We applied several image restoration and enhancement techniques, to CORONAS-I images. We carried out the characterization of the Point Spread Function (PSF) using the unique capability of the Blind Iterative Deconvolution (BID) technique, which recovers the real PSF at a given location and time of observation, when limited a priori information is available on its characteristics. We also applied image enhancement technique to extract the small scale structure imbeded in bright large scale structures on the disk and on the limb. The results demonstrate the capability of the image post-processing to substantially increase the yield from the space observations by improving the resolution and reducing noise in the images

Karovska, Margarita

English

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HIGH RESOLUTION IMAGING OF THE SUN WITH
CORONAS-I
M. Karovska
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138
ABSTRACT
We applied several image restoration and enhancement techniques, to
CORONAS-I images. We carried out the characterization of the Point Spread
Function (PSF) using the unique capability of the Blind Iterative Deconvolution
(BID) technique, which recovers the real PSF at a given location and time of
observation, when limited a priori information is available on its characteristics.
We also applied image enhancement technique to extract the small scall structure
inbeded in bright large scale structures on the disk and on the limb. The results
demonstrate the capability of the image post-processing to substantially increase
the yield from the space observations by improving the resolution and reducing
noise in the images.
1. Introduction
The CORONAS-I mission, launched in 1993, is first of two missions dedicated to solar
observations utilizing the AUOS (Automatic Universal Orbital Station) satellite platform. The
CORONAS-I scientific payload includes two imaging instruments: the Multichannel XUV
Telescope/Coronagraph (TEREK-C), and the Multichannel XUV Imaging Spectroheliograph
(RES-C) (see Oraevsky and Zhugzhda 1991, Zhitnik et al 1993). The TEREK-C is a
normal-incidence telescope consisting of two XUV channels and a coronagraph channel. The
XUV channels consist of (1) high resolution channel (X) for observations in the spectral region
centered at 175 angstrom, and (2) medium resolution channel (MR) for observations in
spectral regions centered at 132, 175 and 304 angstrom. The images are recorded using a 1024
x 1156 CCD detector; pixel sizes for the X and MX channels are 1" and 4" respectively. The
white line coronagraph has a 516 x 1156 pixel detector with a pixel size of 30". The RES-C
instrument provided monochromatic images of the Sun in three spectral regions: 180 - 205
angstrom (XUV channel), 8.41 -8.43 angstrom (Mg XII channel) and 1.85 -1.87 angstrom (Fe
XXV channel). The spectral resolution is 0.015 angstrom/pixel for the XUV channel, 0.002
angstrom/pixel for the Mg XII channel, and 0.0001 angstrom/pixel for the Fe XXV, while the
angular resolution in all three channels is 4". The images are recorded using CCD detectors:
1024 x 1156 pixel for XUV channel, and 576 x 768 pixels for Mg XII and Fe XXV channels.
Both imaging instruments have capability of observing in "Disk" mode and in "Flare" mode.
2In "Disk" mode, imagesof the solar disk are recordedin all channels;in "Flare" mode, the
instruments wait for a flare flag signal from the x-ray spectrometerto start recording (Zhitnik
et al 1993).
During the few months of observations, CORONAS-I returned multiwavelength
observations showing millions degrees plasmas confined within arch-like magnetic field
structures forming active region loops, which are the sites of such explosive events as flares and
coronal mass ejections. These closed magnetic structures coexist with the nebulous regions
defining the quiet sun, and with the coronal holes - tenuous regions with depleted emission
carried along open magnetic field lines. In addition to these distinct large-scale magnetic
structures, the solar atmosphere seen in X-rays is dotted everywhere with compact bright
regions, the coronal bright points.
As part of the collaborative research effort the PI's visited the FIAN institute in Moscow,
and later on, two colleagues from the CORONAS team at FIAN (Dr. Zhitnik's and Dr.
Pertsev), visited the CfA. During these visits, several _sets of images were selected from the
archival TEREK-C data base for postprocessing. The selected sets of images were recorded
using different exposure times in several spectral regions: 132/5 A (FeXXIII-XXIV), 175/9 A
(FeIX-XI) and 304/20 A (HeII+ SiXII).
2. Application of Image Restoration/Enhancement Algorithms to CORONAS-I
Images
CORONAS-I observations contain many complex structures at different spatial scales, and
the contrast between various features is often very high. The characteristics and distribution
of the smallest spatial scale structures cannot be readily extracted directly from these
observations, either because of limited resolution and noise in the images, or because of the
high dynamic range between small-scale and large-scale structures. To improve the resolution
in the images, characterize the PSF and enhance the small structure in the CORONAS-I
images we applied two analysis tools: Blind Iterative Deconvolution and Image Enhancement
Technique.
Blind Iterative Deconvolution (Karovska, Blundell and Habbal 1991) is an image
restoration technique which can be applied to high signal-to-noise images degraded by an
unknown function. This technique finds a pair of functions corresponding to the degrading
function and the undegraded image, which when convolved reproduce the input image within
