diff --git a/docs/JFJOCH_BROKER.md b/docs/JFJOCH_BROKER.md
index c6fcfae70..cb5e75c79 100644
--- a/docs/JFJOCH_BROKER.md
+++ b/docs/JFJOCH_BROKER.md
@@ -254,13 +254,17 @@ Live processing of serial collections is **experimental**. With rugnux live mode
 (non-rotation) collection in the FPGA workflow, in the standard detector mode, is analysed as frames from a
 DECTRIS detector are: the converted frames go to GPU spot finding, indexing and integration, with rugnux's
 stills spot finder (adaptive threshold, single-pixel spots kept). Before any frame is analysed, the first
-100 frames are run through rugnux, in memory, as `rugnux --no-merge` would run on them. That pre-pass
+500 frames are run through rugnux, in memory, as `rugnux --no-merge` would run on them. That pre-pass
 decides the beam centre, refines the detector distance (where the dataset settings give a unit cell) and
 masks the beam-stop shadow and defective pixels; every frame of the collection is then analysed with that
 geometry and mask. The start message waits for the pre-pass, so it states them, and the end message carries
 the centre and distance used (`refined_beam_center_x/y`, `refined_detector_distance_mm`,
 [CBOR](CBOR.md)). The broker's own distance is taken as the beamline's: unlike offline rugnux, the pre-pass
-does not search the whole distance range.
+does not search the whole distance range. It refines the centre and distance for a detector square to the
+beam, as the frames are then analysed. At a serial hit rate the distance needs the 500 frames to be refined at
+all: on fewer, too few index for the geometry refinement to run, and the stated distance is kept. The image
+buffer has to hold them, and the frames that arrive while the pre-pass runs, in half its slots (for a JUNGFRAU
+8M, a slot is the size of an uncompressed frame); a smaller buffer starts the pre-pass on as many as it holds.
 
 Frames arriving during the pre-pass, or faster than the analysis, wait in the image buffer, in at most half of
 its slots; a frame that finds no free slot is dropped and counted as for a full image buffer, so analysis
diff --git a/receiver/StillsGPUAnalysis.h b/receiver/StillsGPUAnalysis.h
index 493a090d7..f20409479 100644
--- a/receiver/StillsGPUAnalysis.h
+++ b/receiver/StillsGPUAnalysis.h
@@ -43,7 +43,10 @@
 // merge = false leaves it out.
 class StillsGPUAnalysis {
 public:
-    constexpr static int64_t PREPASS_IMAGES = 100;
+    // Enough frames for the distance to be refined at a serial hit rate: of 100 frames of a collection indexed
+    // at 18%, 7 indexed at a distance 2.6% off and the geometry refinement did not run; of 500, 34 did and
+    // refined it to 0.4%. They wait in the image buffer, which has to hold them in half its slots.
+    constexpr static int64_t PREPASS_IMAGES = 500;
     constexpr static std::array<double, 2> CHECKPOINTS = {0.5, 0.75};
     constexpr static int MERGE_THREADS = 8;