NEAR-NUCLEUS STUDIES NETWORK
1. THE STUDY OF NEAR-NUCLEUS PHENOMENA
1.1 Goals
In the broadest sense, the goal of near-nucleus studies is to understand
the processes taking place in the coma as they relate to the physical nature
of the cometary nucleus. Assuming that the observed coma distribution results
from the ejection of material from a possibly inhomogenous, rotating nucleus,
coma anisotropy can be used as a tracer of nucleus activity and motion. By
measuring the motions of coma features and extrapolating back to the time of
ejection, it is possible to locate the active areas on the nucleus, and by
observing a sufficient number of features over time, determine or constrain
the nucleus rotational motion. Knowledge of the distribution and evolution
of active areas may provide important clues to the internal structure of the
nucleus and of the comet formation environment. To fulfill these goals for
Comet Halley, the Near Nucleus Studies Network (NNSN) was designed to obtain
data on the spatial and temporal distribution of matter in the coma at the
highest possible resolution, especially during the period of maximum activity
in 1985-86.
1.2 Historical Perspective
When the International Halley Watch (IHW) was formed, cometary near
nucleus
study was an immature field with little quantitative foundation, based largely
on descriptive reports of primarily visual observations of coma morphology.
One of the most extensive of these was Bobrovnikoff's (1931) monograph on the
1910 apparition of Comet Halley. The potential of such studies was underscored
with the publication of the Atlas of Cometary Forms by Rahe et al. (1969),
which illustrated some of the interesting coma patterns in Comet Halley
observed over the previous two apparitions. The application of modern
photographic emulsions in recording the spectacular spiral jets in Comet
Bennett (1970 II) (Larson and Minton, 1972), the analysis of expanding haloes
to estimate nucleus rotation periods by Whipple (1978, 1980), and the
quantitative modeling of fans and jets by Sekanina (1979, 1981a,b) provided
further justification for a dedicated network to observe the expected changing
coma pattern of Comet Halley. From the onset it was clear that the NNSN
strategy for obtaining data would be similar to that of the Large Scale
Phenomena Network (LSPN), but that details, such as optimum detectors, plate
scales, temporal coverage, and acquisition of telescope time needed to be
defined.
2. STRUCTURE AND FORMATION OF THE NEAR NUCLEUS STUDIES NETWORK
2.1 Organization
The Discipline Specialists (DS) selected for the NNSN were Zdenek
Sekanina (Jet Propulsion Laboratory, California Institute of Technology; JPL)
and Juergen Rahe (Dr. Remeis Sternwarte, Bamberg, West Germany), who were to
manage, respectively, the western and eastern hemisphere efforts. Stephen M.
Larson (Lunar and Planetary Laboratory, University of Arizona; LPL) was
selected Deputy DS to assist Sekanina, but after the first year he was
appointed DS. With J. Rahe's responsibilities as co-leader of the IHW and DS
of the LSPN, and later his position at NASA Headquarters, and with Sekanina's
responsibilities as Archive Editor and co-investigator on two Giotto
experiments, it was decided that the day-to-day tasks of the NNSN would be
carried out by Larson at LPL in Tucson. At LPL J. Gotobed initially provided
volunteer computer and programming assistance, N. Connaro supplied part-time
clerical and data input assistance, and the Space Telescope Wide
Field/Planetary Telescope DEC VAX-780 computer housed in the Tucson NOAO
offices was made available on a limited basis by B. Smith (LPL) and R. Lynds
(NOAO). In mid-1985, D. Levy was hired part time to assist in all NNSN
activities. With the influx of data, part-time under-graduate students
assisted in various times with the archiving; S. Movafagh wrote and integrated
archiving software, M. Guengerich and M. Garlick assisted in the tedious data
input and tape handling chores. The NNSN personnel is summarized in Table I.
Table I. Discipline Specialist Team of the Near Nucleus Studies Network
______________________________________________________________________________
Team Member Affiliation Responsibility
______________________________________________________________________________
Stephen M. Larson Lunar and Planetary Laboratory Discipline Specialist
University of Arizona
Tucson, AZ 85721
U.S.A.
Zdenek Sekanina Earth & Space Sciences Division Discipline Specialist
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
U.S.A.
Juergen Rahe Dr. Remeis Sternwarte Discipline Specialist
Universitat Nurnberg-Erlangen
D-8600 Bamberg
Federal Republic of Germany
David H. Levy Lunar and Planetary Laboratory Assistant Discipline
University of Arizona Specialist 1985-1989
Marilyn Guengerich Lunar and Planetary Laboratory Archiving Assistant
University of Arizona 1988-1989
Shahin Movafagh Lunar and Planetary Laboratory Programmer
University of Arizona 1988-1989
______________________________________________________________________________
__
2.2 Recruiting
The effort to recruit observers started in mid-1982 with the first NNSN
Circular letter. Over the next two years more than 200 responses ranging from
general interest to specific plans to monitor Comet Halley were received from
50 countries. The second mailing included a questionnaire inquiring about
anticipated observing plans and equipment and a detailed technical note on
imaging techniques and standardization. The evolving mailing list remained at
about 250 through 1986 with the understanding that fewer than a half of these
were potential contributors. Subsequent NNSN Circulars were issued about every
six months and contained general information on the behavior of P/Halley, news
of the trial run on P/Crommelin, the P/Giacobini-Zinner Watch, technical notes
about imaging techniques, ephemeris information supplied by D.K. Yeomans, and
the results of our study of the 1910 photographs of P/Halley (see below). We
also provided information of a somewhat more general nature to the IHW
newsletters published and distributed by the Lead Center at JPL. We tried to
respond rapidly to individual inquiries which usually dealt with details of
observing techniques.
