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Image processing techniques for gas morphology studies in the coma of comet Halley
Adv. Space Res. Vol. 9, No. 3, pp. (3)217—(3)220, 1989 Printed in Great Britain. All rights reserved.
0273—1177/89 $0.00 + .50 Copyright © 1989 COSPAR
IMAGE PROCESSING TECHNIQUES FOR GAS MORPHOLOGY STUDIES IN THE COMA OF COMET HALLEY G. Schwarz,5 C. Cosmovici,** P. Mackm and W. Ipt
*DP/LR Hauptabteilung Angewandte Datentechnik, 8031 Oberpfaffenhofen, F.R.G. **JFSI CNR, 00044 Frascati, Italy ***SAAO Observatory 7935, South Africa tMPAE, 3411 Katlenburg-Lindau, F.R. G. ABSTRACT
A number of special image processing techniques are known from the literature about comets, from publications about galaxies or from solar corona and remote sensing research. The aim of these processing methods is to enhance small irregular intensity variations or asymmetries, to compare these effects between different spectral bands and to analyze their spatial and temporal behaviour. This paper shows that a number of morphological phenomena can be found in ground—based observations of comet Halley when these or similar techniques are applied to continuum corrected narrow band gas emission images. INTRODUCTION In the following, we concentrate on ground—based observations of comet Halley taken in March 1986 by Cosmovici et al. /1/. These images have a geometrical resolution of about 350 km/pix, a high radiometric resolution and are absolutely calibrated. Various broad and narrow band filters permit the separate analysis of the continuum and molecule emissions as described by Cosmovici et al. /2/. CIRCULAR SYMMETRY OF DUST AND GAS The large dynamic range of typical cometary intensity profiles prevents the immediate recognition of any phenomenon which lies in the range of a few percent of the local signal amplitudes and consequently, such features appear only after image processing. One of the basic features is the degree of intensity symmetry around the nucleus. The global circular symmetry of an image can be tested easily by subtracting the maximum circular symmetric intensity “cone” from the original image. It is also straightforward to compute the relative deviatioi~ i.e. the percentage of deviation from circular symmetry. The symmetric cone is computed by transforming the image after noise reduction into polar coordinates. This is a common method to find the locations of concentric rings or radial vectors. In the transformed image, the columns correspond to concentric rings and the lines to radial vectors (A’Hearn et al. /3/). Next, the minima of all columns are connected to form a line and after smoothing of this profile this becomes the contour of the symmetrical cone to be subtracted from the original image. The application of this technique to continuum images and to continuum corrected gas emission images gives some insight into the markedly asymmetrical dust distibution and the characteristics of gas emissions. An example is shown in Fig. 1. The different sizes of the images are due to the varying amounts of overlap between the narrow band filter images and their continuum counterparts. JETS REVISITED The global asymmetry around the centre of brightness does not yet show any subtle variation of the intensity distribution. Well known effects in the coma of comet Halley are jet structures as described for example by Larson and Sekanina /4/, AHearn et al. /3/, or Cosmovici et al. /2/. These structures depend on the molecules to be analyzed. The jet features are detected either by shift-difference techniques /4,5/ or by ring masking /3/. Both methods are based on the idea of comparing the amplitudes of adjacent pixels at equal distances from the centre of intensity. In the case of perfect circular symmetry, the amplitudes should be equal. Small deviations from this symmetry are enhanced and result in the above mentioned structures. An example of a similar processing technique is the water ion image after enhancement, depicted in Fig. 2. It shows water ion ray structures in the tailward direction. The centre of brightness has been patched up as the original image was recorded with a long exposure time
