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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OrbitalVariationLinePosition
OrbitalVariationLinePosition

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At line 1 changed 33 lines
[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
!!!Orbital drift of the EIS wavelength scale
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. 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. Tests are being performed to compare the two methods but generally inexperienced users should use the housekeeping data method.
NOTE: the ''accuracy'' of EIS absolute velocities obtained with these methods is 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
This method uses line centroid positions measured from a single raster to define the orbit correction for that same raster. It is somewhat risky since it requires an assumption about large-scale velocity flows within the raster. E.g., often one makes an assumption that a quiet Sun region within the raster is at rest.
The method is implemented through the process of performing Gaussian fits to the emission lines in a data-set, and so the reader is referred to the documents listed below:
[Gaussian fitting for the Hinode/EIS mission|https://hyperion.nascom.nasa.gov/svn/eis/release/doc/fitting/eis_auto_fit.pdf]\\
[Fitting examples using the eis_auto_fit|https://hyperion.nascom.nasa.gov/svn/eis/release/doc/fitting/eis_auto_fit_examples.pdf]
The method essentially requires an initial guess to be made for the orbit variation using the routine {{eis_wave_corr}}, and then refinements are made using Gaussian fits to the line of interest.
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. 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. Following testing it will be released to SSW. In the meantime 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]
This page (revision-11) was last changed on 07-Dec-2016 14:14 by PeterYoungTop