Intro:

• When did it occur:
  
• the eye passed over AO on 21sep1998 around 23:30
• What monitor data is available:
•  Wind data was taken on the platform (until i think the anemometer blew away:).
• I have not located it yet (.. the data, not the anemometer).
  
• td tension data:
• there was tension in the tiedowns during the hurricane. I remember seeing the total tension (sum of all tiedowns) oscillate by about 150kips in 3 seconds.. I remember this because i was in the control room talking to paul goldsmith up in ithaca. He asked me how things were going.. I remember glancing at the tiedown tension monitor and telling him i think we might have a problem :).
  
• I have not been able to find the tiedown tension data yet. It must be backed up on tape somewhere.. arun said he'd look for it..
• Tilt sensor data:
• the pitch tilt sensor was running in the dome during the storm. We lost data from 21sep/14:24 to 22sep/01:00 (we must have lost power). So there is no recording when the eye passed.. but there was plenty of oscillations after 22sep/01:00.
• the data is at /share/obs4/tilt/tilt.georgesN
  
• see below for more info.
  

Tilt sensor data

• The tilt sensor had been replaced at some point. I am using the current (pre dome collapse) calibration routine. This data may have been taken with the older sensor.
• When plotting the data I've removed the dc tilt value so it will only show changes. The amplitudes may be off a bit. The frequencies should be  ok.

When was data taken:

• 20sep98 22:18 started
• 21sep98 21:36 .. stopped recording data (power problems?)
• 21sep98 23:30  eye passed
• 22sep98 01:00  started taking data again
• 22sep98 20:30  stopped recording

Plots showing the pitch tilt sensor value and spectra  (.ps)   (.pdf)

• Top Frame: pitch value vs day of month
• the tilt sensor was sampled at 5 Hz.
  
• the vertical scale is uncalibrated degrees (see above).
  
• The x axis is day of month for sep1998 (21,22sep)
• there is no data taken between 21.6 and 22.04
• the pairs of vertical colored lines delimit data used to compute spectra.
• the purple line at 21.95 is when the eye of the hurricane passed over AO.
• spectra
• I computed the spectra of the tilt as: spc=abs(fft(pitch - dcpitch)). (i did't square the result)
• The bandwidth is 2.5Hz.
• Any oscillations greater than 2.5 Hz will alias back into the spectra.
• I used 4.8 hours of data. I then smoothed the spectra by 9 channels to give a spectral resolution of about .5 milliHz.
• The dashed vertical line is at .8 Hz. The dewars have a cold head that pumps helium down to the cold head. the cycles at .8 Hz. There are about 10 of these running on the turret floor. The signal looks strong since the cycling is pretty stable.
  
• Frame 2: (green) 21.0 to 21.2 (00:00 to 04:56) on 21sep
• This was before any strong winds arrived
• Frame 3: (blue) wind causing a few oscillations
• looks like a few bands of the hurricane were passing over ao.
  
• frame bottom: (red) strong oscillations.

Blowup of strong oscillations (.ps) (.pdf)

• This shows the early morning of 22sep after the eye passed.
• I've flagged and labeled the strongest frequencies:
• .633 Hz this is the widest oscillation
  
• .683 Hz
• .723 Hz (pretty narrow)
  
• .8 Hz (this is the crossheads on the floor)

Dynamic spectra of 22sep98 01:12 to 06:00 (.gif)

• 84 1024point spectra were computed from the tilt data 22sep98 01:12 to 06:00.
• the resulting spectra has a freq resolution of 2.5/512=.005 Hz
• You can see the time variation of the strong peaks.
• ignore the .8 hz from the crossheads.
  

SUMMARY:

•  The pitch tilt sensor on the turret floor measured oscillation frequencies of:
• .633 Hz (wide and strong)
• .683 Hz
• .723 Hz (pretty narrow).