Data Set Information
|
DATA_SET_NAME |
WIYN S WI RAW RING PLANE CROSSING V1.0
|
DATA_SET_ID |
WIYN-S-WI-2-RPX-V1.0
|
NSSDC_DATA_SET_ID |
NULL
|
DATA_SET_TERSE_DESCRIPTION |
WIYN images obtained during the Saturn ring plane crossing in
November 1995.
|
DATA_SET_DESCRIPTION |
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.
|
DATA_SET_RELEASE_DATE |
2008-01-31T00:00:00.000Z
|
START_TIME |
1995-11-19T02:00:00.000Z
|
STOP_TIME |
1995-11-23T06:00:00.000Z
|
MISSION_NAME |
SATURN RING PLANE CROSSING 1995
|
MISSION_START_DATE |
1994-01-01T12:00:00.000Z
|
MISSION_STOP_DATE |
1997-01-01T12:00:00.000Z
|
TARGET_NAME |
FLAT FIELD
SATURN
|
TARGET_TYPE |
CALIBRATION
PLANET
|
INSTRUMENT_HOST_ID |
WIYN
|
INSTRUMENT_NAME |
WIYN IMAGER
|
INSTRUMENT_ID |
WI
|
INSTRUMENT_TYPE |
CAMERA
|
NODE_NAME |
Planetary Rings
|
ARCHIVE_STATUS |
ARCHIVED
|
CONFIDENCE_LEVEL_NOTE |
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.
|
CITATION_DESCRIPTION |
Dones, L., M.R. Showalter, R.H. Durisen, R.K. Honeycutt, J.S.
Jurcevic, R. Tripoli, and C. Strom and D. Olson, WIYN S WI RAW
RING PLANE CROSSING V1.0, NASA Planetary Data System, 2010.
|
ABSTRACT_TEXT |
This data set contains images of the Saturn system from the
Wisconsin-Indiana-Yale-NOAO(WIYN) using the WIYN Imager in late
November 1995. These observations were made during and
immediately after the ring plane crossing.
|
PRODUCER_FULL_NAME |
DANIEL M. OLSON
|
SEARCH/ACCESS DATA |
Rings Node Interface
Rings Online Archives
|
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