Document Scope: This document addresses the pipeline
software requirements for the TAUVEX mission.
Overview
TAUVEX is a suite of 3 imagers which
will fly on GSAT-4.
The telescope will point toward the sky at a fixed orientation to the
Earth-satellite axis and will thus scan a fixed line of celestial
latitude with a period of 24 hours. Each individual photon is read and
will be reconstructed into an image of the sky after processing on the
ground.
Problem Description
Data from the instrument require considerable processing before they
can be used for science analysis but because of the amount of data,
this reduction has to be done by an automated pipeline. The output data
products must be those most useful to the user community.
Program Steps
There are four major stages in the pipeline flow, each of which is
further expanded below.
Ingest:
Data produced by the instrument will be sent through the spacecraft
telemetry system to the Master Control Facility in Hasan and will be
processed into Level 0 data. The pipeline will begin with the Level 0
data.
Validation:
A number of checks will be done by the software to ensure that there is
nothing grossly wrong with the data. These checks may include tracking
the voltages, temperatures, total count etc. If a potentially serious
condition arises, an operator will be notified and processing will
continue.
Correction for Instrumental Effects:
Corrections at the instrumental level include:
Distortion corrections -
where the image is mapped to a
different point based on its location in the field.
Flat fielding - The
instrument sensitivity depends on the
location on the focal plane.
Registration:
We get a time-tagged series of photons from the instrument which we
then convert into a map of the sky. In order to ease the pointing
requirements on the spacecraft, we will correct each photon's spatial
position by shifting and registering to other photons from the same
star. Because of the differing sensitivities of the different
instruments, it may be necessary to use the data from one telescope to
correct the others.
Level 2 Data:
Once the registration is complete,
data is ready to be made into images from individual scans. These
images, in standard format (for eg. fits, VOTable), are readable by
any standard astornomical software packages. Different
information on each data set will be writted to a database which can be
queried on line for later access by user community.
Level 3 Processing:
Responsibility of the automated
data pipeline software ends with level 2 data production.
Higher order data products will be dealt with in the due course,
depending on the availability of man power and resources.
Level 3 data shall be combined images from multiple observations and can contain
additional source information useful for variability studies and light
curve analysis. Derived science products like point source catalog,
light curves etc. forms the level 4 data.
The entire pipeline can be run using a shell script (UVS_run_pipeline.sh).
user need to supply the filename for level 1 (or level 0) data. However, this
works only on unix/linux. One need to run individual modules separately on
other platforms. These individual steps may be performed in the following order.
Please check the modules section for more information on
each of them.
UVS_ingest_data: Splits combined data into science and telemetry data files.
UVS_create_level1a: Convert data into FITS binary tables.
UVS_calc_xy: Calculate x and y positions of photons.
UVS_apply_cal: Apply calibration to the data
UVS_flat_field: Correct for variations in the calibration.
UVS_geom_corr: Correct for distortion in the system.
UVS_liearity_corr: Correct for non-linear response of detection.
UVS_register_data: Correct for pointing errors.
UVS_create_image: Create Level 2 FITS files.
UVS_extract_ps: Extract point sources.
UVS_variability: Write time history for each point.
The input of the first step (UVS_create_level1a) is obtained from the spacecraft. For
testing, the input file can be created by running the module 1 and 2 given in the Test Routines
with inputs from the LEVEL0 INPUTS given in the
Data Pipeline Downloads.
Ingest Overview:
Ingest is the first step in the TAUVEX pipeline and will convert the
spacecraft data into scientifically useful data products. Input:
The pipeline will start with the Level 1 data which is
defined in the document:
Data Reception. Output:
The output data is the validated Level 2 data. The format of
the
Level 1a data is binary FITS tables with multiple extensions. The first
extension contains only a header with all observation specific
information. Subsequent extensions contain information about the
individual photon hits and are separated by time stamp; ie., one
extension contains all the photon detections within a 1/8 s time
interval. The headers contain only information specific to that
particular frame.
UVS_create_level1a<Level1_data_file> <Level1a_data_file>NOTE: The output file name would be <Level1a_data_file>_T<n>.fits, where
n = 1,2,3 for each of the three telescopes.
read_level1_data
read_level1_log
create_level1a_header
write_level1a_header
write_level1a_data
Will read TAUVEX Level1
data and will write out FITS binary tables in the
Level 1a format.
Data Correction Overview:
There are several instrumental effects including geometric distortions
and flat fielding that have to be corrected for. Input: The input data is the validated Level 1a data. Output: The output data will be a FITS binary file with each extension representing a different frame - basically the same format as the Level
1 data.
Data Registration Overview: Location of each event may need to be
corrected for instrument scanrate (for scanning instruments),
in-orbit jitter and drifts. This step corrects the positions for
all spacecraft shifts. Input:
The input data is the geometrically and photometrically corrected
photon list. Output
The output data will be an image of the sky (Level 2 data).
Overview:
Level 3 data products are data products derived from the primary data
product - the Level 2 image of the sky. One example would be point
source files with fluxes in multiple bands. Other files will be
determined by the scientific requirements. Input: The input data is the Level 2 FITS image. Output: Output data files are in different formats depending on the file.
Overview:
A suite of automated and manual test routines have to be developed for
the pipeline software so that we can easily check the performance of
the pipeline routines.