2.3 Study of the Photographs of Comet Halley from 1910
In 1983 Sekanina and Larson initiated a study of the high resolution
photographs of P/Halley taken in 1910. The aims were: 1) to develop image
processing methods for enhancing the low contrast coma features and for their
more reliable measurement, 2) to characterize the time scale of changes in the
features, 3) to understand better the coma pattern evolution, 4) to study
quantitatively processes of coma pattern formation, 5) to investigate the
predictability of jets to aid the flight projects, and 6) to provide a basis
for more intelligent design of ground-based imaging experiments. The first
result was a new image processing algorithm designed to enhance density
discontinuities in radial outflow from a rotating nucleus (Larson and Sekanina
1984). The enhanced images made it possible to identify and measure discrete
jets evolving into expanding envelopes over several days which enabled
quantitative particle trajectory modeling using the code developed by Sekanina
during previous years. It was then possible to find a self-consistent model
for the ejection of dust from discrete sources on
the sunward side of the rotating nucleus under the influence of solar
radiation pressure (Sekanina and Larson 1984). The additional images (Larson
and Sekanina 1985) and further modeling (Sekanina and Larson 1986) resulted in
a map of active areas on a spherical nucleus with a simple rotation period of
2.2 days. The limited number of images and short time span precluded
determination of a unique spin vector solution, but established the fact (well
before the space- craft flybys) that most of the dust was ejected from
discrete vents on the sunlit side of the nucleus.
The 1910 photographs revealed an emission phenomenon that produced spher-
ical shells of gas and dust followed by an expanding tailward jet. From the
June 2, 1910 event, it was possible to measure the gas and dust expansion
velocities at 1.4 and 0.4 km/sec respectively. Giving the appearance of a
bright, secondary (sometimes multiple) nucleus before extending tailward by
solar radiation pressure, this type of phenomenon appears to differ from the
usual sunward emission mechanism, and is not yet understood.
The study of the 1910 photographs established the need for images at a
rate of 2-3 per day for the major dust jets, and more if higher resolution
showed smaller overlapping jets. Ideally, the images needed to be scaled and
exposed to resolve the nuclear condensation (defined by the terrestrial
atmospheric seeing), and the outer envelopes up to some 100,000 km from the
nucleus.
2.4 Standard IAU Cometary Filters for Imaging
Standard filters for isolating the principal cometary spectral emissions
of CN, C3, C2, CO+, H2O+ and three continuum bands in the visible region were
defined by a special working group of Commision 15 at the Montreal meeting of
the International Astronomical Union (IAU) in 1979. These were selected as a
means of determining production rates with standard aperture photometers. The
originally distributed 25mm diameter filters were too small and of
questionable optical quality to be used for imaging studies of the spatial
distribution of the gas coma species. With support from the Lead Center, an
order was placed for 15 sets of 38mm diameter, optical quality filters that
would be purchased or borrowed by observers. The transmission specifications
were identical to the photometric filters (see the discussion and curves in
the Photometry and Polari- metry Network section), they had the same optical
thickness (no refocussing for achromatic input) and the size was a compromise
between detector size (field) and cost. After much delay in delivery from the
manufacturers, all the filters were traced in Tucson to confirm their blocking
and bandpass characteristics, and distributed to the groups requesting them in
1984. They proved valuable for those having good tracking capability over the
long exposures needed to produce adequate signal.
2.5 Interaction With the Flight Projects
The Inter-Agency Consultative Group (IACG) was formed to create a
mechanism for coordination between the several flight projects to maximize the
science return (Reinhard 1986). The IACG consisted of delegations from ESA
(Giotto), Intercosmos (VEGAs), Japan (Sakigake and Suisei), NASA (ICE), and
the IHW. The flight projects needed as much near real-time groundbased input
as possible; especially from the Astrometry Network as part of the Pathfinder
Project for spacecraft targeting (see the Astrometry Network section). A sub-
group (which included R.L. Newburn, Z. Sekanina, J. Rahe and D.K. Yeomans of
the IHW) worked on modeling the cometary dust environment that the spacecraft
would fly through (Divine et al. 1986). By 1984, it became clear that NNSN
dust jet study might help provide information on dust jet configurations and
the dust impact hazard, and S.M. Larson was added to the IHW delegation to
brief the IACG on our study of the 1910 photographs, and to explore the
possibility of providing the flight projects with real-time data on the
locations of dust jets. A computer link was established between Tucson and the
European Space Operations Center (ESOC) and a format for data transmission was
established to allow ESA and Interkosmos to predict the location of jets
during the flybys given jet source locations derived from groundbased images.
