Computing the pointing error:

• The rectangle is the platform. The arc is the paraxial surface
• The black lines are before the rotation, the red lines are after the rotation.
• The rotation is about the bottom of the platform where the main cables attach.
• The blue dashed lines go from the center of rotation to the horn (sitting on the paraxial surface). The horn rotates through the same angle as the platform.
• The green dashed lines go from the center of curvature of the reflector to the horn. This is about 435 feet.
• The center of rotation of the platform is about 61 feet above the paraxial surface.
• The purple line is the path the horn moves during the platform rotation.
  

• This shows the pointing error in as you tilt the platform. This is the maximum pointing error. It will occur  when:
• The dome points at the max tilt direction (all error in za)
  
• The dome points 90 degrees from the max tilt direction (all error in az)
  
• The with the azimuth,dome pointing in the direction of the corner that is moving up or down.
  
• The  offsets are described using tiedown offset, platform corner offset, and platform angle. The table contains:
  
• column 1 TdOff::
  
• tiedown offset in inches. + has the tiedowns moving up vertically. The tiedowns are at 192 feet from the center of the triangle.
  
• column 2 PlatCornOff
• The motion of the platform corner (at the tiedown cable radius). The tiedown cable moves 1.7 times the platform motion
• column 3: PlatAngle:
•  platform angle in arcMinutes.
• Positive angles have the corner (where the dome,az is located) moving up vertically above the focus position.
• Different colors are different offset values.
• The top half of the plots show when the platform corner  tilts up vertically.
  
• the bottom half of the plot has platform corner positions below the focus.

Az, Za variation of the platform tilt:

• Unlike the vertical motion of the platform, tilting the platform affects both the az and za error.
  
• If theta is the azimuth of the maximum tilt direction
• pntErrMax is the maximum pointing error
  
• Za error = pntErrMax*cos(az-theta)
• azError= pntErrMax*sin(az-theta)

Comparing the computed and measured tilt errors.

• The source was tracked rise to set with the platform in the nominal position.
• The pointing errors showed an increasing tilt in the azimuth direction when the za was above 12 deg za.
  
• This is examined in a separate page.
• The platform was tilted by moving the tiedowns +/- .5 and +/- 1 inches.
• When the tiedowns move by 1.7 inches, the platform corner moves by 1 inch (the rest is cable stretch and boom rotation).
• The tiedown cables are 192 feet from the center of the platform. The table shows the motions:
• The tilt was toward corner 12. When td12 was moved down 1 inch, then td4,8 were moved up .5 inches so the average height of the platform remained the same.
  
•  The table shows the motions:

• td12 motion inches	corner 12 motion inches	platform angle ArcSeconds
  +0.5 (up)	+.29 (up)	+26.3
  -0.5 (down)	-.29 (down)	-26.3
  +1.0 (up)	+.59 (up)	+52.7
  -2.0 (down)	-.59 (down)	-52.7

  
  

Processing the data:

• Each days pointing error was interpolated to the az,za positions taken when the platform was in the nominal position.
• The pointing error for the nominal position was removed from the errors for each day that had a tilt. This should have removed the pointing errors of the source.

• Page 1: pointing errors as we tracked the source with different tilts
• The top 2 frames are za pointing errors vs za,azimuth
• The bottom 2 frames are az pointing errors vs za,azimuth
• Each of the colors is a different platform tilt
• + B2209+080  has the pointing errors with no tilt.
• You can see the azimuth errors vs za increase with za>13 (3rd frame)
• red B2209+080_+.5  has the errors when td12 was moved up by .5 inches.
• Page 2: arrow plot showing errors and patch across the dish for each day.
• The arrow length is the magnitude of the error (1 tick mark  = 5 arcseconds).
• the direction of the arrow is the direction of the error.
• When the arrow is along a radius, it is pure za error.
•  When it is perpendicular to a radius, it is pure az error.
• the circles are spaced 5degrees in za.
• Page 3: relative pointing errors for each tilt
• the pointing errors for no tilt have been removed from each days errors.
• Za errors:
• as td12 moves up , the point error increases
• as td12 moves down, the pointing error decreases
• As errors.
• As td12 moves up , the slope vs azimuth becomes more negative
• As td12 moves down, the slope vs azimuth becomes more positive
• Page 4 Platform tilt vs azimuth as we move td12 up and down.
• Top: az tilt vs azimuth
• Bottom: za tilt vs azimuth
• The pointing is then computed using these tilts (see section at top of page).
• Page 5,6:Measured and computed pointing errors for various tilts:
• Each frame is a separate tilt.
• The black line is the measured data, the colored line is the computed error.
• Page 5 azimuth errors.
• The computed errors fit the measured data pretty well.
  
• Page 6 za errors.
• The computed za errors follow the measured data except for +.5 inches whose computed error has a larger slope than the data.  There may have been some other problems on this day.

Summary:

• The computed pointing error for a platform tilt track the measured errors pretty well.
• The idl routine:
• platRotToSky() can be used to convert from platform tilt to pointing errors,.
  

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