(3)217
(3)218
0. Schwarz et at.
to obtain a batter signal to noise ratio within the tailward region. The processing steps are simple: after noise reduction, a smoothed version of the original image is generated by rotational blurring of the original image around the centre of intensity. The rotational blurring is done by adding a series of slightly rotated images on top of each other. The blurred image then contains the average intensity of the water ions around the centre of intensity. Finally, the original image is divided by the blurred image and the morphological structures appear. THE QUEST FOR SHELLS
A paper by Schiosser et al. /5/ claiming that the CN production rate of comet Halley varied strongly but regularly led to the idea of analyzing gas emission profiles in the radial direction, as a sudden variation of intensity should result in a shell-like morphology. The method of doing this has to discriminate between the regular non-linear intensity falloff and any irregular phenomenon. A first order solution is the use of logarithmic scaling to produce nearly linear profiles. Some attempts were made to find alternative strategies and the following two methods proved to be useful. The first method is based on unsharp masking in the radial direction. Generally, unsharp masking means dividing an image by a smoothed version of the image (Matuska et ci. /7/). The smooting can be done along specific directions, for example along concentric rings around the centre of intensity. In our case, we transform the images after noise reduction into polar co—ordinates, but before any unsharp masking is applied, the radial falioff is removed. This is done by dividing the amplitudes by their local mean which produces an image with nearly constant amplitude levels. Then, a subsequent unsharp masking process in the radial direction and a retransformation into cartesian co-ordinates yields results as shown in Fig. 3. The images contain areas with steady signal levels and areas with clearly visible ripple—like structures. These ripples are due to intensity changes which are not aligned with the radial enhancement direction (edges of curved jets) and to the low signal to noise ratio in some image areas. As an aid to interpretation, the depicted images contain in their upper right corners inserted jet enhanced images to the same scale. This method is useful in analyzing effects which extend over a few pixels, but it does not necessarily reveal extended smooth variations. Therefore, a second method was used based on radial profile ratioing. The general principle is to compare “ideal” radial intensity profiles with actual image profiles. The comparison can be made either by ratioing or by subtraction. The ideal curve can be a mathematical function or the modelling of a physical relationship. In principle, similar techniques were used to analyze images of galaxies (Lauer /8/, Schweizer and Seitzer /9/) in order to search for structures which deviate from ideal intensity profiles. As our aim was to search for shell—like effects, we selected for the ideal curve a low order “non wiggling” polynomial fit with negative exponents. Profile ratioing results are shown in Fig. 4. CN images showed no similarly evident structures. Again, polar co—ordinates were used to obtain the best approximation for each radius vector. The centres of the images were excluded in order to obtain a better fit in the outer regions. DETACHED BLOBS A common image processing technique used for the analysis of geomorphological satellite images is local histogram adaptation, a histogram remapping method based on the local histogram around each pixel (Tom /10/). Each pixel amplitude is substituted by a new amplitude derived from histogram equalization in a local window around the corresponding pixel. This method allows the detection of irregular intensity patterns, for example detached bright blobs as reported by Dao—han /11/. If we take the logarithm of the intensity and select a window size of about 20 by 20 pixels, then nearly all enhanced images show jet structures, single pixel noise and stars. If, however, the filter wavelength lies above 6800 A. then in a few images detached blobs become visible which do not appear on other images taken with different filters. An example is shown in Fig. 5. The blob is not a processing artifact as can be seen from the distribution of the lower bits in the original image and no Star appears at a comparable location within images which were taken at about the same time. However, no proof of the gaseous nature of the observed phenomenon can be given. DATA INTEGRITY The results presented here depend on a number of factors, as for example the correct pointing of the telescope, the radiometric correction of the detector pixels, the absolute calibration factors, the selection of a noise reduction methcd, the precise co—alignment of narrow band and continuum images, the selected image processing method and its potential artifacts. In order to avoid artifacts, two aspects deserve special attention: noise reduction and un— sharp masks. For noise reduction nonlinear weighted methods as described by Wang et al. /12/ produce good results. The image morphology is preserved while single outliers are reduced. This is especially important during ratioing in the outer regions of the images where the signal to noise ratio becomes a problem.
.
Image Processing Techniques for Gas Morphology
(3)219
Unsharp masks have to be smooth in all directions and directional unsharp masks must not have streaks perpendicular to the main unsharp direction. A simple solution to overcome this difficulty is to generate these masks not simply linewise or columnwise but for overlapping segments of multiple lines or columns and to merge these segments into a final unsharp mask. We consider that the simple image processing methods presented in this paper lead to reliable and reproducible results which support a number of findings already published for continuum images. REFERENCES 1. C.B. Cosmovici, P. Mack, H. Craubner and G. Schwarz, CCD—Observations of comet Halley from South Africa with the Giotto HMC- and IHW-filters, in: Eur. Space Ag. Spec. Pubi., 250(2),151—155 (1986) 2. C.B. Cosmovici, G. Schwarz, W. Ip and P. Mack, Gas and dust jets in the inner coma of comet Halley, Nature, 332, 705—709 (1988) 3. M.F. A’Hearn, S. Hoban, P.V. Birch, C. Bowers, in comet P/Halley, Nature, 324, 649—651 (1986)
R.
