Use
Case: Offline: Reduce & Image Data for a Close Approach Comet
Interferometric Use Case for ALMA (based on ALMA SW Memo
11, Science Requirements and Use Cases, modified to include offline
processing requirements).
Goal: Reduce and image data for a close-approach comet
that requires multi-field mosaic imaging in multiple spectral lines.
(ACA and single dish dish data are required in post processing). This
Use Case is also applicable for observations of near-Earth and
main-belt asteroids.
Contact Author: D. Shepherd
Role(s)/Actor(s):
Primary: The PI (follows the basic course for this UseCase)
Secondaries:
Archive (needed to download data into offline package or
disk/tape.)
Offline (to do the actual data reduction and imaging)
Local User Resource: 2-body ephemeris on local disk
Priority:
Major (Needed for observing some Solar System objects)
Performance:
Response to user inputs in near real time. For a standard
multi-field mosaic (e.g. N pointings, Nyquist spacing, all fields with
similar uv coverage, mosaic boundaries extend at least one primary
beam beyond the emission region), processing time over that of a
signal field should increase no more than linearly with the number of
uv points to be processed.
Frequency of Use:
Perform this Offline Use Case for each comet or asteroid
observation (possibly several observations per year will be made).
Preconditions:
- Observations have been made by ALMA. On-line corrections that
should have been done (and thus do not need to be considered by the
offline system?):
- Near-field corrections
- General felativity corrections for the actual 3D path through
the solar system.
- The user has an updated 2-body ephemeris on their local disk which
is in the proper format to be read into the offline package.
- WVR corrections have been made on-line and stored in the Archive
file.
- Archive file includes all necessary calibration & source
observations (e.g. not just a partial scheduling block).
- Interferometric uv data has been downloaded from the Archive or is
available on tape or disk in ALMA FITS format.
- If combining single dish and interferometric data: calibrated,
single dish image is available on disk (downloaded from the
archive or reduced in a previous UseCase.
Basic
Course:
NOTE: All steps in the Basic Course should be able to be saved to
a master script. Alternatively, the user can start from a master
pipeline script downloaded from the Archive, and modified to run
this entire sequence or just parts of the sequence.
Almost all Data Reduction steps are the same as for reducing a large
mosaic image with interferometer + ACA data and combining this data
with Total Power images. The steps given below highlight the
differences needed for processing comet and asteroid data that have
poorly known positions at the time of the observation. Thus, the
ephemeris must be updated during post-processing. This Use Case is
not appropriate for planet and moon observations because the planetary
ephemerides are accurate enough that corrections do not have to be
made after the fact.
- After the data is filled from the Archive, tape, or disk into the
Data Reduction Package, perform the following steps:
- Read a new 2-body ephemeris from the local disk (6 orbital
elements).
- Using the new ephemeris elements, calculate the position of
the source as a function of time, determine the needed phase
shift and make the correction versus time.
- Using the ephemeris elements, calculate the 'cometocentric'
velocity as a function of time. Compute the velocity shift
and shift the line centers appropriately. The line should
be significantly oversampled (at least 4 or 5 channels
should have emission).
- Note: Velocity corrections are often not needed because
changes in radial velocity will generally be small
(~1/100th of a km/s), thus the orbital elements in the
first ephemeris are likely to be accurate enough. A few
cases may made need velocity correction such as the
passage of a comet through perihelion where the
velocity is significantly higher and velocity
differences between the initial ephemeris estimate
(before observations) and the actual observation can be
high.
- The software should report back on what was done and the
extent of the shifts.
- Continue with the data processing as per any other complicated
ALMA+ACA+SD mosaic reduction.
Postconditions:
- Publication-quality figures are generated.
- Image statistics and analysis results are available to user.
- Calibrated data & images are stored for later use if needed.
- Calibration & imaging script recording session is available (if
desired by the user).
Issues
to be Determined or Resolved: None at this
time.
Notes: This Use Case was created by D. Shepherd
based on conversations with Pat Palmer and Eric Greison. It was
created to help test the Offline software. Relavent SSR Use Cases
from SSR Memo 11 are: 4.6.2 (Retrieve Archived Data); 4.7.1 (Reduce
Multi Field [in pipeline mode]).
Last modified: 23sep03