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Ott, S., Metcalfe, L., Pollock, A., & Tuffs, R. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data
Analysis Software and Systems IX, eds. N. Manset, C. Veillet, D. Crabtree (San Francisco: ASP), 543
Innovative Cosmic Ray Rejection in ISOCAM Data
S. Ott, L. Metcalfe, A. Pollock
ISO Data Centre, Astrophysics Division, Space Science Dept. of
ESA, Villafranca, P.O. Box 50727, 28080 Madrid, Spain
R. Tuffs
Max-Planck-Institut für Kernphysik, Heidelberg,
Germany
Abstract:
Cosmic ray rejection (deglitching) is one of the challenging data analysis
problems for ISOCAM, the infrared camera on board ESA's Infrared Space
Observatory (ISO).
Artifacts, caused by undetected glitches, limit the calibration accuracy and
sensitivity, and
directly influence the quality of the final data products.
We present five new deglitching algorithms suited for different types of ISOCAM
data and discuss their applicability.
We also show examples demonstrating the improvements gained by these
deglitching techniques.
Glitches (remnants of cosmic ray impacts) have been found to be a powerful (and
often dominant) limiting factor for all aspects of the data analysis of ISOCAM
(Cesarsky et al. 1996), the infrared camera on board ESA's Infrared Space
Observatory (ISO) (Kessler et al. 1996)1.
Standard deglitching techniques2 like multi-resolution (Starck
et al. 1998) or sigma clipping work ``blindly'' on a data vector only and
therefore ignore valuable information about:
- ISOCAM's current configuration,
- Our knowledge of glitch distribution in time,
- The current flux level per pixel.
Therefore the standard deglitchers have often to rely on badly chosen
thresholds, which consequently result in:
- Too high noise level due to un-suppressed glitches,
- Masked out sources or gradients during stabilisation,
- No deglitching for observations with few read-outs.
- field of application:
- dark current calibration with highly redundant
data. The signal (trends) are buried in the noise, and the value of the final
result is known, as a perfectly dark-corrected dark must be zero
- algorithm:
- repeated application of multi-resoltution deglitching and
conventional sigma clipping
- result:
- reduction of the noise level by a factor of five and of the
standard error by a factor of four. The number of rejected pixels is a strong
indicator of deviations due to hidden features and trends
- caveats:
- true features are partially clipped out. To avoid this,
iterations are necessary
- field of application:
- all datasets with stabilised and highly redundant data
(especially faint source data)
- algorithm:
- after conventional deglitching, each detector pixel, for each
raster position, is
projected into the sky to generate a ``sky cube''. Afterwards sigma-clipping is performed
for each sky-cube-pixel
- result:
- excellent rejection of remaining faders and dippers (Siebenmorgen
et al. 1999) not caught by earlier deglitching
- caveats:
- none known yet
- annotation:
- as sky-cube deglitching relies on the comparison between
detector- and sky-pixel, the actual ISO pointing and the distortion corrections for
ISOCAM have to be accurately known. Correction for ISOCAM's long and short term
drift will result in further improvements
Figure 1:
CAM survey data after multiresolution deglitching (left) and
after sky-cube deglitching (right).
![\begin{figure}
\plottwo{P2-51a.eps}{P2-51b.eps}
\end{figure}](img1.gif) |
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- field of application:
- all datasets with relatively well stabilised
data
- algorithm:
- iterative method; the data-cube is normalised by
estimations of flux level per state3
- result:
- stringent deglitching, including a very good rejection of
faders and dippers. The low residual noise permits additional treatments for
faint sources
- caveats:
- pixels with a high gradient (e.g. due to cross-talk in the
neighbourhood of strong extended sources) or strong point sources with high
noise can be masked out and have to be recovered by a NOSC post-processing
step (see Section 5 below)
- annotation:
- Overruling of the on-target-flag (normally set to bad
during slews of the satellite) enables the recovery of a significant amount of
otherwise ignored data for raster observation of a flat sky
Figure 2:
CAM survey data after multiresolution deglitching (left) and
after BISC deglitching (right).
![](P2-51c.gif) |
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Figure 3:
BISC deglitched CAM survey data with additional faint source
processing.
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- field of application:
- all datasets with stabilised data and very few
(down to four!) read-outs
- algorithm:
- per step, 25% of the read-outs (alternately with the
highest and then with the lowest signal within a state) are ignored. On the
remaining sample, sigma clipping is performed using modified values for mean and
variance
- result:
- spectacular results for CAM parallel, on its own and as BISC
postprocessor
- caveats:
- imperfect glitch rejection in case of higher than expected
number of strong glitches during a state
Figure 4:
CAM paralled exposure with six readouts after multiresolution
deglitching (left) and after NOSC deglitching (right).
![\begin{figure}
\plottwo{P2-51f.eps}{P2-51g.eps}
\end{figure}](img3.gif) |
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- field of application:
- in principle, all datasets
- algorithm:
- Currently, there are two algorithms under evaluation:
- Polynomial or spline fitting of the data vector and
clipping of the outlying points,
- Spline interpolation and rejection of points where the second derivative
has a significantly high value.
- result:
- promising examples on data with high gradients
- caveats:
- still under development
References
Cesarsky, C., et al. 1996, A&A, Vol. 315, L32
Kessler, M., Steinz, J.A. et al. 1996, A&A, Vol. 315, L27
Siebenmorgen, R. et al. 1999, ISO Handbook Volume III (CAM),
Version 1.0, ESA Document, Reference Number SAI-99-057/Dc
Starck, J-L., Murtagh, F. & Bijaoui, A., 1998, Image
Processing and Data Analysis: The Multiscape Approach, Cambridge
Univeristy Press, Cambridge (GB)
Footnotes
- ... 1996)1
- ISO is an ESA project with instruments funded by ESA member
states (especially the PI countries: France, Germany, the Netherlands and the
United Kingdom) and with the participation of ISAS and NASA.
- ... techniques2
- See Siebenmorgen et al. 1999 for the effects of cosmic ray
impacts and a description of conventional deglitchers.
- ... state3
- state: the finest division time division of ISOCAM activities.
All CAM and satellite (pointing) parameters are fixed during a state.
© Copyright 2000 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: Compression of Mosaic CCD Images with CompFITS2
Up: Data and Image Processing
Previous: Cosmic Ray Rejection and Data Compression for NGST
Table of Contents -
Subject Index -
Author Index -
PS reprint -
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