The concept proved overly optimistic given our limited understanding of the
spin vector and because the mechanism for rapid image transmission,
enhancement, measurement and analysis required much more time and resources
than were available. Such infor- mation might not have influenced the flight
profile given the high encounter velocity and short encounter time, but would
have been useful in helping interpertation of the spacecraft data. The most
that could be provided was a correct qualitative prediction of relative jet
activity during the three encounters given the roughly two day cycle of
activity observed at the Boyden Observatory two weeks before the VEGA-1
encounter. In spite of the limited input of the NNSN, the IACG experience
represented a milestone in international cooperation and data exchange on many
levels, and helped usher in a new era of openness. This spinoff may, in the
long run, become recognized as one of the most meaningful benefits of this
return of Comet Halley.
3. NNSN DATA PROCESSING
The primary goal of the archiving task was to make sure that all the
necessary data were accurately included with the images as FITS headers in the
form prescribed by the Lead Center. Image array data have always presented
special problems due to their sheer volume which must be reduced to one form
or another. Most first-time CCD observers found themselves overwhelmed with
the flat field and photometric reduction tasks. The result of underestimating
the problem caused most of the data to be submitted near the deadline, without
the prescribed headers, and without photometric calibration in most cases.
3.1 Data Requirements
We requested that observers send their data in the Flexible Image
Transport System (FITS) format, with additional header keywords specific to
NNSN data. In practice, less than 20 percent of observers were able to
generate the full NNSN headers; many images were submitted in unusual formats
produced by local image processing packages. Likewise, we requested 16-bit
integer data, but often received 32-bit integer and 32-bit real data numbers.
Most of our original requests stemmed from our own specialized and limited
image handling software which was later replaced by more general software.
Although photometric calibrtion was encouraged, it was not a requirement.
3.2 Data Flow
After much experimentation and many false starts, we adopted the
following data handling procedure, much of which was dictated by available
hardware. Upon receiving a tape, a folder was opened to hold correspondence
and any accompanying hard copy. The data set was entered on a job status board
that included boxes that would be checked off at appropriate milestones. The
tape was then read into the ST/NOAO VAX 780 computer using the program DOMAIN,
and each image was inspected, selected, graded, had orientation determined,
and comet maximum and sky background level measured. At LPL, permanent
file numbers and system codes would be assigned to the images selected for
inclusion in the archive. Unique system codes were entered in a file for all
combinations of data related to the observatory, telescope, detector, array
size, filter and observers. This minimized the times these data had to be
entered for each of 250 configurations used. The file number, date, time
(either beginning or mid-time), system code, exposure, filter, observer(s),
orientation, quality, comet maximum, sky background and comments would then
be entered into the INFORMIX database program in a Charles River Data Systems
Computer. The database program had 50 fields per record including all of the
FITS keyword entries. After all of the entries for a data set were in the
data- base, a chronological report was written with the most critical entries
arranged on one line for easy proofreading. After proofreading, a program was
run that searched by file number and utilized the database entries and system
code file to run D.K. Yeomans' two-body comet ephemeris generating program to
calculate the location of the comet and the airmass using the observatory
coordinates. The program used the osculating elements most appropriate for
the time of observation and placed the selected computed values in the
remaining database fields. The program also ran an error check on the few
redundant entries in the system code file and the database. With the database
fields completed, a report was written that had the precise form of the
required headers. The headers were then printed out and proofread before being
transmitted by modem (twice for error checking) to the VAX and combined with
the renamed images for transmission to JPL on magnetic tape. This seemingly
orderly procedure was usually interrupted by the need to obtain some missing
information from the observers, or for the management of limited disc space.
The adopted procedure was in place and operational only during the last year
when all of the images were prepared for the archive. Previous attempts to
edit headers were simply not accurate enough, and program debugging took
much of one year prior to the final production run.
3.3 Data Selection
The NNSN data are of uneven quality for many possible reasons, and sub-
standard images were often included in cases when there were no other data.
The rationalization for including substandard images is that it may be
possible, with future sophisticated image reconstruction software, to
compensate for imperfect focus or guiding that currently limits the value of
these images. It was felt that a present-day archivist cannot accurately
predict what data may be useful in the future as far as the next apparition of
Comet Halley. In some cases, the images are very weak, but these were usually
the only narrow- band images available. In most cases, information on
identified defects is included in the header comments. Discussion of the
quality ratings can be found in Sec. 7.1.
