  A challenge for nuclear physics is to measure masses of exotic nuclei up to
the limits of nuclear existence which are characterized by low production cross
sections and short half-lives. The large acceptance Collector Ring (CR) at FAIR
tuned in the isochronous ion-optical mode offers unique possibilities for
measuring short-lived and very exotic nuclides. However, in a ring designed for
maximal acceptance, many factors limit the resolution. One point is a limit in
time resolution inversely proportional to the transverse emittance. But most of
the time aberrations can be corrected and others become small for large number
of turns. We show the relations of the time correction to the corresponding
transverse focusing and that the main correction for large emittance
corresponds directly to the chromaticity correction for transverse focusing of
the beam. With the help of Monte-Carlo simulations for the full acceptance we
demonstrate how to correct the revolution times so that in principle
resolutions of dm/m=1E-6 can be achieved. In these calculations the influence
of magnet inhomogeneities and extended fringe fields are considered and a
calibration scheme also for ions with different mass-to-charge ratio is
presented.

Accelerator Physics;Nuclear Experiment