The Eiger2 Si 500K's multi-threshold JFJ preview messages key the frame under a threshold name (e.g. "threshold_1"), not "default" as eiger_1_5/ eiger_9 do -- confirmed by capturing the live JFJ ZMQ preview stream. _preview_callback hardcoded "default", so it silently dropped every frame for eiger2_si_500k (251+ "without data" errors in DeviceServer.log, matching the empty BEC image widget). Now prefers "default" when present and falls back to whichever single channel the broker actually sent. Also document the JFJoch Web UI (http://sls-jfjoch-001.psi.ch:8080) and the Eiger 2 Si 500K detector server address (129.129.122.159) in bl_detectors.yaml's Eiger section. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>