| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains images of comets observed with NASA's Solar and
Heliospheric Observatory (SOHO). The vast majority of these comets are
sungrazing comets which were discovered from publicly available SOHO
images. Approximately 950f the SOHO discovered comets are members of
one of the following families of comets:
Family References
------ -----------------------------------------------------
Kreutz Kreutz, H. (1888) Publ. Sternwarte Kiel, No. 3, 1-111
[KREUTZ1888]
Kreutz, H. (1891) Publ. Sternwarte Kiel, No. 6, 1-67
[KREUTZ1891]
Kreutz, H. (1901) Astron Abhand., No. 1, 1-90
[KREUTZ1901]
Marsden, B. (1967) Astron. J., 72, 1170-1183
[MARSDEN1967]
Marsden, B. (1989) Astron. J., 98, 2306-2321
[MARSDEN1989]
Biesecker et al. (2002) Icarus, 157, 323-348
[BIESECKERETAL2002]
Marsden Marsden, B. (2002) MPEC 2002-E18 [MARSDEN2002A]
Meyer (2003) ICQ, July 2003, 115-122 [MEYER2003]
Meyer Marsden, B. (2002) MPEC 2002-E18 [MARSDEN2002A]
Meyer (2003) ICQ, July 2003, 115-122 [MEYER2003]
Kracht Marsden, B. (2002) IAU Circ 7832 [MARSDEN2002B]
Meyer (2003) ICQ, July 2003, 115-122 [MEYER2003]
The comet images in this data set were taken with the Large Angle
Spectrometric Coronagraph (LASCO) onboard SOHO. LASCO consists of
three coronagraphs, C1, C2, and C3 with nested fields-of-view, centered
on the Sun. The innermost coronagraph, C1, has a narrow passband
Fabry-Perot interferometer tuned to hot coronal emission lines so
detection of comets was not expected. It has not been used regularly
since June 24, 1998. Thus, no comets were ever detected in C1, and no
C1 images are archived here. Some comets in this data set were seen in
only C2 or C3, however others were seen in both coronagraphs. Both C2
and C3 are externally occulted coronagraphs with annular fields-of-view
from about 2.5 to 6 solar radii for C2 and 4 to 32 solar radii for C3.
Both coronagraphs have 1024x1024 pixel CCD cameras, resulting in pixel
sizes, projected onto the sky, of 11.9 arcseconds for C2 and 56
arcseconds for C3.
A small group of synoptic programs occupy most of the observing day.
Originally, the white light synoptic program obtained an image from C1
and C2 every 30 minutes, and an image from C3 every hour, with most
images having a reduced field of view. However, C1 has not been used
regularly since June 24, 1998. Thus, the telemetry has been
re-allocated, and typically three full-resolution (1024x1024) C2 orange
and two full-resolution (1024x1024) C3 clear images are taken per hour.
In addition, a polarization sequence using C2 and C3 is taken 1-2 times
per day. A color sequence is taken with C2 and C3 once per day. The
synoptic programs utilize approximately 850f the available daily
telemetry. The remaining available telemetry can be devoted to special
observations.
Since the malfunction of the High Gain Antenna in mid-2003, it has been
necessary to roll the spacecraft by 180 degrees every three months. As
a result, the spacecraft goes through a 'keyhole' period lasting about
three weeks every time the space craft is rolled. During a portion of
this time some instruments are shut down, freeing up bandwidth/memory
and allowing an increased cadence of the C2 camera. For the days on
either side of the roll maneuver five C2 images are taken per hour
rather than the nominal three. This has resulted in a slightly higher
number of comet discoveries during these periods.
A more extensive discussion of the data can be found in Biesecker et
al., 'Sungrazing Comets Discovered with the SOHO/LASCO Coronagraphs
1996-1998', Icarus, 157, 323-348, 2002 [BIESECKERETAL2002]. A complete
description of LASCO can be found in Brueckner et al., 'The Large Angle
Spectroscopic Coronagraph (LASCO): Visible Light Coronal Imaging and
Spectroscopy', Solar Physics, 162, 357-402, 1995 [BRUECKNERETAL1995].
A more in depth description of C3 can be found in Morrill et al.,
'Calibration of the SOHO/LASCO C3 White Light Coronagraph', Solar
Physics, 233, 331-372, 2006 [MORRILLETAL2006].