4. THE CONTRIBUTED DATA
Correspondence indicated the existence of about 4000 images that might
qualify for the archive, and about 3700 were received. Most observers
had done a good job of filtering out the unusable data, and 3540 images
were cataloged and reformatted for the archive. Digital CCD images comprise
98% of the collection. Many observers, faced with the task of flat fielding
large numbers of CCD images for the first time, submitted images up until
the deadline. With the exception of 65 photographs digitized at the NNSN
Center, only reduced data were accepted. It was assumed that the observers
were the best qualified and equipped properly to flat field their images.
Improved detectors, more telescopes, and dedicated observers overcame the
poor observing circumstances of Comet Halley's 1986 apparition to obtain
some truly remarkable images that show much finer detail than the best images
taken during the very favorable 1910 apparition. This, and the fact that
there is nearly continuous coverage for 2 months both pre- and post-
perihelion, provides an unprecedented data set for the study of near-nucleus
phenomena for any comet.
It should be noted that the jet structure is typically of low contrast
superimposed on a steep intensity gradient radial to the central condensation,
and as such, spatial filtering algorithms usually must be applied to enhance
the visibility of these features. There are a number of image processing
packages in use that have adequate utilities to enhance the digital images in
this archive.
4.1 Time Distribution of Images
The Halley Archive contains 3540 NNSN images, which range in time from
recovery in 1982 through the 1989 observing season. The number of images is
near the middle of our range of early estimates. The majority of images (about
three quarters) were taken from mid-October 1985 through May 1986, during
the period of major jet activity, when the comet was within 2 AU of the Sun.
The coverage (see Table II) was excellent during the spacecraft encounters.
There are some gaps during full moon periods pre-perihelion, as well as
solar conjunction Jan. 29 through Feb. 27, 1986.
Table II. Daily Number of NNSN Images From October 18, 1985 to May 18, 1986
______________________________________________________________________________
Day of 1985 1986 Day
of
month Oct. Nov. Dec. Jan. Feb. Mar. Apr. May
month
______________________________________________________________________________
1 3 2 0 0 19 20 21 1
2 4 2 26 0 0 8 19 2
3 11 1 11 0 18 16 4 3
4 4 2 44 0 26 21 6 4
5 0 2 29 0 23 35 4 5
6 0 0 38 0 40 52 13 6
7 4 20 36 0 41 30 20 7
8 5 17 50 0 52 11 18 8
9 5 12 11 0 29 22 11 9
10 20 10 6 0 12 15 7 10
11 11 12 23 0 45 19 0 11
12 10 61 6 0 40 16 2 12
13 9 5 28 0 49 3 0 13
14 24 17 5 0 32 33 8 14
15 2 23 0 0 12 41 4 15
16 12 15 5 0 5 29 3 16
17 5 10 11 0 9 21 16 17
18 15 53 2 1 0 13 13 10 18
19 16 7 4 8 0 6 15 19
20 4 7 25 7 0 6 5 20
21 2 36 7 1 0 16 7 21
22 18 1 5 2 0 26 14 22
23 15 1 1 2 0 55 34 23
24 14 1 5 1 0 23 20 24
25 0 0 2 1 0 31 9 25
26 0 0 9 1 0 9 19 26
27 0 0 5 0 18 17 24 27
28 2 1 7 2 31 23 41 28
29 2 1 2 0 0 30 29
30 2 2 1 0 12 12 30
31 5 1 0 19 31
______________________________________________________________________________
4.2 Source Distribution
Images obtained by 80 observers in 25 groups working at 23 observatories
in 10 countries are represented in the archive (Table III). Several observers
used equipment at more than one observatory during the apparition. Apertures
from 0.3 to 5 meters were employed at most of the major observatories. The
largest longitude gap was between the European and Chilean observatories.
Table III. Observatories and Observers Contributing to the NNSN IHW Archive
______________________________________________________________________________
OBSERVING STATION
_________________________________ ELEV. APER. OBSERVERS
(M) (M)
E LONG LAT OBSERVATORY
______________________________________________________________________________
0244500 +414200 BNAO-ROZHEN 1750 0.7 SHKODROV,V/IVANOVA,V/BONEV,T
/BELLAS,Y
0262418 -290218 BOYDEN OBS. 1378 1.5 TAPIA,S/SENAY,M/LARSON,S
0344548 +303548 WISE OBS. 900 1.0 SCARROTT,S/WARREN-SMITH,R
0691500 +381800 SANGLOK 2302 1.0 KISELEV,V/SIKLITSKY,V/CHERNOVA,G
/AMIRKHANJAN,V
0724300 +243900 GURUSHIKAR OBS. 1700 0.4 CHANDRASEKHAR,T/DEBIPRESAD,C
/ASHOK,N
