There are three double focusing locations along Hall C beamline. At each double focusing point, the beam forgets its former behavior, therefore, the beam optics tune at each section is decoubled with the previous. Any local disturpancy cannot be transfered to the next section. This is the major requirement of optics tune in order to provide high reliability and reproducibility.
During experiments, users want not only to have a well-focused beam on the
target, but also to have the beam spot size and the incident angle are
independent to beam momentum change, is defined as a term "double
achromatic focusing", i.e. R
= R
= 0. This function is executed
mainly by Hall C 
arc achromat.
Some users want to have continuous beam transportation when the Moeller
polarimeter is on. The 3 tesla Helmholtz coils give axial field only.
It doesn't effect the global beamline optics. The two Moeller quads
have certain few percent effect on the downstream optics. With fine
adjustment of 3C18, 19, and 20A, the effect can be reduced and a set of
compromising tuning parameters can be found.
A new energy measurement optics will be verified in 2003 by CASA. The idea is to use the first half of achromat (dispersive section) to determine the absolute beam energy and to use the second half achromat recombining beam achromatically. If this is doable, the Hall C beamline transporation optics will be greatly simplified by applying only optics model. The impact of such modification is to make beam energy measurement on-line, no optics alternation is necessary.