If you string together consecutive raster images into a movie, you will find that the field of view moves around in time. For rasters of duration 30mins or more, this will translate into an apparent distortion (flexing) of the image. Some examples are available at:
the pointing wobble seems to be about 3-5 arcsecs in X and Y.
Shimizu-san (private communication) has stated that XRT and SOT are co-aligned to within 0.5 arcsec. For EIS it is only 5 arcsec.
It seems that there are some distortions occurring within EIS (structure, mirror, grating?) over time that are causing this wobble in the EIS images.
Note that this means sit-and-stare observations with EIS are not recommended! It is preferably that you use small rasters (5-10 mirror positions) so that you generate small images that can be cross-correlated with each other.
EIS Pointing Jitter#
Following on from what was mentioned at the EIS UK meeting in July... Here is an example of the apparent pointing jitter that appears in the y-axis. The effect of the orbital pointing drift appears visible as a slow variation throughout the observations. However, an additional jitter appears as a kind of "sawtooth" oscillation in the y-axis with a period of around 3-4 minutes. This could have a big effect on sit-and-stare Doppler observations, where there is a velocity gradient across the slit.
These pointing offsets were produced by performing a 2D cross-correlation on a Mg VII 278 slot movie, and the x-axis has units of seconds.