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[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
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Generally the EIS emission lines have a Gaussian shape and, by fitting a Gaussian function to the spectrum, one can derive line intensity, line width and velocity maps. The high sensitivity of EIS means that for many lines it is possible to perform good quality fits at each pixel in the image. The routine eis_auto_fit in Solarsoft takes the output from eis_getwindata (Worksheet 5) and fits a single Gaussian to each pixel in the image. It is called as
Generally the EIS emission lines have a Gaussian shape and, by fitting a Gaussian function to the spectrum, one can derive line intensity, line width and velocity maps. The high sensitivity of EIS means that for many lines it is possible to perform good quality fits at each pixel in the image. The routine EIS_AUTO_FIT in Solarsoft takes the output from eis_getwindata (Worksheet 5) and fits a single Gaussian to each pixel in the image. The calling procedure is:
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IDL> eis_auto_fit, wd, fit, refwvl=195.12
{{{
IDL> eis_auto_fit, wd, fit
}}}
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refwvl is used to specify a reference wavelength for the emission line, so that velocities can be calculated. Reference wavelengths for lines can be obtained from the CHIANTI database, e.g.,
The output from EIS_AUTO_FIT is an IDL structure called FIT. Intensity, velocity and line width arrays can be extracted from FIT by doing:
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IDL> which_line,’fe_12’,195
{{{
int=eis_get_fitdata(fit,/int,calib=calib)
vel=eis_get_fitdata(fit,/vel,calib=calib)
wid=eis_get_fitdata(fit,/wid,calib=calib)
}}}
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If a wavelength window contains more than one emission line, then it is possible to use a reduced wavelength range with the wvlpix= keyword. E.g., wvlpix=[194.9,195.4] to use only pixels between 194.9 and 195.4 Å.
The optional input 'calib' specifies which radiometric calibration to apply to the data. Please check the [calibration discussion page|EISCalibration] for more details. The recommended options are 1 or 3 for the Del Zanna (2013) or Warren et al. (2014) calibrations. Neglecting to specify calib means that the original laboratory calibration (as performed in the call to eis_prep) will be retained. Although 'calib' does not need to be specified for the velocity and line width arrays, it's good practice to always specify it.
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To see what the output structure contains, do:
Error arrays for each parameter can be obtained by adding the optional output 'error=error'. Make sure that 'calib' takes the same value as it did when you extracted the intensity array.
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IDL> help,fit,/str
{{{
int=eis_get_fitdata(fit,/int,error=int_error,calib=calib)
}}}
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the tags int, cen, vel and wid correspond to intensity (units: erg cm-2 s-1 sr-1), centroid (units: Å), velocity (units: km s-1) and Gaussian width (units: Å). To get the FWHM, multiply the Gaussian width by 2.35. For each of the tags, there is an associated 1σ error array.
This example shows the most simple use for eis_auto_fit. The routine can also be used to perform multiple Gaussian fits and a full description of the capabilities are given in two documents: '[Gaussian fitting for the Hinode/EIS mission|ftp://sohoftp.nascom.nasa.gov/solarsoft/hinode/eis/doc/eis_notes/16_AUTO_FIT/eis_swnote_16.pdf]' and '[Gaussian fitting examples using eis_auto_fit|https://ftp://sohoftp.nascom.nasa.gov/solarsoft/hinode/eis/doc/eis_notes/17_AUTO_FIT_EXAMPLES/eis_swnote_17.pdf]'.
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The results of the fitting process can be checked using the widget-based routine eis_fit_viewer:
Some points to note about the line fit parameters:
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IDL> eis_fit_viewer, wd, fit
The top 3 graphic windows in the GUI show line intensity, line velocity and line width (FWHM). Note the vertical bands in the velocity window – this is due to the orbital variation of the lines on the detector (see Worksheet 7a).
With the ‘Zoom’ button depressed, the mouse can be used to draw a ‘rubber-band’ box to zoom into the images. With the ‘Pixel’ button depressed, the line profile from the selected pixel will be drawn in the bottom-left graphic window.
The bottom-right window shows a histogram of the selected images pixels for either intensity, velocity or line width.
!!Exercise
#Some horizontal stripes can be seen in the velocity and line width maps. What could be causing these?
#Use eis_fit_viewer to find the approximate variation in line centroid position caused by the orbital motion of Hinode.
# The velocity map has been calibrated such that the average velocity across the whole raster is set to zero. Alternative methods of calibrating the velocity map are described in the fitting documents listed above. If velocity maps are used in scientific analysis, then scientists should consider carefully how the maps are calibrated.
# When the line width array is extracted with eis_get_fitdata, it is automatically corrected for the instrumental line width. To keep the instrumental component, use the /keep_inst_width keyword. The thermal component of the line width can be removed by using the thermal_wid= optional input - see the routine header for details on how to use this keyword.
This page (revision-12) was last changed on 27-Nov-2017 16:52 by Peter YoungTop