1160806 -320029 PERTH OBSERVATORY 407 0.6 A'HEARN,M/HOBAN,S/BIRCH,P/CANDY,
M/MARTIN,R
1310445 +311500 KAGOSHIMA SP. CTR. 228 0.6 TAKAGISHI,K/TOMITA,K/WATANABE,J
/EIRAKU,M
1333606 +343426 OKAYAMA AST. OBS. 372 1.9 WATANABE,J/KAWAKAMI,H/KINOSHITA,
H/NAKAMURA,T/NORIMOTO,Y/OKITA,K
/SHIMIZU,M/TOMITA,K
1391141 +360021 DODAIRA AST. OBS. 879 0.9 WATANABE,J/AOKI,T/HIRAYAMA,T
/KAWAKAMI,H/MURATA,Y/NAKAMURA,T
1393229 +354021 TOKYO AST. OBS. 59 0.7 HATANAKA,Y
1490358 -311637 ANGLO-AUSTRALIAN 1164 3.9 GREEN,S/HUGHES,D
1702754 -435915 MT. JOHN 1029 0.6 GILMORE,A
2043140 +194934 MAUNA KEA 4214 2.2 STORRS,A/BUIE,M/GOGUEN,B
/CRUIKSHANK,D/LARK,N/HAMMEL,H
/BELTON,M/MEECH,K/ALVAREZ,L
2043140 +194935 MAUNA KEA 4215 3.6 GOLDBERG,B/HALLIDAY,I/AIKMAN,C
2430810 +332122 PALOMAR 1706 5.1 JEWITT,D/DANIELSON,G
2430829 +332056 PALOMAR 1706 1.5 PORTER,A/SELMAN,I
2482006 +351214 LOWELL OBSERVATORY 2204 0.6 A'HEARN,M/HOBAN,S/WANG,Z/
SCHLEICHER,D/FEIERBERG,M/LUTZ,B
/SAMARASINHA,N
2482408 +315729 KITT PEAK NATIONAL 2096 2.1 JEWITT,D/MEECH,K/BELTON,M/
ALVAREZ,L/WEHINGER,P/MCCARTHY,D
2482402 +315750 KITT PEAK NATIONAL 2120 4.0 MEECH,K/JEWITT,D/DJORGOVSKY,S
/SPINRAD,H/WILL,G/BELTON,M
2485940 +321248 TUMAMOC 0950 0.5 LEVY,D/LARSON,S/MAGEE,M
2491243 +322633 MT. LEMMON 2790 1.5 FINK,U/LEVY,D/WISNIEWSKI,W
2491605 +322501 CATALINA 2510 1.5 LARSON,S/LEVY,D/HOBAN,S/FINK,U
/DISANTI,M/SCHULTZ,A/MARCIALIS,R
/FINK,R
2885850 -325854 CERRO EL ROBLE 2220 0.7 TORRES,C
2891106 -300956 CERRO TOLOLO 2225 1.5 LARSON,S/TAPIA,S
2891106 -300956 CERRO TOLOLO 2225 1.5 MEECH,K/JEWITT,D
2891605 -291518 EUROPEAN SOUTHERN 2347 1.5 FRANDSEN,S/REIPURTH,B
/GAMMELGAARD,P/PEDERSEN,H/WEST,R
/JOERGENSEN,H/KJAERGAARD,P
/HAEFNER,O
2891802 -290023 U. TORONTO S. OBS. 2276 0.6 LARSON,S/TAPIA,S/SHELTON,I
______________________________________________________________________________
5. IN RETROSPECT
The operation of the NNSN was an experiment in many ways. Vastly improved
technology and communications since 1910 provided new tools as well as new
challenges. The NNSN was started just as CCD's began replacing the
photographic emulsion as the areal detector of choice in astronomy. Because
of this changeover, it was difficult to predict and plan for the outcome.
Most observers severely underestimated the time and effort needed to
decalibrate their CCD images, and financial support diminished after the
excitement of the flybys. The details of preparing the data for the archive
changed many times during the course of the campaign, and most of it was
learned and designed as we went. The trial runs were invaluable in providing
experience with the new detectors at the telescope, as well as demonstrating
the inadequacies of our early concepts of data handling.
With the advantage of hindsight, it is probable that the IHW would have
been more efficient over its lifetime if the networks could have used the same
computer hardware and shared the same data handling software developed by one
group (say at the Lead Center). As it was, personnel of each network
implemented their own systems to the same end with considerable duplication of
effort. On the other hand, such a strategy requires definitions of the needs
well in advance, and may restrict flexibility for later changes.
The NNSN solicited data from anyone who might have had the capability
to acquire images, even as an adjunct to other programs, but a look at the
statistics of the NNSN shows that the bulk of the data came from relatively
few people using dedicated systems on moderate aperture telescopes. The
lesson may be that future imaging networks would best spend their resources
contracting with a few observers who have easy access to telescopes and can
dedicate more time to the task.
The Halley Archive does not contain all of the useful images available
for near-nucleus studies, since some observers were not able to prepare their
images by the time of our deadline. Had the NNSN continued for another year,
perhaps 10-15% more data could have been archived, and it still would not have
been complete. Future investigators may find a few years from now that the
NASA Planetary Data System contains additional Comet Halley images.
6. SCIENCE HIGHLIGHTS
The science results obtained from NNSN data are too numerous to sumarize
completely here, so we mention only a few of the highlights.