Martin and D.A. Klinglesmith, CN jets
4. S.M. Larson and Z. Sekanina, Coma Morphology and Dust-Emission Pattern of Periodic Comet Halley, Astronomical Journal, 89, 571—578, 600-606 (1984) 5. D.A. Klinglesmith, The interactive astronomical data analysis facility — image enhancement techniques applied to comet Halley, in: Modern Observational Techniques for Comets, JPL Publ. 81—68, 223 (1981) 6. W. Schlosser, R. Schulz and P. Koczet, The cyan shells of comet P/Halley, in: Eur. Space Ag. Spec. Publ., 250(3), 495—498 (1986) 7. W. Matuska, D.H. Janney, J.A. Farrell and C.F. Keller, Enhancement of Solar Corona and Comet Details, Optical Engineering, 17, 661—665 (1978) 8. T.R. Lauer, Boxy isophotes, discs and dust lanes in elliptical galaxies, Mon. Not. R astr. Soc., 216, 429—438 (1985) 9. F. Schweizer and P. Seitzer, Ripples in disk galaxies, Astrophysical Journal, 328, 88—92 Plates 1—4 (1988) 10. V.T. Tom, Adaptive filter techniques for digital image enhancement, in: SPIE, 528, Digital Image Processing, 29—42 (1985) 11. C. Dao—han, L. Zong-li, Z. Jia-qing, Y. Lin-shan, A.C. Gilmore, Possible nucleus splitting of comet Halley in March, 1986, in: Eur. Space Ag. Spec. Publ., 250(3) , 317—318 (1986) 12. D.C. Wang, A.H. Vagnucci and C.C. Li, Gradient Inverse Weighted Smoothing Scheme and the Evaluation of its Performance, Computer Graphics and Image Processing, 15, 167—181 (1981)
(3)220
0. Schwarz ci al.
Fig. 1. Circular Asymmetry: Filter 1MW 5,139A Continuum Corrected, Scale see Fig. 2. In all Figs. the Sun is on the Top.
Fig. 2. Azimuthal Unsharp Masking: Filter MPI 6,193A, Continuum Corrected.
Fig. 3. Radial Unsharp Masking: Filter see Fig. 1, Scale see Fig. 2.
Fig. 4. Radial Unsharp Masking: Filter IHW 3,871A, Scale see Fig. 2.
—
~1.T*II1~..
-
Fig. 5. Profile Ratioing: Filter see Fig. 2.
___
Fig. 6. Detached Blob (right) 7,100A, Scale see Fig. 2.
: Filter MPI
0273—1177/89 $0.00 + .50 Copyright © 1989 COSPAR
IMAGE PROCESSING TECHNIQUES FOR GAS MORPHOLOGY STUDIES IN THE COMA OF COMET HALLEY G. Schwarz,5 C. Cosmovici,** P. Mackm and W. Ipt
*DP/LR Hauptabteilung Angewandte Datentechnik, 8031 Oberpfaffenhofen, F.R.G. **JFSI CNR, 00044 Frascati, Italy ***SAAO Observatory 7935, South Africa tMPAE, 3411 Katlenburg-Lindau, F.R. G. ABSTRACT
A number of special image processing techniques are known from the literature about comets, from publications about galaxies or from solar corona and remote sensing research. The aim of these processing methods is to enhance small irregular intensity variations or asymmetries, to compare these effects between different spectral bands and to analyze their spatial and temporal behaviour. This paper shows that a number of morphological phenomena can be found in ground—based observations of comet Halley when these or similar techniques are applied to continuum corrected narrow band gas emission images. INTRODUCTION In the following, we concentrate on ground—based observations of comet Halley taken in March 1986 by Cosmovici et al. /1/. These images have a geometrical resolution of about 350 km/pix, a high radiometric resolution and are absolutely calibrated. Various broad and narrow band filters permit the separate analysis of the continuum and molecule emissions as described by Cosmovici et al. /2/. CIRCULAR SYMMETRY OF DUST AND GAS The large dynamic range of typical cometary intensity profiles prevents the immediate recognition of any phenomenon which lies in the range of a few percent of the local signal amplitudes and consequently, such features appear only after image processing. One of the basic features is the degree of intensity symmetry around the nucleus. The global circular symmetry of an image can be tested easily by subtracting the maximum circular symmetric intensity “cone” from the original image. It is also straightforward to compute the relative deviatioi~ i.e. the percentage of deviation from circular symmetry. The symmetric cone is computed by transforming the image after noise reduction into polar coordinates. This is a common method to find the locations of concentric rings or radial vectors. In the transformed image, the columns correspond to concentric rings and the lines to radial vectors (A’Hearn et al. /3/). Next, the minima of all columns are connected to form a line and after smoothing of this profile this becomes the contour of the symmetrical cone to be subtracted from the original image. The application of this technique to continuum images and to continuum corrected gas emission images gives some insight into the markedly asymmetrical dust distibution and the characteristics of gas emissions. An example is shown in Fig. 1. The different sizes of the images are due to the varying amounts of overlap between the narrow band filter images and their continuum counterparts. JETS REVISITED The global asymmetry around the centre of brightness does not yet show any subtle variation of the intensity distribution. Well known effects in the coma of comet Halley are jet structures as described for example by Larson and Sekanina /4/, AHearn et al. /3/, or Cosmovici et al. /2/. These structures depend on the molecules to be analyzed. The jet features are detected either by shift-difference techniques /4,5/ or by ring masking /3/. Both methods are based on the idea of comparing the amplitudes of adjacent pixels at equal distances from the centre of intensity. In the case of perfect circular symmetry, the amplitudes should be equal. Small deviations from this symmetry are enhanced and result in the above mentioned structures. An example of a similar processing technique is the water ion image after enhancement, depicted in Fig. 2. It shows water ion ray structures in the tailward direction. The centre of brightness has been patched up as the original image was recorded with a long exposure time
