Date: Tue, 30 Mar 1999 20:48:16 -0800
From: tarbell@sag.lmsal.com
Subject: Michelson Tuning 

SOI-MDI technical mail message # 155  on  Tue Mar 30 20:40:21 PST 1999
Mail from: tarbell@sag.lmsal.com
Subject: Michelson Tuning
Original Date: Tue, 30 Mar 1999 20:38:36 -0800
---------------
I analyzed the detuned of 11-MAR-99, which were good data and a long run
to provide an excellent average detune set.  For comparison, I used some
full disk velocities from 15-MAR-99 (since the 11-MAR ones had been
cleaned off our disk by now).  As usual, the detunes provide a very good
prediction of the measured velocities.  But the measured FD images
include the extreme limb velocities, especially in the W & NW, which are
cropped from the detunes.  The observed velocities are summarized in the
table below.

The present continuum tuning is (M1, M2) = (24, 41).  Using
disk-averaged detune wavelengths, cotuning on 11-MAR was (23.3, 41) in
OBS mode and (23.7, 41) in CAL mode.  So the cotuning is really about
right now, which surprised me at first.  The reason is that a big
relative tuning change was made in Oct-98, which probably
overcompensated the M1 drift since Jan-98:

#               Oct-98  Jan-98  Aug-97  Mar-97  22-Jul-95
MTM1    FC      24      #29     #27     #32     #34
MTM2    FC      41      #42     #41     #42     #41

Here is a table showing measured velocities of various types on 15-MAR,
along with what would have been measured if we changed the tuning in
three ways.  The first and third improve the cotuning slightly, while
the second leaves it alone.  I hope your mail program doesn't mangle
this table too badly.

                      Min      Max      Disk Ctr   Disk Avg  CAL Mode

Actual  (24, 41)    -1900      4100       250       610       326  

Retuned (25, 42)    -2180      3790       -40       320        27

Retuned (26, 42)    -2300      3640      -160       200       -89

Retuned (27, 43)    -2560      3340      -455       -80      -388

The actual measured velocities are too positive these days, though not
extremely so.  The orbital velocity on 11-MAR was about +445 m/s, using
an average for the same day in 97 and 98.  By 15-JUN, it will drop to
about +220 m/s, and to -300 m/s by mid-August.  I think either of the
first two retunings (25 or 26, 42) would be fine.  The first would keep
us happy longer, the second is optimized a bit more for the dynamics
period.  Maybe retuning during dynamics is out of the question, anyway.

Ted