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Diolaiti, E., Bendinelli, O., Bonaccini, D., Close, L., Currie, D., & Parmeggiani, G. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data
Analysis Software and Systems IX, eds. N. Manset, C. Veillet, D. Crabtree (San Francisco: ASP), 623
Starfinder: a Code for Crowded Stellar Fields Analysis
E. Diolaiti, O. Bendinelli
Dipartimento di Astronomia, Universitá di Bologna,
Via Ranzani,1, 40127 Bologna Italy
D. Bonaccini, L. Close, D. Currie
European Southern Observatory,Karl-Schwarzschild-Str.2,
D-85748 Garching b. Munchen, Germany
G. Parmeggiani
Osservatorio Astronomico di Bologna, Via Ranzani, 1, 40127 Bologna,
Italy
Abstract:
Starfinder is an IDL code for the deep analysis of stellar fields,
designed for well-sampled images with high and low Strehl factor. The
Point Spread Function (PSF) for the analysis is extracted directly from
the CCD frame, to take into account the actual structure of the
instrumental response and the atmospheric effects. An important feature
is represented by the possibility to measure the anisoplanatic effect in
wide-field Adaptive Optics (AO) observations and exploit this knowledge
to improve the analysis of the observed field. A description of the
method and applications to real AO data are presented.
If the PSF is constant across the frame, the observed stellar field may
be considered a superposition of shifted scaled replicas of the PSF itself,
lying on a smooth background originated by faint unresolved stars and other
possible sources. Actually the PSF is not always constant: in
wide-field AO imaging, for instance, off-axis stars appear blurred and
radially elongated with respect to the guide star; this anisoplanatic
effect is mainly due to the partial correction of the wavefront
tip-tilt. A further complication in the analysis of nearly
diffraction-limited images is represented by the detailed structure of
the PSF, which is generally difficult to model analytically and may
produce false detections. Starfinder (see also Diolaiti et al. 1998)
seeks to consider all these aspects.
If it is not known, the PSF for the analysis must be extracted from the
image. In our code the user selects a set of stars, which are cleaned
from the most contaminating sources, background-subtracted, centered
with sub-pixel accuracy, normalized and superposed with a median
operation. The halo of the retrieved PSF is then smoothed, applying a
variable box size median filtering technique.
The PSF estimate represents a template to analyze the field stars;
sub-pixel positioning is accomplished by interpolating the PSF array,
which must be well-sampled. A similar approach has been described in
Véran et al. (1998).
To overcome anisoplanatic effects in AO imaging we
use an approximation of the local PSF given by the convolution
of the reference source, commonly referred to as guide star, with a
radially elongated elliptical gaussian. The parameters of this blurring kernel
are derived from a polynomial fit. To do this, first a set of stars at various
distances from the reference
source is
selected then the parameters (elongation and width) of the
convolving elliptical gaussian, which gives the best match to the observation, are
determined for each one.
This set of measurements is fitted with a polynomial,
which will be used to determine the local PSF for the analysis of each
presumed star in the field.
The image background is estimated by interpolating a set of local measurements
relative to sub-regions arranged in a regular grid (see Bertin et al. 1996). If the brightest
stars in the field can be removed, a very similar estimate may be obtained
by the application of a median smoothing technique to the input frame.
The starting point is a list
of presumed stars, whose observed intensity in the
background-removed image is greater than a prefixed detection threshold.
Preliminary smoothing
reduces the incidence of noise spikes.
The objects are listed by decreasing intensity and analyzed one by one
by the following sequence of steps:
- Re-identification after subtraction of the known stars, in order
to reject spurious detections due to PSF features of bright sources,
- Cross-correlation with the PSF, as a measure of similarity with
the template,
- Astrometric and photometric analysis by local fitting.
Each new accepted star is added to a synthetic stellar field, updated at
every step. When all the objects in the list have been analyzed, a final
re-fitting is performed to improve their astrometry and photometry; then
they are temporarily removed to upgrade the background estimate.
The basic step described above (detection and analysis) may be repeated:
a new list of presumed stars is formed after subtracting the previously
detected ones and the analysis is started again on the original
image. This iteration is very useful to detect stars in crowded groups,
down to separations comparable to the Rayleigh limit for the detection
of close binaries.
An optional deblending mode is available. All the objects somewhat more
extended than the PSF are considered blends. The deblending strategy
consists of an iterative search for residuals around the object and
subsequent fitting; the iteration stops when no more residual is found
or the fit of the last residual was not successful.
Figure 1:
Left: PUEO image of the Galactic Center (upper image) and
reconstructed image given by the sum of the synthetic stellar field
(more than 1000 stars) and the estimated background, the display stretch
is square root (lower panel). Right: the obtained luminosity
function.
 |
The algorithm has been run on a K-band PUEO image of the Galactic Center,
as an example of a well-sampled high-Strehl AO observation of a
stellar field.
We have evaluated the astrometric and photometric accuracy of the
algorithm adding to the image, for each magnitude bin in the retrieved
luminosity function (Figure 1), a total of 10 % of synthetic stars
located at random positions, with the only constraint that the minimum
distance of each simulated star from all the previously detected ones
must be greater than 1 PSF FWHM (about 4 pixels). The plot of the
photometric errors shows accurate and unbiased photometry (see Figures
2a, 2b).
Our method has also been applied to two well-sampled low-Strehl images of the
globular cluster 47 Tuc, observed at the ESO 3.6m telescope with the
ADONIS AO system. The PSF FWHM is about 6 pixels.The two frames, that
have a large overlap area, have been analyzed with the same procedure.
The results (Figures 2c and 2d) present a good internal astrometric and
photometric accuracy.
Starfinder seems to be able to analyze well-sampled images of very crowded
fields observed, for instance, with ground-based AO systems. According to our
experience it may be successfully applied also to adequately sampled HST
images, like those obtained with dithering strategies. It is reasonably
fast (only few minutes on a Pentium II PC for the analysis of the
Galactic Center) and a widget interface makes it accessible to users
unfamiliar with IDL. In the near future the tools for space-variant
analysis will be improved. A further work will give a complete
description of the code.
Figure 2:
Galactic Centre: a) photometric errors of detected synthetic
stars, b) astrometric errors, representing the distance between the true
and the calculated position in FWHM units. 47 Tuc: c) off-centering
between corresponding stars in the two images, d) magnitude
difference. The first 4 points correspond to repaired saturated stars.
 |
Acknowledgments
Francois Rigaut is acknowledged for kindly providing the PUEO image of the
Galactic Center and for supporting the initial development of this method.
This work was partly supported by the Italian Ministry for University and
Research (MURST) under grant Cofin 98-02-32.
References
Bertin E., Arnouts S. 1996, A&AS, 117, 393
Diolaiti E., Bendinelli O., Bonaccini D., Parmeggiani G.,
Rigaut F. 1998,
in ESO/OSA Topical Meeting on Astronomy with Adaptive Optics, ed. D.Bonaccini (Garching b. Munchen, ESO), p.175
Véran J.-P., Rigaut F. 1998, Proc. SPIE 3353, 426
© Copyright 2000 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: Recent Advances in Parameter Estimation in Astronomy with Poisson-Distributed Data
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Previous: Data Analysis and Simulations of Spectroscopic and Continuum Mapping with the ISO PHOT Interactive Analysis ( PIA)
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