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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Enhanced HST Pointing and Calibration Accuracy:
Generating HST Jitter Files at ST­ECF
M. Dolensky, A. Micol, B. Pirenne and M. Rosa
Space Telescope ­ European Coordinating Facility,
Karl­Schwarzschild­Str. 2, D­85748 Garching, Germany
Abstract. After providing on--the--fly re­calibration of HST data to
archive users, ST--ECF has embarked on a new project aiming at group­
ing, cosmic ray cleaning and drizzling images taken by most HST instru­
ments. To perform this task in an unattended way, very precise pointing
information is required.
In addition, several other spacecraft parameters contained in the en­
gineering data stream are required as inputs to new, enhanced calibration
algorithms. An example is the estimation of the particle induced back­
ground count rate in the FOS digicons using actual HST magnetometer
readings to scale geomagnetical shielding models. The production of the
missing jitter files combined with the extraction of additional engineering
parameters will greatly enhance the homogeneity and the scientific value
of the entire archive.
We therefore started the generation of the missing jitter files. In this
article we explain the various aspects of the task and we give an example
of a project which will take advantage of the results.
1. The Pipeline
In order to generate the missing jitter files a pipeline was set up (Figures 1 and
2).
The heart of the pipeline is the Observation Monitoring System (OMS). It
correlates the time­tagged engineering telemetry with the Mission Schedule. The
Mission Schedule is a list of all commands and events that drive HST activities.
The engineering telemetry is stored in the Astrometry and Engineering Data
Processing (AEDP) subset files.
Both the Mission Schedule and the AEDP subset files are retrieved from
the Data Archive and Distribution System (DADS) at STScI.
In order to reprocess old AEDP subset files it is necessary to set up OMS ac­
cording to the spacecraft configuration and telemetry format valid at that time.
So, configuration files that change over time such as spacecraft characteristic
files are retrieved from the Project Data Base PDB.
OMS runs on a VAX/VMS platform. ECF maintains and operates the
pipeline remotely using the Space Telescope Operations Support Cluster (STOSC)
at STScI, i.e., ECF and STScI are sharing resources.
The AEDP subset files are used in two ways:
304

Generating HST Jitter Files at ST­ECF 305
FTP
PDB
OMS
Automatic
Quality
Control
ST­ECF
Archive
Manual
Quality
Control
XOMSAT
STScI
CADC
Configuration Files
AEDP
Subsets
Mission
Schedule
Observation
Log Files
AEDP
Subsets Selected
Parameters
ST­ECF
DADS
Figure 1. Pipeline reprocessing AEDP Subset Files
VMSTAR,
GZIP
FTP
PREPRC
OMSANL
(main)
AEDP
Subset Files
YECMSC
UNCOM­
PRESS
Mission
Schedule
Event
Files
Telemetry
Error File
ASCII Tables
Obs Logs
AEDP Subset Files
Parameter List
MAIL
SMTP
Report,
Notification
Configuration: Telemetry Format, Spacecraft
Characteristics, Ephemerides, etc.
Obs Logs
Mission
Schedule
XOMSAT
Automatic
Quality
Control
DCL­Scripts,
Utility
Listhead
Figure 2. Detailed Data Flow Graph of the Pipeline

306 Dolensky, Micol, Pirenne and Rosa
­18
­15
­12
­9
­6
­3
0
3
­9 ­6 ­3 0 3 6 9 12
V2_avg[marcsec]
V3_avg[marcsec]
­18
­15
­12
­9
­6
­3
0
3
6
9
0 100 200 300 400 500
Time[sec]
Jitter
[marcsec]
Jitter along V2 Axis Jitter along V3 Axis
Figure 3. Jitter Ball and Jitter vs. Time for Dataset X22X0103
1.1. Generation of Observation Logs
Observation Logs contain a set of pointing and specialised engineering data
(Leitherer et al. 1995; Lupie, Toth & Lallo 1997). The exact pointing of the
telescope, also called jitter information (Figure 3), is a prerequisite for further
archive services such as grouping, cosmic ray rejection and drizzling HST images
(Micol 1998).
1.2. Extraction of Selected Parameters
Xomsat is a stand­alone utility of OMS. Its purpose is to read an AEDP subset
file and to extract a user defined set of parameters. Nowadays the new Control
Center System (CCS) is used for RT­Analysis of selected parameters. The same
capability for historical telemetry is planned for a future release of CCS. It will,
however, require more time before it is available.
2. Quality Control
There is a semi­automatic procedure for the quality control of Observation Logs
(Figure 3). The automatic part consists of a test series which is performed at
STScI. A report is sent to the ECF via e­mail. In order to reduce data volume
only the comparatively small jitter files are packed and compressed using vmstar
and gzip respectively.
After an additional manual inspection at the ECF the logs finally end up
in the archive and become available to the community.

Generating HST Jitter Files at ST­ECF 307
ECF's archive interface includes a Java applet 1 that allows the visualization
of any two columns of an obs log as an X/Y plot.
3. Outlook
The pipeline started working in October 97. The first goal is the generation
of obs logs for the time period between the first Servicing Mission in Dec. 93
and Oct. 94. In Oct. 94 STScI started producing obs logs on a regular basis.
The file format was changed in August 95. For homogeneity the next step is to
compute jitter files for the time span until Aug. 95. Finally, the same pipeline
could be adapted to get obs logs back to launch in 1990.
Acknowledgments. Setting up this pipeline was only possible with the in­
valuable help of the DP­, OPUS­ and PDB­Teams at STScI. Special thanks to
R. Perrine, P. Goldstein, P. Hyde, K. Steuerman and J. Sandoval who sorted out
a lot of technical questions as well as DPT group head D. Swade and his deputy
M.A. Rose, who arranged for a very productive environment during my visit at
STScI. Many thanks also to D. Fraquelli and J. Scott of the OPUS group who
showed me a lot of HST's operations and quality control mechanisms. For his
collaboration in the early stages of the project many thanks to Joachim Paul of
MPE.
References
Leitherer, C. et al. 1995, HST Data Handbook, STScI, 107
Lupie, O., Toth, B. & Lallo M. 1997, Observation Logs, STScI
Micol, A., Bristow, P., & Pirenne, B. 1998, this volume
1 ``http://archive.eso.org/archive/jitview.html''