| We use the MEDICIS correlation program in order, firstly to validate the geometrical parameters given in the headers of the STEREO-Secchi-Euvi A (ahead) and B (behind) images, and secondly to present comfortable stereo pairs.
The relative centering between A and B is obtained within half a pixel by integer X and Y shifts. The relative magnification between A and B, very close to unity, is determined within 0.0002, i. e. 0 .14 pixel at the solar limb. At any time, up to mid February, 2007, the relative roll angle between A and B is adjusted within .01°, i. e. 0.12 pixel at the solar limb. On February 3, 2007, at a very moderate separation between A and B, the roll angle of, for instance, A is determined within 2°.
Static stereo pairs prepared according to the above prescription are viewed from computer screens by different methods. On small (800x1200, or 1200x1600) screens, anaglyphs have the advantage of displaying a large field of view. On high quality (IBM 2400x3840) screens, left/right or up/down stereo pairs viewed with the adequate convenient left/right or up/down Viewmagic TM devices allow simultaneously for a high quality and a large field of view. The amount of disparities, such as cosmic ray impacts, between the A and B images, is very small. For prolonged viewing sessions, we note that the up/down Viewmagic device with fixed mirrors, large X, small Y field of view is preferred to the left/right Viewmagic with large X, large Y field of view with adjustable mirrors.
When overplotting pixellized coordinate grids, solar images have to be enlarged in order to obtain smooth parallels and meridian lignes. On small screens, the field of view has to be more restricted than on large screens.
Even with a small angle, as of February, 3, prominences are easily located in front of or behind the solar limb. It remains to be investigated if the apparent mottles of the HeII 304 Ǻ chromosphere are real.
We then begin to investigate the differential rotation and the meridional velocities of the solar chromosphere from STEREO-A , STEREO-B and SoHO-EIT HeII 304 Ǻ images. We pay a special attention to adjusting a common grid of Carrington coordinates to theses three sets of data.