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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.
Data Analysis Concepts for the Next Generation Space
Telescope
M.R. Rosa 1 , R. Albrecht 1 , W. Freudling and R.N. Hook
Space Telescope European Coordinating Facility, European Southern
Observatory, D­85748 Garching, Germany
Abstract. Several data analysis concepts, developed in response to the
nature of HST data, are applicable to the optimum exploitation of data
from NGST as well. Three examples are discussed in this paper: simula­
tion of PSFs in combination with image restoration techniques, automatic
extraction of slitless or multi--object spectra, and predictive calibration
based on instrument software models. These tools already greatly en­
hance HST data analysis and the examples serve to demonstrate that
S/W solutions to many of the challenging data­analysis requirements of
NGST can be derived straight forwardly from the HST software environ­
ment.
1. Introduction
The volume of data flowing from the Next Generation Space Telescope (NGST)
will in all likelihood exceed by huge factors that currently being received from
HST. The anticipated operational scenarios will at the same time enforce even
more the reliance upon calibration pipelines, automatic data analysis pipelines
and software supported observation planning at the users level.
The NGST core science program serves as an example of the type of science
exposures regularly to be obtained with such an instrument. This program im­
plements the recommendations of the HST­and­Beyond (Dressler) Report with
emphasis on targets at high redshifts. The main observing modes will be very
deep imaging and low resolution multi (several hundreds) object spectroscopy.
Targets are typically galaxies, barely resolved stellar clusters and luminous point
like sources such as supernovae in high redshift galaxies. Embedded sources and
faint cool objects in the immediate neighborhood of bright stars, i.e., planets
and very low mass stars, will certainly also have a share in the schedule. The
study assumed a 10 % mission time overhead for calibrations, implying maxi­
mum retrieval of information from raw data. This low overhead and stringent
demands can only be met if loss­less data combination and analysis is combined
with robust, noise­free calibration strategies.
The budget to be allocated for NGST operations including observation plan­
ning, data calibration and data analysis is ultimately linked to the complexity
1 A#liated to the Astrophysics Division of the Space Science Department of the European Space
Agency
173

174 Rosa, Albrecht, Freudling and Hook
of the observational procedures and to the requirements imposed by operational
aspects. To name a few: field rotation and plate scale change between successive
re­observations of the same field, field position dependent PSFs, non­availability
of calibration reference sources during certain periods, low ceilings on overheads
available for calibration observing time.
We present below three areas of software development for the current HST
data exploitation which lend themselves directly to the support of optimum
NGST science output without charging NGSTs budget .
2. PSF Modeling and Image Restoration
The combination of the rather broad PSF in the IR, the faint limit of the NGST,
and the fact that the deep images will be crowded with fore­ and background
galaxies will make photometry with NGST image data di#cult.
We have developed a preliminary NGST PSF generator based on Tiny Tim,
the widely used PSF generator for the HST (Krist & Hook 1997). At this time
the PSF is generated for the NGST ``reference design''. Under operational condi­
tions such PSFs will be predicted using future developments of the code relying
on information about the wavefront errors. Such highly accurately simulated
PSFs will allow the application of techniques which have been developed and
are successfully in use for HST data. These are in particular:
Photometric restoration, a software package producing the independent restora­
tion of point sources as well as a non­uniform background. It also handles
the problem of crowding, which will be much more significant for NGST
deep images than it is for HST, mainly because of the broader IR PSFs
and because of the limiting magnitudes (Lucy 1994; Hook & Lucy 1994)
Improving the resolution of the NGST at wavelengths where a deployable pri­
mary mirror is not di#raction limited, i.e., the classical image restoration
domain.
Homogenization and combination of images taken with di#erent PSFs. Given
a low mass, deployable mirror, NGSTs PSF might vary with time. Data
fusion techniques such as those developed for HST will allow the lossless
combination of these frames (Hook & Lucy 1993).
3. Software for NGST Multiple Slit Spectroscopy Planning
One of the most important scientific motivations of NGST is the spectroscopy
of very faint galaxies detected in a previous deep imaging exposure. The current
instrument plans include a multi--object spectrograph fed by a micro­mirror
array. This device will provide flexible, software controlled apertures of arbitrary
shapes and locations within the imaged field.
The operational concept will routinely make use of the deep survey images
of the field for subsequent multi­object spectroscopy of objects selected according
to special criteria (eg. blue color drop­outs). Because of the huge number of

