Worksheet 6 - Fitting Gaussians to the data #

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:

IDL> eis_wave_corr, l1name, offset
IDL> eis_auto_fit, wd, fit, offset=offset

The first command is used to determine the wavelength offset at each spatial pixel due to the tilt of the EIS slit and orbit variation of emission line centroids. Correcting for these effects yields a much more accurate velocity map.

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:

int=eis_get_fitdata(fit,/int)
vel=eis_get_fitdata(fit,/vel)
wid=eis_get_fitdata(fit,/wid)

Error arrays for each parameter can be obtained by adding the optional output 'error=error':

int=eis_get_fitdata(fit,/int,error=int_error)

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' and 'Gaussian fitting examples using eis_auto_fit'.

Some points to note about the line fit parameters:

1. 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.
2. The Gaussian line widths include a significant instrumental component of approximately 55 mA.