Processing
----------
The SOHO team calculated an offset bias and exposure correction
factor for each image. The exposure correction factor is the ratio
of the true exposure time to the exposure time recorded in the header
for a given image, typically very close to 1. The offset bias for a
given exposure is subtracted from the raw image. Next, the exposure
correction factor for the exposure is multiplied by the exposure time
in the FITS header, resulting in the true length of exposure. The
bias subtracted image is then divided by the true length of exposure
to convert it to a rate (DN/sec). This bias subtracted, normalized
image is then multiplied by the vignetting function for the telescope
and filter used in the image (the C3 vignetting differs before and
after the 1998 interrupt, however the C2 vignetting did not change).
A background image is calculated by taking the median (the mean of
the middle two values at each pixel) of four images (each processed
as above); two prior to the image of interest, and two after the
image of interest. The images used in the background calculation are
chosen to be as close in time to the image of interest as possible
without contamination from either the comet, background stars, or
other abnormalities. Only images with the same telescope
configuration (detector, filter, polarizer, summing, and rotation)
are used for processing. Occasionally fewer than four images are
available for the background calculation, in which case the maximum
number available are used. The background image is subtracted from
the processed image yielding the final processed image which is in
DN/sec.
The processing is summarized by this equation:
processed image = [(raw image - bias) / (exposure time
* exposure factor) * vignetting]
final processed image = processed image
- median of neighboring processed images
Since the vast majority of SOHO comets are very faint and are hard to
locate even in fully reduced images, 41 x 41 pixel subimages centered
on the brightest pixel of the comet of interest have been extracted
from the final processed image and are archived here.
The offset bias, exposure time, exposure correction factor,
vignetting, X and Y positions of the comet in the original image,
and the minimum and maximum X and Y positions of the subimage in
the original image are recorded in the FITS header. Additionally,
the offset bias, exposure time, and exposure factor for each image
used to calculate the background image are included in the FITS
header.
Filters
-------
The following filters and polarizers are used in C2 and C3
imaging:
Filter Coronagraph Nominal Bandpass (nm)
-------------------- ----------- ---------------------
Blue C2 and C3 420 - 520
Orange C2 and C3 540 - 640
Light Red C2 620 - 780
Deep Red C2 and C3 730 - 835
H alpha C2 and C3 2.0 nm at 656.3 nm
Infrared C3 860 - 1050
Clear C3 400 - 850
Polarizer at 0 deg C2 and C3 400 - 850
Polarizer at +60 deg C2 and C3 400 - 850
Polarizer at -60 deg C2 and C3 400 - 850
Calibration
-----------------
The processed data can be converted to approximate V magnitudes by
the following relation:
V = a - 2.5 * log_10(totcts/sec)
where V is the V magnitude, totcts/sec is the DN/sec value described
above within the aperture of interest, and a is the magnitude
correction computed using F, G, and K type stars observed in LASCO
images. The calibration can be applied for each of the C3 filters,
and for the C2 orange filter. Values of a for each telescope/filter:
Telescope Filter a
--------- -------- -----
C2 Orange 11.62
C3 Clear 10.95
C3 Orange 9.72
C3 Deep Red 9.90
C3 Blue 8.67
C3 IR 8.71
The error in the magnitude due to the zero point calculation (a)
varies by year and filter, but is estimated to be 0.05 magnitudes.
Data
----
The data consist of images from SOHO-LASCO, from 1996-2005. The
images are grouped into directories by year and comet and numbered
sequentially for each image with a given coronagraph for a given
comet. The file names thus indicate the IAU name of the comet
(four-digit year plus letter and number designation, for example
1996a2), the telescope used to take the image (c2 or c3), and the
three-digit image number in the sequence, with the first image for a
given comet in a given telescope numbered 001. An example of a
typical filename is: 1996a2_c3_008.FIT. The time recorded in the
FIT header is the UTC time that the light was received at the
spacecraft.
Each processed *.FIT file has a detached PDS label (*.LBL) file which
contains information about the image in standard FITS keywords.
Occasionally, images were encountered that failed processing.
Failures were due to a variety of causes, most frequently because the
image was of size 0x0 or had an exposure time of 0 seconds. These
images were not included in the data set.
Month by month plots of exposure correction factors can be found in
the document/calib_docs/exp_fac/ directory of this archive. Yearly
plots of the sun position and roll angle (the angular deviation in
degrees of the image from heliographic north) can be found in the
document/sun_position/ directory of this archive.
The current version of this data set contains all Kreutz comets
observed from 1996-2005. Later updates to this data set will include
Kreutz comets discovered since 2005, non-Kreutz comets observed by
SOHO, and photometry of each comet as calculated by Matthew Knight
(UMD), once available. The photometry of many of the earliest
discovered SOHO comets were published by Biesecker (2002)
[BIESECKER2002].
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