Data Analysis Concepts for the Next Generation Space Telescope 175
Figure 1. NGST PSFS at 2 µm ­ generated with a modified version
of the HST PSF tool Tiny Tim. Left for an 8 m filled pupil, right for
the notched GSFC design.
sources to be selected and identified, and because the field will be accessible
in general only for a few weeks, object selection and the configuring data for
the micro­mirror array have to be performed within a very short time (days at
most). This can only be achieved by completely automatic image analysis and
subsequent object classification.
The software requirements are very similar to those for batch processing of
slitless spectroscopy, which uses direct imaging for object detection, wavelength
calibration and weighting of spectrum extraction by size and orientation of the
objects (Freudling 1997). A completely automatic ``pipeline'' to process such
data for the HST NICMOS camera has been developed (Freudling & Thomas
1997), and this program (Calnic­C) could be very easily adapted to become both
the micro­mirror configuration program and the spectral extraction code.
4. Calibration of NGST Multiple Slit Spectroscopy Data
The multi­object spectroscopy concept mentioned above serves the scientific re­
quirements very well. However, such highly flexible instrument configurations
are very demanding on calibration. It is obvious that classical calibration con­
cepts will not be able to cope with an almost unlimited variety of on/o# target
apertures that can be constructed across the field. Clearly, one can not possibly
hope to obtain useful sensitivity curves by observing standard stars even in only
a limited subset of these slit­lets. The aperture­mirror configuration for the field
to be studied will destroy the current ``instrument setup'' that was unique to
a particular position in the sky. Along the same lines, dispersion relations and
wavelength zero points will become obsolete as soon as the micro­mirrors are
reorganized.
The predictive calibration methods currently developed for HST (Rosa
1994) and ESO VLT (Rosa 1995) instrumentation are, however, ideally suited

176 Rosa, Albrecht, Freudling and Hook
to this situation. The kernels are instrument software models based on first
principles (e.g., grating equations). Configuration data, usually measurable en­
gineering quantities, are regularly verified on dedicated calibration exposures
for a few selected instrument configurations. Such models demonstrably permit
very accurate predictions of dispersion relations and sensitivity curves for modes
not actually covered by calibration data (Rosa 1997; Ballester & Rosa 1997).
Once ``calibrated'' on empirical calibration data, the software models can
be made integral parts part of the data calibration and data analysis pipelines
in two ways:
Predictive calibration: Calibration reference data (eg. dispersion relations,
sensitivity curves) are generated on­the­fly for the observational data set
under processing, permitting high­performance pipelines. Only configura­
tion data need to be accessed, rather then multiple reference files for each
mode. In addition, the model­generated reference data are noise free.
Forward analysis: An even more advanced application is the use of these soft­
ware models to match astrophysical ideas and observations in the raw data
domain. For example, an optimization package would browse an archive of
theoretical galaxy spectra at various evolutionary stages, very accurately
simulate raw observational data with eg. luminosities and redshifts as free
parameters, and seek for statistically significant matches.
Predictive calibration and forward analysis are currently being explored
for HSTs STIS. Utilizing these methods NGST operations in multi­object spec­
troscopy mode will need to allocate only very small amounts of time for specific
calibrations. Only wavelength zero­point shifts need to be verified observation­
ally for a given instrumental setup. Aperture specific sensitivity curves require
only infrequent checks on configuration data that have been obtained for a suit­
able subset of the micro­mirrors from standard stars.
References
Ballester, P., & Rosa, M.R., 1997, A&AS, in press (ESO prepr No 1220)
Freudling, W., 1997, ST­ECF Newsletter, 24, 7
Freudling, W., & Thomas, R., 1997, ``http://ecf.hq.eso.org/nicmos/calnicc/caln
icc.html''
Hook, R.N., & Lucy, L.B., 1993, ST­ECF Newsletter, 19, 6
Hook, R.N., & Lucy, L.B., 1994, in ``The Restoration of HST Images and Spec­
tra'', R.J. Hanisch & R.L. White, Baltimore: STScI, 86
Krist, J.E. & Hook, R.N., 1997, ``The Tiny Tim Users Manual'', STScI
Lucy. L.B., 1994, in ``The Restoration of HST Images and Spectra'', R.J.
Hanisch & R.L. White, Baltimore: STScI, 79
Rosa, M.R., 1994, CAL/FOS­127, Baltimore: STScI
Rosa, M.R., 1995, in ``Calibrating and Understanding HST and ESO Instru­
ments'', P. Benvenuti, Garching: ESO, 43
Rosa. M.R., 1997, ST­ECF Newsletter, 24, 14