The public domain image processing programme ImageJ is recommended for schools in our "Astronomie & Internet" project.  When the exact time of an observation is important, ImageJ reads the FITS header to produce a Julian Date at the time of mid-exposure.  The old STELLA FITS headers contained a "JD" card which was useless and resulted in incorrect times.  This card has been removed from the FITS headers in all data (including images obtained in the past).  If you need to correct old data files, we offer a special ImageJ plugin which renames "JD" to "JD-END" and thereby forces ImageJ to calculate correct times.