given physical constraints, A necessary condition is that the degrading function is invariant
over the image field, and the degradation is linear operation. To satisfy these conditions we
applied the BID to selected small regions from the images of the full Sun containing compact
bright features.
Figure 1 (left) shows an example of a region selected from the full-frame TEREK-C
31024x1152pixels imagerecordedin 175A (FelX-XI) on 5/02/1993 at UT 22:21. The region
was selected in the center of the Solar disk. Figure 1 (right) shows the restored image using
the BID technique. The reconstructed PSF is shown in Figure 2. The PSF is slightly larger
than one pixel (4 arcseconds), and therefore the deconvolution process could not produce a
dramatic improvement of the resolution, although it did converge to a solution.
We also developed an image enhancement algorithm (IEA) to sharpen low-contrast
small-scale structures, particularly when they are embedded in large bright structures on the
disk, or located at or above the solar limb. It is based on a modified version of an algorithm
that calculates the maxima of the multi-directional second derivatives of the intensities in a
given image (Koutchmy et al. 1988; Karovska, Blundell, and Habbal 1994). Our algorithm
detects the location of a significant change ill grey level in the image by calculating the
maxima of the multidirectional second derivatives. Although this method strictly speaking
does not improve the resolution in the images, the sharpening of the fine structure allows
detailed study of the spatial characteristics and temporal variability of the small-scale
structures, often hidden within the large-scale bright features ill the original images. Figure 3
(right) shows the same region as in Figure 1 (shown again in Figure 3- left) after being
enhanced using the IEA. The enhanced image shows several small-scale loops that could not
be easily discerned in the original images. We also applied the IEA to the multiwavelength
TEREK-C images of regions at the limb. In several images we detected coronal loops
extending above the limb as well as structure in the coronal holes.
We explored the dynamical evolution of the fine structures in the active regions at
different temperatures and the morphology of the structures at or near the limb using a time
series of multiwavelength TEREK-C images. In Figure 4, we show He II and Fe IX-XI images
taken 11 minutes apart on 1994 April 15. A correction has been made to the He II image to
subtract contaminating light from the second spectral order. The images in Figure 4 were
enhanced and the results are shown in the lower panels of the figure. The edge-enhancement
has successfully accentuated many of the structures less visible in the original images. We also
looked at similar images made on the two previous days, April 13 and 14, and we constructed
movies of the April 13-15 data. The movies and the edge-enhancement techniques were both
useful in distinguishing real features from those created by instrumental artifacts and photon
noise. The restored and enhanced images show fine loop structures in active regions, and the
substructure within the coronal bright points which could be a manifestation of preferentially
heated plasma.
Finally, we studied the short time-scale variability of the fine structures in the TEP_C-C
images, using a sequence of nine Fe IX-XI images obtained on 1994 June 2. These images were
taken on average once every 38 seconds. In Figure 5, we show the sequence of images after
edge-enhancement. Supergranualar boundaries appear to brighten and fade from image to
image on a time-scale shorter than a minute. We also detected a small prominence eruption at
the end of the sequence in the bottom left corner of the field of view.
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Fig. 1._ A 4 x 4 arcminutes (64 x 64 pixels) region selected from the full-flame TEREC-C
1024x1152 image recorded in 175 _(FeIX-XI) near the center of the solar disk: (left) before the
deconvolution, (right) after the deconvolution using the BID
-5-
Fig. 2.-- A TEREC-C PSFat 175__in the centerof the field restored using the BID techniques.
The image has been magnified to show the structure within the PSF
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Fig. 3.-- A 4 x 4 arcminutes (64 x 64 pixels) region selected from the full-frame TEREC-C
1024x1152 image recorded in 175/_(FeIX-XI) near the center of the solar disk: (left) before the
enhancement, (right) after the enhancement using the IEA
7Fig. 4.-- The upper panelsare He II )`304 (upper left) and Fe IX-XI ),175 (upper fight)
CORONAS images from 1994 April 15, displayed using a square-root intensity scale. The lower
panels are edge-enhanced versions of these images
Fig. 5.-- A sequenceof edge-enhancedFeIX-XI ),175CORONAS images taken on 1994 June
2. The average time difference between the images is 38 seconds
9The results of this project demonstratethe capability of imageprocessingto substantially
increasethe yield from the spaceobservationsby improving the resolution and reducingnoise
in the images. _ _
REFERENCES
Karovska, M., Blundell, S. F. & Habbal, S. R. 1994, ApJ, 428, 854
Koutchmy, O., Koutchmy, S., Nitschelm, C., Sykora, J. & Smartt, R. N. 1988, in the Proc. 9th
Sacramento Peak Summer Syrup., ed. R. C. Altrock (Sunspot: AURA), 256
Oraevsky and Zhugzhda 1991, CORONAS Information Series No. 1.
Zhitnik, I., Umov, A., Ivanchuk, V., and Zharkova, V. eds. 1993, The CORONAS Mission:
Scientific Objectives and Technical Description.
This preprint was prepared with the AAS It,TEX macros v4.0.



High Resolution Imaging of the Sun with CORONAS-1

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