165 science images were obtained. Most observations of
    the Mimas-Hyperion region were taken in the R filter to maximize
    signal-to-noise. However, observations of satellites interior to
    Mimas were taken through the 890-nm methane filter to minimize
    scattered light from Saturn.  Exposure times ranged from 5 seconds
    to 5 minutes, with 1-2 minutes being typical.
 
    The night of 19 November 1995 was devoted to instrumental checkout
    and the next four nights were devoted to Saturn observations. The
    night of 22 November 1995 was devoted exclusively to narrowband
    methane imaging of the main rings and inner satellites; this work
    will be described elsewhere.  On the other three nights (20, 21,
    and 23 November), the team took both broadband and narrowband
    frames.
 
    The team recognized early on the night of the 20th that the tiny
    satellites Helene, Telesto and Calypso were extremely bright in
    the images.  These bodies have radii of 10-16 km. This raised
    their awareness that, in principle, they should be able to detect
    moons in the data that are substantially smaller than 10 km.  For
    this reason, they devoted much of the 20th, 21st and 23rd to long
    exposures of the region outside the main rings. Typical exposure
    times were 30--140 seconds and the R filter was emphasized because
    it had the finest sensitivity.
 
    NOTE: Inspite of careful image processing, ghosts from previous
    images remain in some images.
 
    Parameters
    ==========
    The PDS label for each file contains a broad variety of additional
    parameters enabling the user to determine image geometry and to
    convert pixel values to physically meaningful quantities.
 
  Data
  ====
    The data provided here are images in FITS format. For each data
    file, a detached PDS label is provided containing additional
    parameters.
 
  Ancillary Data
  ==============
    Additional calibration files are provided to assist in the
    analysis and interpretation of the data.
 
    As mentioned above, the appropriate subdirectories under the
    CALIBRATION subdirectory contain bias and flat field files.
 
  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.
 
 
  Media/Format
  ============
    This data set is archived on DVD 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
  =========================
 
    Inspite of careful image processing, ghosts from previous images
    remain in some images.
 
    Calibration of the images involved several steps. For the known
    satellites and for three stars that repeated from image to image,
    the brightness was integrated and divided by each image's exposure
    time, yielding values of DN/sec for each target.  DN is the
    integer data number found in each pixel of an image. These
    measurements were obtained from the original, unfiltered images to
    ensure that the filtering had not truncated the DN sums.  In each
    case, a region surrounding the target body was used to determine
    the local background, which was then subtracted from the DN
    summation. Although this calibration carried out for the R, B and
    V filters, it became apparent quickly that the R filter was by far
    the most sensitive, and the other filters were excluded from
    further analysis.
 
    From these individual determinations, the team then did a series
    of careful comparisons to verify that the visible moons and stars
    maintained constant brightness ratios.  Because the moons are
    elongated, their brightness from one night to the next could
    change significantly, but this could be roughly matched to their
    known axial ratios (Thomas etal 1983) and this could be further
    complicated because one or more of the stars might be variable.
    The brightest star, identified as S1 and as 5249-00969 in the
    Hubble Space Telescope's Guide Star Catalog, was indeed found to
    vary significantly on time scales of hours. As a result, it was
    eliminated from further consideration as a calibration reference.
 
    In the end, after allowing for the non-sphericity of Helene,
    Telesto and Calypso, all three moons were found to have very
    constant brightness ratios relative to the next brightest stars,
    S2 and S3. They chose S2 as their reference because it is the
    brighter of the two. Scaling to the dimensions of the three moons,
    the team inferred that S2 is equivalent in brightness to a moon of
    21 km radius. The primary source of uncertainty arises from the
    fact that the three moons have different albedos, providing a size
    uncertainty of ~20%.
 
    The other piece of the question is how bright an object needs to
    be in order to say with confidence that it should not have escaped
    notice in the search procedure. For this estimate, the team
    generated images in which false moons were added to the data prior
    to the filtering procedure. These simulated moons have the same
    point spread function as the actual moons and stars, which is
    found to be well matched by a two-dimensional Gaussian with a
    standard deviation of 1.34 pixels. They concluded that a moon with
    total DN = 3000 would be consistently visible, even if it falls
    rather close to the planet.  Further out where the noise is lower,
    a total DN = 2000 should be detectable.
 
    These can be translated into sizes assuming that they match the
    other moons in albedo.  As noted above, S2 is the equivalent of a
    21-km moon. The instrument records at least 1200 DN/sec for this
    star, corresponding to 144,000 DN in the minimum exposure time for
    satellite search images of 120 sec. This is can be interpolated to
    a 3000-DN moon in a 120 second exposure corresponds to a radius of
    3.0 km and a 2000-DN moon corresponds to a radius of 2.5 km. These
    detection limits fall well below those of previous ground-based
    searches for moons in the Saturn system (Kuiper 1961, Baron and
    Elliot 1983).
 
  Review
  ======
    This data set passed peer review on 1/25/2010. The members of the
    peer review panel were J. Bauer, L. Dones and C. Olkin.
 
  Data Coverage and Quality
  =========================
    The team have demonstrated that unknown moons larger than 2.5--3.0
    km in radius should have been detectable in the data set, yet none
    were. The question remains of how thorough the orbital coverage
    was during the three nights of observing.
 
    The team suggest that the minimum requirement for detection is
    that a moon appear in three different images.  For each image,
    they tabulated the range of distances from the planet where a moon
    could fall without being obscured by the occulting mask or one of
    the known moons. They then generated a large set of hypothetical
    moons, orbiting Saturn on circular, equatorial orbits, positioned
    at radial intervals of 2000 km in semimajor axis and 2 degrees in
    initial longitude. Each simulated moon is advanced along its orbit
    and the number of images in which it should have been visible is
    determined. If that number exceeds three, then the team assume
    that a moon 3 km or larger should have been detected. By counting
    how many of the 180 moons at each orbital radius should have been
    detected, we determine the detection probability.
 
    The requirement for detecting a 2.5-km moon is more stringent. For
    these tinier moons, it is necessary that a minimum of three
    detections all fall at least 300,000 km from the center of the
    planet. This is the rough boundary where the background noise and
    variations settle down to a low enough level that a 2.5-km moon
    could be reliably detected.
 
    Fractional coverage fluctuates as a function of radius, with
    typical values in the 60-80% range between Enceladus and Titan.
    The overall coverage between the orbits of Mimas and Hyperion is
    71% for 3-km moons and 57% for 2.5-km moons. While the possibility
    remains that the team missed a few moons above the detection
    threshold, they feel it is unlikely that they missed several.  If
    the fractional coverage is f, then the probability of missing N
    moons is (1-f)^N. Thus, the probability of missing one 3-km moon
    is 29%; for two, the number drops to 8.4%; for three, it drops to
    2.8%. For 2.5-km moons, the probabilities are 43% that one was
    missed, 19% that two were missed , and 8% that three were missed.
    The team concluded that the inner Saturn system beyond the orbit
    of Enceladus is relatively free of undiscovered moons 2.5-3 km or
    larger in radius.
 
 
  Citing this dataset
  ===================
    The following is the recommended information to include in a journal
    citation of this dataset: Dones, L., M.R. Showalter, R.H. Durisen,
    R.K. Honeycutt, J.S. Jurcevic, R. Tripoli, and C. Strom, and D. Olson
    WIYN Observations of the November 1995 Saturn Ring Plane Crossing,
    WIYN-S-WI-2-RPX-V1.0, USA_NASA_PDS_RPX_0401,
    NASA Planetary Data System, 2010.