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)
To see what the output structure contains, do:
IDL> help,fit,/str
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.
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.
