Weekly Report 19Nov2010 26Nov2010

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===RHESSI Website Migration===
===RHESSI Website Migration===
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The new [http://hesperia.gsfc.nasa.gov/rhessi2/ RHESSI website] now has more content and increased functionality for adding various kinds of content.
===Goals for Next Week===
===Goals for Next Week===

Revision as of 14:04, 30 November 2010

Contents

Pixon Albedo Imaging

Using Eduard Kontar's simulated eventlist files, Pixon Images were created for a compact circular gaussian source with an albedo contribution near disk center, and near the limb. The following parameters were used for each image:

The following plots are extensions of the plots from the previous report.

The plot below shows the gaussian model of the compact source (orange),the original data map with the minimum and maximum value at each radial distance (Black), the Pixon image with the minimum and maximum value at each radial distance (Purple), and the clean image with the minimum and maximum value at each radial distance (Cyan)for a source near disk center.

Original Data Map (Black), Pixon Map (Purple), Clean Map (Cyan), Model Gaussian (orange) for a source near the disk center

The gaussian model falls off as we would expect the flux to fall in a circular gaussian. For 2 arcsec the model and the map are consistent, after this point the model falls off much more sharply, while the map shows a falloff that is not gaussian. The pixon image shows a tight envelope around the compact source, as does the clean image. As we move into the region where the albedo is present clean falls off in the same manner as the original map. The envelope around the the average of the flux for clean widens immediately after the compact source but then remains relatively constant. The pixon image immediately goes from a tight envelope around the compact source to a wider envelope in the region of the albedo. The size of the envelope remains constant radially but the average moves up and down in an unpredictable manner. For instance at approximately 3.5 arcsec from the peak of the compact source pixon images what appears to be a 'pileup' of flux. At the edges of the image the flux does not drop away to zero but seems to level off.


The plot below shows the gaussian model of the compact source (orange),the original data map with the minimum and maximum value at each radial distance (Black), the Pixon image with the minimum and maximum value at each radial distance (Purple), and the clean image with the minimum and maximum value at each radial distance (Cyan) for a source near the limb.

Original Data Map (Black), Pixon Map (Purple), Clean Map (Cyan), Model Gaussian (orange) for a source near the limb)

In the case of the source near the limb the averages of each the original data, the clean image, and the pixon image fall off in a similar manner to the averages near the disk center. There is more variation at any given point with the average moving up and down, but the trend is similar. However, looking at the min and max for the original data map, and the imaging algorithms the foreshortening effect shows up. There is a much larger envelope around the average for each method. The min and the max are much more widely separated. The min falls off to zero more rapidly and the max is higher in each case, at a given radial distance. This is what we would expect in the case of foreshortening. Foreshortening creates an asymmetry in the source and the albedo. The plots below clearly show this asymmetry.

The following plot shows the the average and the min and max for the clean images near the disk center and near the limb.

The clean profile for an image near disk center (Cyan), and near the limb (Purple), with the minimum and maximum values also shown for each radial distance.

The purple plot shows the values for the image near disk center. The cyan plot shows the image near the limb. The image near the disk center shows a tight envelope around the average value for the extent of the compact source. As we move radially away from the compact source the envelope around the average flux gets wider but remains symmetric. Looking at the cyan plot, the average flux away from the compact source falls of slightly more than near disk center, but with a similar slope. It does not fall off in the same smooth fashion as the image near disk center, showing higher and lower fluxes as it approaches the edge of image causing a 'spiky' pattern. The more interesting aspect is the behavior of the min and max traces. They envelope formed by these around the average is wider than in the case of the image at disk center, with a sharper falloff for the minimum compared to the maximum. This is the expected behavior when foreshortening is present.The minimum values at a given radial distance away from the disk center will be smaller than the values toward the disk center. This means the minimum value and maximum values should differ more than in a case with radial symmetry.


The following plot shows the the average and the min and max for the clean images near the disk center and near the limb.

The pixon profile for an image near disk center (Cyan), and near the limb (Purple), with the minimum and maximum values also shown for each radial distance.

RHESSI Image Test - Clean v. Pixon

The following is an example from the RHESSI Imaging Test. The original map is two point sources, 20 arcsecs apart, 2 arcsecs in extent. The plot below shows the original map as the background image, the pixon contours in blue, and the clean contours in green, using a clean beam width factor of 2.

The original source map (background), with the pixon contours(Blue) for an image using detectors 1 to 4, and clean contours (Green) for an image using detectors 1 and 2.

In the case above the contours are all at the default percentage levels. The pixon contours at the 10% level are larger in extent than the same level for clean. This is due in part to pixon breaking down if only detectors 1 and 2 are used, with clean, the image was made with only detector 1 and 2. At contours above the 50% level both methods do a good job of getting the correct size and shape of the original map.

The plot below shows the flux profile for the original data map (Black), the clean image (Red), and the pixon image (Green), along a line parallel to the heliocentric X axis through the line symmetry of the original map in that direction.

The profile in the Heliocentric X direction through the center of the map. The profile for the original map is black, the clean profile is red, and the green profile is red.

The original map shows two peaks which are symmetric and 20 arcsecs apart. The clean profile and the pixon profile are both slightly offset from the center of the original data map. The peaks are offset toward the disk center by ~ 1 arcsec. Thye biggest difference between the images and the original data map is the relative flux of the two sources. In the original map the flux is equal for both sources. Clean shows a similar value of the flux for the source closer to disk center but the flux for the secon source is approximately ~10% lower. Pixon shows the flux for both sources at a lower value than the original map, with the source farther away from disk center having a higher value. I've looked at multiple configurations for the original data map and the flux measurement is usually wrong if multiple sources are present, regardless of the imaging algorithm. If the original sources are compact and separated clean and pixon can image the size of the source down to the 50% contour pretty well, but do a poor job of reproducing the flux in each source. This may be due to the fact that the algorithms do not preserve the total counts in the original eventlist used. Is this constraint is not used because it restricts the algorithm too much while it is iterating and prevents it from converging to an image?

RHESSI Website Migration

The new RHESSI website now has more content and increased functionality for adding various kinds of content.

Goals for Next Week

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