Start bumps with the cantilever _off_ the surface.

Photodiode sensitivity (and thus, bump slope) decreases as the
zero-deflection photodiode input moves away from zero Volts.  In order
to mitigate the effect of this on our calibration, we need to keep
track of the zero-deflection signal.  By starting our bump off the
surface, our bumps should always start and end in the zero-deflection
regime.

This will make analysis a bit more complicated though.  I'll get to
that next.

diff --git a/calibcant/calibrate.py b/calibcant/calibrate.py
index f78520ffe5ed6f4f41a7c54b4a9c61330b8f9b17..269dafa18cbe50420a03e42871cc89b0964ed5da 100755 (executable)
--- a/calibcant/calibrate.py
+++ b/calibcant/calibrate.py
@@ -230,13 +230,14 @@ def vib(**kwargs) :
 # with proper spacing relative to the surface.
 
 @splittableKwargsFunction()
-def move_just_onto_surface(stepper, zpiezo, Depth_nm=100, setpoint=2) :
+def move_just_onto_surface(stepper, zpiezo, Depth_nm=-50, setpoint=2) :
     """
-    Uses z_piezo_utils.getSurfPos() to pinpoint the position of the surface.
-    Adjusts the stepper position as required to get within stepper_tol nm
-    of the surface.
-    Then set Vzp to place the cantilever Depth_nm onto the surface.
-
+    Uses z_piezo_utils.getSurfPos() to pinpoint the position of the
+    surface.  Adjusts the stepper position as required to get within
+    stepper_tol nm of the surface.  Then set Vzp to place the
+    cantilever Depth_nm onto the surface.  Negative Depth_nm values
+    will place the cantilever that many nm _off_ the surface.
+    
     If getSurfPos() fails to find the surface, backs off (for safety)
     and steps in (without moving the zpiezo) until Vphoto > setpoint.
     """