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This page was created on 16-Jun-2010 22:23 by PeterYoung

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Spatial offset of intensity and velocity features
Spatial offset of intensity and velocity features

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At line 15 changed one line
wavelengths (redshift); and wherever there is a increasing intensity gradient from north
wavelengths (redshift); and wherever there is an increasing intensity gradient from north
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The second feature noted by Tian et al. (2010) is that all the bright points found in the coronal hole have redshifts on one side and blueshifts on the other. Inspection of the images shows that the bright points are blueshifted on the north side and redshifted on the south side. This is again explained by the tilted point spread function.
The second feature noted by Tian et al. (2010) is that all the bright points found in the coronal hole have redshifts on one side and blueshifts on the other. Inspection of the images shows that the bright points are blueshifted on the north side and redshifted on the south side. This is again explained by the tilted point spread function (PSF).
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The velocity shifts due to the PSF are around the 5 km/s level. I.e., if a velocity of V km/s is measured at the intensity peak (along the Y-direction), then a velocity of up to V+5 km/s will be measured to the south of the brightening, and a velocity of up to V-5 km/s will be measured to the north of the brightening. These shifts occur at about 3-4 pixels away from the intensity peak (see the footpoint image earlier in this document}.
The velocity shifts due to the PSF are around the 5 km/s level. I.e., if a velocity of V km/s is measured at the intensity peak (along the Y-direction), then a velocity of up to V+5 km/s will be measured to the south of the brightening, and a velocity of up to V-5 km/s will be measured to the north of the brightening. These shifts occur at about 3-4 pixels away from the intensity peak (see the footpoint image earlier in this document).
In general users should be cautious about showing velocity maps in publications, as a reader's eye tends to be drawn to the features that are most redshifted or most blueshifted. These features could be artificially enhanced by the tilted PSF and give a misleading impression of the velocity field of the structure.
When deriving velocities, the values obtained where the intensity peaks (in the case of bright points), or where the intensity is fairly uniform should be correct.
The EIS team is investigating the PSF function and whether a software correction can be performed. The issue is complicated by the fact that the PSF probably changes shape along the slit.
This page (revision-16) was last changed on 11-Dec-2017 16:06 by Ignacio Ugarte-UrraTop