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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.
On­The­Fly Re­Calibration of HST Observations
B. Pirenne and A. Micol
Space Telescope -- European Coordinating Facility, ESO, Garching,
D­85748
D. Durand and S. Gaudet
Canadian Astronomy Data Centre, DAO, Victoria, BC
Abstract. On­the­fly re­calibration of HST data (OTF) has now been
available at the CADC and ST­ECF for about 18 months. With the
installation of STIS and NICMOS on the spacecraft, a new breed of data
types and data organisations was challenging our existing data retrieval
and processing service. We briefly describe the OTF method below.
1. What is On­the­fly re­calibration?
HST observations are normally calibrated and stored in the archive immediately
after reception by the ground system. The calibration can only be executed
using the calibration reference files which are available at that time. Some
may be missing or may not be the most appropriate ones as new calibration
observations might be obtained later on. Moreover, the calibration software of
those instruments hardly ever stabilizes. In other words, the longer one waits
before calibrating an observation, the better the results should be.
This is the concept that we decided to implement. The recipe is simple in
principle: recalibrate only at the time of the data delivery to the user. This is
``Just­in­time'' HST data!
2. Implementation
The implementation is best explained by considering the data flow model pre­
sented in Figure 1.
In this model, users perform the traditional catalogue browsing activities:
selection of data according to search criteria, examination of the results, refine­
ment of the selection etc., until the proper set of observations has been identified
from the science database, perhaps helped by looking at quick­look samples of
the data. Subsequently, users will mark those records for retrieval and will
typically select the on­the­fly reprocessing of the datasets.
After proper identification, selection of the output media etc, the request
is registered in the archive database. Then, a first automatic process reads the
files required from the data repository, while a second process starts the actual
re­calibration.
341

342 Pirenne, Micol, Durand and Gaudet
browse,select,
request obs.
science
DB
archive
DB
Process
Observations.
Process
Datasets.
Retrieve
Files.
Calibration
DB
science
Data
Web
Figure 1. On­the­fly re­calibration data flow: from request submis­
sion to data delivery.
Currently, this processing step involves the selection of the best calibration
reference files using a specialized database. This database relates the current
best calibration reference files to any given HST observation. Using this infor­
mation, the actual calibration reference files (flat, bias, dark, etc.) are retrieved
from magnetic disks and applied to the science data being re­calibrated.
As soon as all exposures belonging to a request have been reprocessed, the
data is made available to the user.
On­the­fly re­calibration has another advantage for our archive sites: we
only need to keep the raw data and calibration reference files on­line for the
process to be automatic. As a matter of fact, original calibration, engineering

On­The­Fly Re­Calibration of HST Observations 343
data and other auxiliary data need not be available to the re­calibration process
and can remain on secondary storage.
3. Recent Improvements
The ST­ECF and CADC are currently working towards improving this system
through two major new facilities.
. First, the integration of the calibration pipeline for the two new HST
instruments (NICMOS and STIS) complements our existing service.
. The science catalogue is now showing associations of WFPC2 exposures
that can be retrieved and re­calibrated as one single entity, enabling further
processing steps such as cosmic ray rejection and image co­addition. These
latest improvements are described at length in Alberto Micol's article in
this volume.
4. Conclusions
The mere fact that the service is used a lot is for us both proof of the usefulness
of the OTF concept and an encouragement to develop it further: The integration
of more advanced processing steps (cosmic­ray removal, co­addition of frames)
pushes OTF even further by allowing users to concentrate more on data analysis
and leave the reduction tasks to automated procedures.
References
Micol A., Bristow P., & Pirenne B. 1998, this volume
Crabtree, D., Durand, D., Gaudet, S., & Pirenne, B., 1996,``The CADC/ST--
ECF Archives of HST data: Less is More'', in ASP Conf. Ser., Vol. 101,
Astronomical Data Analysis Software and Systems V, ed. George H. Ja­
coby & Jeannette Barnes (San Francisco: ASP), 505