The detector CTI characteristics are subject to change and require assessment on a regular basis in flight.
The combination of bright emission lines in Capella and the smooth continuum of Mkn421 provide the means for
this, usually in conjunction with gain measurements. Once every two years, Mkn421 is observed offset by 
armin
in the cross-dispersion direction near the edges of the detectors at 
armin for purposes of monitoring
parallel transfers.