Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~phil/pnt/platMotion/pntErrVsPlatRotation.html
Дата изменения: Wed Jun 20 22:17:26 2012
Дата индексирования: Mon Feb 4 21:22:45 2013
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rotaPlatToSky

Pointing error as the platform is rotated.

jun12

Links:
Computing the pointing error
comparing the computed error with the measured error.

The platform can be rotated by pulling on the tie downs or redistributing weight on the platform.

Computing the pointing error:

The picture below shows the different parts of the platform rotation:

rotating the platform about the main cables.

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.

The distance the horn moves increases as the zenith angle increases. The sky angle is the purple distance divided by 435 feet. The platform angle is the distance divided by the blue line. The sky rotation will be about 6 times smaller than the platform rotation because of the ratio of the distances.

The picture below shows the ratio of SkyRotationAngle/PlatFormRotationAngle as a function of zenith angle:

SkyAngle/PlatformAngle vs za

The ratio changes because the distance from the center of rotation of the platform to the horn changes with za.
The idl routine platrotosky will compute the positions of the horn as the platform rotates as well as the rotation angle on the sky.

The plots show the computed pointing error for different platform rotation angles (.ps) (.pdf):

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)

processing: x101/120618/pntErrVsRot.pro

Comparing the computed and measured tilt errors.

A test was done using the source B2209+080 to check the tilt computations.

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.

The plots show the pointing errors as we tilted the platform (.ps) (.pdf):

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,.

processing: x101/120606/x102sbn.pro

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