Nucleus spin vector -- The observed dust jet curvature indicates that the
nucleus rotates in a prograde sense and the sources have an instantaneous
apparent period of around two days. The dust jet morphology repeats quite
accurately with the 7.4 day light curve (Millis and Schleicher, 1986; Larson
and Sekanina, 1987), indicating that the nucleus orientation in space repeats
with that frequency. This implies complex rotation for which a unique
solution has not been identified as of December 1989.
Dust jets--Measured outward projected velocities of dust in the jets
range from 0.2-0.6 km/s (Larson and Sekanina, 1988). There is evidence for
variable size distribution in the dust jets (Hoban et al, 1989).
Gas jets--Discrete jets of gas (CN, C2 and C3) were observed for the
first time in a comet (A'Hearn et al., 1986a,b), but they do not correlate
well with the dust jets (Larson et al., 1987; Larson and Sekanina, 1987). The
exact origin and mechanism of the gas jets are still debated (Larson, 1988).
Similarity with 1910--The type of coma morphology in 1910 and 1986 was
very similar, including the straight "tailward" jets (Larson et al., 1987).
7. THE NNSN ARCHIVE
This section is intended to assist investigators in the use of the NNSN
images contained in the archive by defining and explaining the network-
specific FITS header keywords, index entries, filter bandpasses, and any
peculiarities in specific data sets. There is also information on known data
sets that for various reasons could not be included in this archive.
The 3540 uncompressed NNSN images in the archive are arranged chronologi-
cally on the CD-ROM discs together with the other non-LSPN data and are
accessed as FITS files in the same manner as the other data on the discs. Each
image is accompanied by a FITS header intended to provide nearly all of the
information a user needs to know about the image. Every effort was made to
ensure accuracy of the entries, but users are advised that the source of any
apparent inconsistency should be investigated by them. Mistakes could have
been made anywhere from the observers' logs through the NNSN database entry,
and even perhaps in the CD-ROM mastering process. The submitting institution
and observer's names should allow users to track down and resolve apparent
inconsistencies.
There are no calibration frames included in the archive. Calibration
information, such as step wedge input intensities and output counts, are
included in the HISTORY or COMMENT lines. In some cases, there might be
conversion factors from counts to magnitude or flux units. In other cases,
total counts, times and exposures of standard stars are given.
7.1 The NNSN FITS Header
The headers that accompany the images begin with the five mandatory FITS
keywords plus 35 additional entries. The header keywords are listed in Table
IV.
Table IV. The NNSN Header Keywords
______________________________________________________________________________
SIMPLE = T /T conforms to standard FITS format
BITPIX = 16 /16 (or 32)-bit data
NAXIS = 2 /number of axes in array
NAXIS1 = ____ /number of pixels in X axis (samples)
NAXIS2 = ____ /number of pixels in Y axis (rows)
OBJECT = 'P/HALLEY' /object name
FILE-NUM= 4_____ /file number
DATE-OBS= '__/__/__' /mid-UT date of observation (dy/mo/yr)
TIME-OBS= ._____ /mid-UT decimal part of day
DATE-REL= '__/__/__' /date released to archive (dy/mo/yr)
DISCIPLN= 'NEAR NUCLEUS' /IHW network
LONG-OBS= '___/__/__' /observatory east longitude (deg/min/sec)
LAT--OBS= '___/__/__' /observatory latitude (+-deg/min/sec)
SYSTEM = '4_______' /observing system code
OBSERVER= '__________________' /observer's names (see ADD. OBS.:in COMMENT)
SUBMITTR= '__________________' /submitter's names
SPEC-EVT= _ /T if jets present (10/85-6/86)
DAT-FORM= 'STANDARD' /type of data
OBSVTORY= '__________________' /observatory name
ELEV-OBS= ____ /elevation of observatory in meters
TELESCOP= '__________________' /telescope used
APERTURE= _.__ /telescope aperture in meters
TELEFL = _.___ /effective focal length in meters
PLTSCALE= __.__ /plate scale in arcsec per mm
CROTA1 = ___._ /position angle of sample axis, north -> east
SENSE = _ /PA counterclockwise (T) or clockwise (F)
DETECTOR= '__________________' /detector used
DIGITIZE= '__________________' /type of digitizer used if not detector
APSIZE = _.___ /original pixel size of detector/digitizer
FILTER = '______ ' /filter used (see Table IV)
EXPOSURE= ____._ /exposure duration in seconds
AIRM-MID= _.___ /calculated airmass at mid-exposure
QUALITY = '_________' /general quality of image
DATE-WRT= '__/__/__' /date this file written
ORIGIN = '__________________' /institution sending data to NNSN
BUNIT = '__________________' /intensity units (note that BSCALE=1, BZERO=0)
COMETMAX= ______ /approximate maximum value in comet image
SKYMIN = ______ /approximate background sky brightness
COMMENT (any comments relating to the contents of the image and additional
observers)
HISTORY (any comments relating to the reduction process)
END
_____________________________________________________________________________
The 250 system codes are specific to any detector, array size, telescope,
optical configuration and filter, and are of the form 4NNNXXYY, where 4
denotes the NNSN, NNN is the IAU observatory code, XX the telescope, and XX
the filter/ scale/array size configuration. A list of observers are associated
with each system code, so more than one observer group may share a system
code.
