Recording a movie of spectrum while the Sun is drifting thru the slit allow to build a sequence of spectroheliograms
where spacing of wavelengths is a function of dispersion and spacing of pixels of the CCD array. cf. Spectroheliography and video . Then, it is easy to choose an image in the blue wing of Hα line and another one symmetrically in the red wing, and make difference to reveal the spectral shifts (a dopplergram).
For images in 256 gray levels, operation is :
[red image] - [blue image] + 127
It gives blueshifts in white and redshifts in dark. If no shift, color is middle gray. We can repeat this method with all couples of images from center of Hα line to few Angströms distance. This gives a collection of images showing spectral shifts more and more high, thus radial speeds more and more important. Stacking these images to make a movie allows to see a large range of radial speeds projected on the solar disk.
Observation on 18 september 2011
Solar observations has began with linear array, in the core of Ha line.
Clouds was bothering me to get images but let me to observe visually the spectrum. Some important spectral shifts appear (right) and i have connected the webcam. Four recordings was taken from 15:17 to 15:43 UT.
The reconstruction of spectroheliograms from spectra allows to determine the position of the phenomenon on the disk, at a place without sunspot but where are 2 small filaments. Below, the superimposition of an image in Ha, at 14h58 UT, and a spectroheliogram Hα-1A showing the active region.
The range of speeds is large and a unique dopplergram can't restore all the phenomenon. I thus build for each recording, a complete collection of dopplergrams - i.e. for all couples (Ha-x, Ha+x) - that I use to make a small movie. after all, each video shows how is distributed the plasma speed in a range of about ±200km/s .
Obviously, spectra are rather noisy, and lines near Ha bother the computing of dopplergrams. This produces stripes, darkening and pseudo shifts. Part of these defects could be attenuated with a better processing
Recording at 15h17m UT
Recording at 15h26m UT
Recording at 15h38m UT
The 4th Recording at 15h43 UT does not show any radial motion in this area.
Image linear array at 15h49m UT
We can see on this last image (except clouds) that the 2 filaments are almost invisible and 2 bright ribbons appeared. The 2 filaments have been disturbed by a flare and they flew away in a little more than one half an hour , thus disappearing at the sight of the instruments with thin pass band.
The clouds did not enable me to prolong the observation to see whether the filaments were reformed or not.