(3)217
(3)218
0. Schwarz et at.
to obtain a batter signal to noise ratio within the tailward region. The processing steps are simple: after noise reduction, a smoothed version of the original image is generated by rotational blurring of the original image around the centre of intensity. The rotational blurring is done by adding a series of slightly rotated images on top of each other. The blurred image then contains the average intensity of the water ions around the centre of intensity. Finally, the original image is divided by the blurred image and the morphological structures appear. THE QUEST FOR SHELLS
A paper by Schiosser et al. /5/ claiming that the CN production rate of comet Halley varied strongly but regularly led to the idea of analyzing gas emission profiles in the radial direction, as a sudden variation of intensity should result in a shell-like morphology. The method of doing this has to discriminate between the regular non-linear intensity falloff and any irregular phenomenon. A first order solution is the use of logarithmic scaling to produce nearly linear profiles. Some attempts were made to find alternative strategies and the following two methods proved to be useful. The first method is based on unsharp masking in the radial direction. Generally, unsharp masking means dividing an image by a smoothed version of the image (Matuska et ci. /7/). The smooting can be done along specific directions, for example along concentric rings around the centre of intensity. In our case, we transform the images after noise reduction into polar co—ordinates, but before any unsharp masking is applied, the radial falioff is removed. This is done by dividing the amplitudes by their local mean which produces an image with nearly constant amplitude levels. Then, a subsequent unsharp masking process in the radial direction and a retransformation into cartesian co-ordinates yields results as shown in Fig. 3. The images contain areas with steady signal levels and areas with clearly visible ripple—like structures. These ripples are due to intensity changes which are not aligned with the radial enhancement direction (edges of curved jets) and to the low signal to noise ratio in some image areas. As an aid to interpretation, the depicted images contain in their upper right corners inserted jet enhanced images to the same scale. This method is useful in analyzing effects which extend over a few pixels, but it does not necessarily reveal extended smooth variations. Therefore, a second method was used based on radial profile ratioing. The general principle is to compare “ideal” radial intensity profiles with actual image profiles. The comparison can be made either by ratioing or by subtraction. The ideal curve can be a mathematical function or the modelling of a physical relationship. In principle, similar techniques were used to analyze images of galaxies (Lauer /8/, Schweizer and Seitzer /9/) in order to search for structures which deviate from ideal intensity profiles. As our aim was to search for shell—like effects, we selected for the ideal curve a low order “non wiggling” polynomial fit with negative exponents. Profile ratioing results are shown in Fig. 4. CN images showed no similarly evident structures. Again, polar co—ordinates were used to obtain the best approximation for each radius vector. The centres of the images were excluded in order to obtain a better fit in the outer regions. DETACHED BLOBS A common image processing technique used for the analysis of geomorphological satellite images is local histogram adaptation, a histogram remapping method based on the local histogram around each pixel (Tom /10/). Each pixel amplitude is substituted by a new amplitude derived from histogram equalization in a local window around the corresponding pixel. This method allows the detection of irregular intensity patterns, for example detached bright blobs as reported by Dao—han /11/. If we take the logarithm of the intensity and select a window size of about 20 by 20 pixels, then nearly all enhanced images show jet structures, single pixel noise and stars. If, however, the filter wavelength lies above 6800 A. then in a few images detached blobs become visible which do not appear on other images taken with different filters. An example is shown in Fig. 5. The blob is not a processing artifact as can be seen from the distribution of the lower bits in the original image and no Star appears at a comparable location within images which were taken at about the same time. However, no proof of the gaseous nature of the observed phenomenon can be given. DATA INTEGRITY The results presented here depend on a number of factors, as for example the correct pointing of the telescope, the radiometric correction of the detector pixels, the absolute calibration factors, the selection of a noise reduction methcd, the precise co—alignment of narrow band and continuum images, the selected image processing method and its potential artifacts. In order to avoid artifacts, two aspects deserve special attention: noise reduction and un— sharp masks. For noise reduction nonlinear weighted methods as described by Wang et al. /12/ produce good results. The image morphology is preserved while single outliers are reduced. This is especially important during ratioing in the outer regions of the images where the signal to noise ratio becomes a problem.
