Weekly Report 25Mar2011

RHESSI Albedo Imaging Test

The RHESSI Albedo Imaging test continued.

The following steps were attempted with Pixon an Clean:

• Pixon and Clean images were made from maps that contained two circular Gaussian sources.
• The sources were centered at locations 6 arcsecs apart.
• One source was compact (standard deviation of 2 arcsecs) while the other was extended (standard deviation of 12 arcsecs).
• Pixon and Clean images were made with three detector combinations 1-9, 1-4, and 4-9.
• The Clean images were made with a clean beam width factor of 2.

The map below is the original map used to create the calibrated eventlist files. The two sources are normalized to the same value. The counts in each source are equal.

The table below shows the detector combinations, the imaging algorithms, and the results for the centroid and standard deviations in each case.

Original Map Configuration	Imaging Algorithm	Detectors Used	Image Centroid X	Image Centroid Y	ST DEV X	ST DEV Y
Two Circular Gaussian Sources (offset by 4 arcsec)	Pixon	1 to 9	907.47	256.34	9.64	9.64
Two Circular Gaussian Sources (offset by 4 arcsec)	Pixon	4 to 9	907.21	256.18	9.38	9.56
Two Circular Gaussian Sources (offset by 4 arcsec)	Pixon	1 to 4	910.37	255.26	2.27	2.12
Two Circular Gaussian Sources (offset by 4 arcsec)	Clean	1 to 9	907.59	256.18	8.98	9.02
Two Circular Gaussian Sources (offset by 4 arcsec)	Clean	1 to 4	910.87	255.92	5.97	5.82
Two Circular Gaussian Sources (offset by 4 arcsec)	Clean	4 to 9	907.83	256.21	9.75	9.83

The centroids for both the Pixon and Clean images were in the same approximate locations. The centroids and standard deviations were all calculated using the flux tool. A circle of radius 30 arcsecs was placed around the image.

• For the images that involved the coarser detectors both Clean and Pixon found a centroid at ~907 arcsec.
• This location is around halfway between the center of the compact source and the center of the extended source.
• For the images using grids 1 to 4 only the centroid was located at ~910 arcsec.
• This was the location of the compact source in the original. The fine grids are seeing the compact source in both image algorithms. The extended source is not affecting the centroid.
• Using the coarser detectors the flux tool yields a standard deviation of ~9.5 arcsec in the X and Y directions. The extended source has a standard deviation of 12 arcsecs both directions. With the additional flux in the image from the compact source the overall standard deviation should be smaller than 12 arcsec so this is consistent with expectations.
• When using detectors 1 to 4 Pixon yields a standard deviation of slightly larger than 2 arcsec in both directions. Pixon is reproducing the parameters of the compact source when using the finer detectors. Clean finds a much larger standard deviation. With a maximum of 500 iterations Clean finds components outside of the compact source increasing the standard deviation. This may be a result of over cleaning the image.

The plot shows the flux profiles of images made with Clean from a map with two offset Gaussian sources, one with a standard deviation of two arcsecs and one with a standard deviation of 10 arcsec. The seperation of the two sources is 4 arcsecs. The plots were made from images using detectors 1 to 9, detectors 1 to 4, and detectors 4 to 9.

The plot shows the flux profiles of images made with Pixon from a map with two offset Gaussian sources, one with a standard deviation of two arcsecs and one with a standard deviation of 10 arcsec. The seperation of the two sources is 4 arcsecs. The plots were made from images using detectors 1 to 9, detectors 1 to 4, and detectors 4 to 9.