The quality rating is only a rough, qualitative guide that includes the
effects of seeing, focussing, guiding, the signal-to-noise ratio, and
decalibration. The four rating categories are excellent, good, fair, and poor:
excellent refers to images with no obvious flaws and poor denotes images
included only because of the lack of better ones on that day. The user will
have to gain some experience to know what to expect from the different grades.
The airmass at mid-exposure (AIRM-MID) is calculated from current epoch
topocentric comet coordinates (RA, DEC) and observatory coordinates (LONG,
LAT)
as:
AIRM-MID = sec Z - 0.0018167 (sec Z - 1) - 0.002875 (sec Z - 1)**2
- 0.0008083 (sec Z - 1)**3,
where the zenith distance Z follows from:
sec Z = 1/[sin(LAT)*sin(DEC) + cos(LAT)*cos(DEC)*cos(local hour angle)].
The airmass at the zenith is 1, so it does not include a correction for the
local elevation. Also note that there is no correction for apparent and true
zenith distance.
7.2 NNSN Index Entries
For each image in the digital archive, there is an entry in the NNSN
index that includes useful information the users will need in order to
determine if that image may satisfy their needs. All items are derived from
the NNSN extended FITS headers. Each entry item is described in Table V.
Table V. List of Index Entries
___________________________________________________________________________
Heading Description
____________________________________________________________________________
Date(UT) Date (day & fraction of day) of middle of observation
NNSN# Near Nucleus Studies Network file number
Filtr Filter used (see Table VI)
Detector Detector used
Field Angular field of axes (arcmin) derived from the array size
and the plate scale. The field may actually be smaller due to
field stops or vignetting
PAX Position angle (N through E) of NAXIS1 (degrees)
ExpS Exposure duration (seconds)
Pixl Angular scale of picture element (arcsec)
Ap Telescope aperture size (meter)
Scale Effective plate scale at the detector (arcsec per mm)
System Observing system code (see sec. 7.1)
Observer(s) Name(s) of the observer(s)
Notes Notes from HISTORY or COMMENT keywords, footnotes
______________________________________________________________________________
7.3 Printed Archive Images
The printed archive contains one representative halftone image every few
days to give the user a general idea of the appearance of the comet. The
images are reproduced with the same orientation (north up, east to the left)
and scale (200,000 km on a side at the comet). To permit greater visibility of
the near-nucleus region as well as some of the outer coma, the base 10
logarithm of the counts is displayed. The final prints have similar densities
and contrasts to maximize visibility of the comet, but the halftone process
may degrade the dynamic range further. For detailed study, the user should use
images from the digital archive.
7.4 Filters
Table VI lists the filters used and their wavelengths at 50 percent (and
for some also at 10 percent) of maximum transmission. These numbers do not
take into account detector responses or atmospheric extinction. For more
information on transmission characteristics, the observers should be contacted
directly. There are five general categories of filters:
(1) Four standard broadband photometry filters sets are not specifically
intended to isolate cometary emissions. During the period of maximum activity
(January-April 1986) the dominant source was dust, but the V and B bands have
sizable contributions from C2 and CN emissions. Some filter data were obtained
from Thuan and Gunn (1976) and from Bessell (1979).
(2) Schott glass filters were used primarily as short-wavelength cutoff
filters. The table indicates the 10 percent and 50 percent cutoff wavelengths,
while the effective peak and the long wavelength cutoffs were defined by the
detectors.
(3) IHW/IAU cometary filters are imaging quality versions of the standard
photometry bandpasses. The values in the table are the mean of all those
measured and are correct to 1 nm. Further details can be found in the Photo-
metry and Polarimetry Network summary. Our IHW filter names are intended to
indicate the effective peak wavelengths (in nm) and the emissions they should
isolate. The continuum bands are indicated by BC (blue continuum), MC (mid-
continuum) and RC (red continuum).
(4) Giotto Halley Multicolor Camera filters were used to provide ground-
based calibration during the flyby. The filter characteristics are taken from
Keller et al. (1982).
(5) Special bandpass filters are the remaining miscellaneous filters that
observers usually selected to investigate special spectral features. A parti-
cularly useful example is the 618H2O+, which isolates the (0,8,0) emission of
H2O+ and provides excellent images of the ion tail.
Table VI. Characteristics of Filters Used for NNSN Images.