.
Image Processing Techniques for Gas Morphology
(3)219
Unsharp masks have to be smooth in all directions and directional unsharp masks must not have streaks perpendicular to the main unsharp direction. A simple solution to overcome this difficulty is to generate these masks not simply linewise or columnwise but for overlapping segments of multiple lines or columns and to merge these segments into a final unsharp mask. We consider that the simple image processing methods presented in this paper lead to reliable and reproducible results which support a number of findings already published for continuum images. REFERENCES 1. C.B. Cosmovici, P. Mack, H. Craubner and G. Schwarz, CCD—Observations of comet Halley from South Africa with the Giotto HMC- and IHW-filters, in: Eur. Space Ag. Spec. Pubi., 250(2),151—155 (1986) 2. C.B. Cosmovici, G. Schwarz, W. Ip and P. Mack, Gas and dust jets in the inner coma of comet Halley, Nature, 332, 705—709 (1988) 3. M.F. A’Hearn, S. Hoban, P.V. Birch, C. Bowers, in comet P/Halley, Nature, 324, 649—651 (1986)
R.
Martin and D.A. Klinglesmith, CN jets
4. S.M. Larson and Z. Sekanina, Coma Morphology and Dust-Emission Pattern of Periodic Comet Halley, Astronomical Journal, 89, 571—578, 600-606 (1984) 5. D.A. Klinglesmith, The interactive astronomical data analysis facility — image enhancement techniques applied to comet Halley, in: Modern Observational Techniques for Comets, JPL Publ. 81—68, 223 (1981) 6. W. Schlosser, R. Schulz and P. Koczet, The cyan shells of comet P/Halley, in: Eur. Space Ag. Spec. Publ., 250(3), 495—498 (1986) 7. W. Matuska, D.H. Janney, J.A. Farrell and C.F. Keller, Enhancement of Solar Corona and Comet Details, Optical Engineering, 17, 661—665 (1978) 8. T.R. Lauer, Boxy isophotes, discs and dust lanes in elliptical galaxies, Mon. Not. R astr. Soc., 216, 429—438 (1985) 9. F. Schweizer and P. Seitzer, Ripples in disk galaxies, Astrophysical Journal, 328, 88—92 Plates 1—4 (1988) 10. V.T. Tom, Adaptive filter techniques for digital image enhancement, in: SPIE, 528, Digital Image Processing, 29—42 (1985) 11. C. Dao—han, L. Zong-li, Z. Jia-qing, Y. Lin-shan, A.C. Gilmore, Possible nucleus splitting of comet Halley in March, 1986, in: Eur. Space Ag. Spec. Publ., 250(3) , 317—318 (1986) 12. D.C. Wang, A.H. Vagnucci and C.C. Li, Gradient Inverse Weighted Smoothing Scheme and the Evaluation of its Performance, Computer Graphics and Image Processing, 15, 167—181 (1981)
(3)220
0. Schwarz ci al.
Fig. 1. Circular Asymmetry: Filter 1MW 5,139A Continuum Corrected, Scale see Fig. 2. In all Figs. the Sun is on the Top.
Fig. 2. Azimuthal Unsharp Masking: Filter MPI 6,193A, Continuum Corrected.
Fig. 3. Radial Unsharp Masking: Filter see Fig. 1, Scale see Fig. 2.
Fig. 4. Radial Unsharp Masking: Filter IHW 3,871A, Scale see Fig. 2.
—
~1.T*II1~..
-
Fig. 5. Profile Ratioing: Filter see Fig. 2.
___
Fig. 6. Detached Blob (right) 7,100A, Scale see Fig. 2.
: Filter MPI