This page (revision-11) was last changed on 07-Dec-2016 14:14 by PeterYoung

This page was created on 14-May-2007 13:16 by Louise Harra

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8 26-Jan-2009 05:53 1 KB David R Williams to previous | to last Deleted the current-time stamp which gave the wrong date for David's contribution.
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OrbitalVariationLinePosition
OrbitalVariationLinePosition

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There is a shift of the spectral line position during the orbit. This is due to the thermal changes occuring across the instrument during the orbit and was an expected effect. We are currently collecting data that will allow us to model this accurately for correction. As well as orbital variation, there will be seasonal variation as well. In the meantime, especially when dealing with rasters, you must correct for this. An uncorrected velocity map looks like the attachment, where you can clearly see the change in red and blue shift during the orbit. Software is being produced to correct for this but is not within eis_prep. It is now released to SSW (eis_orbit_correction). You can correct for the variation by modelling the line position along the time direction and subtracting that component which is sinusiodal in shape. [Orbitfiles | OrbitalVariationLinePosition/orbital.jpg]
!Orbital drift of the EIS wavelength scale
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There is a shift of the spectral line position during the 98.5 minute Hinode orbit that is due to the thermal changes occuring across the instrument during the orbit and was expected before launch.
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The effect is clearly seen if a velocity map is made from an EIS raster, such as the example below.
[Orbitfiles | OrbitalVariationLinePosition/orbital.jpg]
Wide, alternating bands of red and blue shift are seen that have an amplitude of about 35 km/s and thus mostly dominate the real solar Doppler shifts.
The amplitude is approximately fixed in wavelength/pixel space to 0.0223 angstroms/1 pixel, and thus the velocity amplitude varies with wavelength. E.g, for Fe XII 195.12 it is 35 km/s, while for Fe XV 284.16 it is 24 km/s.
Two methods are available to users for correcting the orbital drift: one uses instrument housekeeping data, while the other uses the spectral data themselves. Both have drawbacks, and a general rule of thumb is that EIS absolute velocities are accurate to no better than 4 km/s.
!!Instrument housekeeping data correction
This method is described in detail by [Kamio et al. (2010)|http://adsabs.harvard.edu/abs/2010arXiv1003.3540K,] and basically makes use of temperature readings within the EIS instrument to model how the Fe XII 195.12 line drifts on the detector over the course of the mission. Unlike the correction method based on measured centroid positions (see below) there is no need to make an assumption about the large scale velocity structure in a single raster.
[Software implementation of the housekeeping data correction method|HKmethod]
!!Measured centroid correction
We are currently collecting data that will allow us to model this accurately for correction. As well as orbital variation, there will be seasonal variation as well. In the meantime, especially when dealing with rasters, you must correct for this. An uncorrected velocity map looks like the attachment, where you can clearly see the change in red and blue shift during the orbit. Software is being produced to correct for this but is not within eis_prep. It is now released to SSW (eis_orbit_correction). You can correct for the variation by modelling the line position along the time direction and subtracting that component which is sinusiodal in shape.
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This page (revision-11) was last changed on 07-Dec-2016 14:14 by PeterYoungTop