About a decade ago the Chicago String Ensemble's chief maestro Alan Heatherington left for Buffalo, and the ensemble ended up cobbling together a couple of uneven seasons with guest conductors. Then Heatherington returned and built the CSE into a well-regarded orchestra distinguished by imaginative programming. In August Heatherington again severed his ties with the group, this time apparently for good. And again the ensemble has to reshuffle its programs and hire guest conductors, with an eye out for a successor. The candidate to be scrutinized in the opening series is David Loebel, now affiliated with the Saint Louis Symphony. An up-and-comer with a light touch, Loebel has already led several other prestigious orchestras, including the Chicago Symphony Orchestra. The program is a perennial favorite, consisting of baroque concerti plus a Purcell appetizer. The Purcell is the Fantasia Upon One Note, to be followed by three of Bach's violin concerti, which will showcase the CSE's David Katz, Alan Ellsworth, and Guillame Combet. The entire group will work up a sweat in Handel's Concerto Grosso in B-flat. Friday, 8 PM, Saint Pauls United Church of Christ, 2335 N. Orchard. Saturday, 8 PM, First United Methodist Church, 1630 W. Hinman, Evanston. Sunday, 3 PM, Emmanuel Episcopal Church, 203 S. Kensington, La Grange. 332-0567.