Overall instrument focus (Grating focus)#
The EIS focus quality was tested during the week starting the 15th September 2008. The grating focus test was undertaken in two grating movements on 2 consecutive days (nominal grating position+58 steps and nominal grating position+116 steps). Two specially designed studies (slit and slot rasters) were run at each focus position. No improvements in the focus were observed and the grating position remains as for pre-launch, i.e. EIS within focus.
After the grating was moved back to nominal position, it was expected that a small error may occur in the line positions (caused by hysteresis which is a common phenomenon in mechanisms) and this will be absorbed by the orbital variation (around 2 pixels). However the measured wavelength shift was around 4.5 pixels in the short wavelength direction, as shown below.
The planning tool/EIS-PREP wavelength calibration parameters were adjusted to account for the new line positions and the adjustment is transparent to users.