______________________________________________________________________
Category Cut-on [nm] Cut-off [nm]
____________________ _______________ Central ________________
Wavelength
IHW name Other name 10% 50% [nm] 50% 10%
______________________________________________________________________
Standard Broadband
B Johnson B 386 440 494
V Johnson V 491 548 605
R Johnson R 585 650 715
I Johnson I 729 825 921
Gunn G 458 493 528
Gunn R 610 655 700
Gunn I 690 780 880
Mould B 386 442 498
Mould V 501 546 591
Mould R 585 647 708
Mould I 732 829 927
Cousins B 390 440 530
Cousins V 500 550 600
Cousins R 620 640 700
Cousins I 700 790 900
Wide B 400 450 500
Wide R 550 560 - - -
Schott Glass
GG7 475 460 - - -
GG11 420 480 - - -
GG13 440 490 - - -
GG455 450 455 - - -
GG495 490 495 - - -
OG530 525 530 - - -
RG1 600 610 - - -
RG610 600 610 - - -
RG665 660 665 - - -
IHW/IAU Comet Imaging
309OH 305 306 309 313 314
365BC 360 362 365 368 370
387CN 384 386 387 390 401
406C3 401 402 406 409 410
426CO+ 422 423 426 429 430
485MC 481 482 485 487 489
514C2 507 508 514 519 520
684RC 678 679 684 688 689
703H2O+ 689 691 703 713 714
Giotto Multicolor Camera
314OH C10 293 295 298 323 336
408C3 C11 399 403 410 419 422
HMCB C5 320 336 395 488 489
450BC C8 441 443 453 457 458
509C C12 499 502 511 520 524
HMCO C4 578 585 652 702 714
731RC C9 715 718 728 743 746
HMCR B/C3 695 705 - - -
Special Bandpasses
315OH 310 315 320
457CO+ 454 457 459
598NH2 596 598 600
600HN2 580 600 620
619H2O+ 617 619 621
625CONT 624 625 625
630OI 628 630 632
630TLT 627 629 631
H ALPHA 646 656 666
701RC 691 701 711
852CONT 826 852 878
860CONT 854 860 866
910CN 900 910 920
918CN 912 918 924
______________________________________________________________________
7.5 Notes on Specific Data Sets
Listed below some features of some specific data sets that the user
should be aware of.
The Wise Observatory polarization images of Eaton (402601-21) are broken
into separate fields separated by blank spaces.
The COMETMAX and SKYMIN values for the images taken when Comet Halley
was at large heliocentric distances (400001-012, 403801-950, 406001-124) are
only guideline values to produce a good display, since the comet is sometimes
only a few counts above the background.
The photographs from Mt. Johns Observatory and Cerro El Roble (402501-
546, 402551-566) were digitized using an LPL CCD camera and macro lens
adjusted to give an appropriate scale. When there is sensitometric
calibration, the digitized step values and their relative input intensities
are given in the header comments. The Cerro El Roble films had multiple
exposures, so when there are two images per field, the time and exposure of
the faintest image is given in the header comment. The user should be aware
that there will be two sets of field stars in these images.
8. ADDITIONAL DATA SETS
Table VII lists the data sets that for various reasons do not appear
in the archive, but might be potentially useful for future near-nucleus
studies. Some images in the Large Scale Phenomena Network, digitized to about
4 arcsec per pixel, might also be useful in studying evolved coma features.
Table VII. Data Sets not Listed in the Archive.
_____________________________________________________________________
Institution Observer Observatory Aperture (m)
_____________________________________________________________________
Univ Bejing Liu,Z Yunnan 1.0
Univ Calif Spinrad,H et al. Lick 3.0, 0.5
Univ. Catania Cristaldi, S. Catania Obs. 0.3
U Coll London Rees,M et al. Table Mt (JPL) 0.6
Univ Hamburg Kohoutek,L ESO 2.2
Klet Mrkos,A Klet 0.6
Univ Kyoto Akabane,T Hida 0.6
Univ Liege Dossin,F et al. Haute-Provence 0.6
Notre Dame Univ Rettig,T AAO 2.3
Meudon Kohl,J et al. Haute-Provence 1.9
Meudon Lecacheux,J et al. Pic-du-Midi 2.0
Osmania Univ Kilambi,G et al. Japal 1.3
Univ Padova Barbieri,C et al. Mt Ekar 1.8
SAAO Mack,P et al. SAAO 1.9
Univ Texas Barker,E et al. McDonald 2.1, 0.9
US Naval Obs Luginbuhl,C et al. USNO Flagstaff 1.5
_____________________________________________________________________
9. ACKNOWLEDGEMENTS
The contribution of the Near-Nucleus Studies Network to the Halley
Archive was made possible by the many dedicated observers worldwide and their
unselfish cooperation. Participation in the NNSN has been strictly voluntary.
The results benefit cometary and space science and have helped foster
international cooperation. We thank Ya. Yatskiv (Ukrainian Academy of
Sciences) and I. Williams (Queen Mary College, University of London) for
organizing and transmitting data from observers in their countries. We thank
B.A. Smith (LPL) and R. Lynds (NOAO) for use of the ST VAX780 computer at
NOAO. E. O'Neil (NOAO), S. Movafah (LPL), and J. Gotobed (LPL) provided
assistance with the VAX. M. Aronsson of the Lead Center provided helpful
suggestions and assistance. M. Guengerich (LPL) did an extraordinary job with
data entry in our database and constructing the FITS headers.
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