| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains photometric measurements of comet 9P/Tempel 1
from images taken with the Medium Resolution CCD Instrument during the
approach phase of the Deep Impact mission (from 1 May 2005 to 4 July
2005). These data, based on circular apertures ranging from 5 to 30
pixels in diameter, were derived from both science and navigation
images taken through clear filters.
Observations
------------
These tables consist of measurements performed on images taken
from 1 May 2005 to about six hours before impact, defined here as
05:44:34.265 UT 4 July 2005 at the flyby spacecraft. All dates
provided in the tables refer to the mid-point of the observation.
The heliocentric distance of 9P/Tempel 1 goes from 1.64 AU to 1.51
AU during approach. A total of 3014 images were analyzed: 595
science images and 2419 optical navigation images.
Related Data Sets
-----------------
The following PDS data sets were used to derive these data:
DIF-C-MRI-3/4-9P-ENCOUNTER-V2.0/DATA/RADREV : Calibrated, radiance
MRI encounter data,
version 2.0
DIF-C-MRI-2-NAV-9P-ENCOUNTER-V1.0 : Raw MRI navigation
data
Please note that MRI reversibly calibrated, radiance (RADREV) science
images were used in this analysis. These data were not processed to
remove artifacts, such as cosmic rays, which sometimes affect the
tabulated results. Also, measurements derived from navigation images
were acquired from the raw data.
Data
====
Tables of Photometric Data
--------------------------
The nav_phot.tab and sci_phot.tab files are fixed-width, ASCII
tables that contain photometric measurements of comet 9P/Tempel 1
taken with 5, 7, 9, 15, 20, 25, and 30 pixel diameter apertures.
Observation dates are included in the tables, as is the centroid
position of the comet in the image. These files additionally
contain a column specifying the filter used to take the image and
another column listing the centroid's distance from the horizontal
boundary between the CCD quadrants. For more information about
the MRI CCD instrument and its filters, Deep Impact instruments
publication by Hampton, et al. (2005) [HAMPTONETAL2005] provided
in the Deep Impact documentation data set,
DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V1.0, which is archived by the PDS.
Processing
==========
The following assumes that the images are displayed with lines
increasing up and samples to the right.
Science Data
------------
Photometric measurements were derived from the reversibly calibrated
RADREV images taken through the MRI clear filters. An additional
background subtraction algorithm was applied to these data in order
to remove the horizontal ''striping'' caused by electrical
interference. This noise, whose amplitude is on the order of a few
DN, most adversely affects the data taken early on in the approach
sequence.
In many of these images, the comet was located on or close to the
boundary between the upper and lower halves of the CCD. The pixels
in the rows surrounding this boundary are 1/6 of a pixel smaller
than the other pixels of the CCD. This increases the point spread
function of objects that overlap this boundary by 1/3 of a pixel.
Flux measurements similarly tend to be greater at the boundary.
In about 80 percent of the images, the comet centroid lies less than
20 pixels away from this boundary. For a more detailed discussion
of how this affects the data and what corrections were applied,
please see the Deep Impact MRI photometry report included in the
document directory of this data set.
For more information about how the RADREV science data were
calibrated, see the Deep Impact (Science) Calibration Pipeline
Summary included in the document directory of the data set or
the Deep Impact instrument calibration report by Klaasen, et al.
(2006) [KLAASENETAL2006] included in the Deep Impact documentation
data set, DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V1.0, which is archived
by the PDS.
Optical Navigation Data
-----------------------
An optical navigation image consists of a number of ''snippets'',
each containing an object of interest to the navigation team (for
example, a guide star). Comet 9P/Tempel 1 is always contained in one
of these ''snippets'', which varied in size from 350x350 pixels for
most of the approach sequence to 150x150 pixels for the last week
before impact. These images do not have serial and parallel
overclock pixels and therefore needed to be treated differently from
the science images. Only the raw navigation images are used in this
analysis.
A background subtraction procedure was applied only to the snippet
containing the comet. This procedure also removed the horizontal
striping previously discussed for all images taken more than one week
before impact. In data taken between UTC 2005-06-27T05:44:09.225 and
impact, the comet's coma fills the snippet, which could therefore not
be used to remove the background. For this time frame, the
background was measured in the relevant image quadrant from a raw
science image taken close in time to the navigation image of
interest.
For a detailed description of the navigation data, please see the
Deep Impact navigation images report included in the document
directory of this data set. For additional information on the
reduction processes applied to these data, please see the Deep Impact
MRI photometry report included in the document directory of this data
set.
This data set overview was provided by F. Bastien. For a detailed
discussion of this data set, please refer to the Deep Impact MRI
photometry report included in the document directory of this data set.
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