Intro  top

The Measurements   top

• Page 1 the top 2 plots show the za Error and az Error versus azimuth. The sources are color coded. The bottom 3 plots show the tensions in tiedowns 12,4,8 while these measurements were being made. Tiedown 8 lost tension around az=100 with B1140+223 and at az=50 with source B1317+179. These points are not included in the following plots.
• Page 2 Pointing errors Vs az,za The top two plots show za error vs za and az. The bottom two plots show the az error vs za and az. The black + B0340+048 have a large az error offset. You can see a 1 azimuth term in the za error vs az and a za slope in the za error vs za. The pointing errors are:
• ZaError: offset=1.41 Asecs, rms: 7.65 Asecs
• AzError: offset=7.08 Asecs, rms: 7.90 Asecs
• Page 3 Pointing errors color coded by receiver These are the same plots as page 2 except they are now color coded by receiver (lbw black, sbn red). The za dependence of the azimuth errors is more pronounced for the lbw (black) points than the sbn (red).
• Page 4 1az and 1az, sin(za) fits to the errors. A 1 azimuth term in the pointing errors can occur if the platform tilt changes from the value it had when the pointing model was made. This could occur after one of the tiedown encoder failures (if the new encoder was not set to the correct position). If the azimuth encoder was not reset correctly then there would be an azimuth error times the sine of the zenith angle (since the reported errors are on the sky). To test for this i fit the following models to the za and az errors:
• zaError= C0 + C1*cos(az) + C2*sin(az)

The top plot shows the 1 az fit to the za errors. The amplitude is 9.5 Asecs and the phase angle (of peak) is 45.4 degrees. The fit rms is 2.9 Asecs.
• azError= C0 + C1*cos(az) + C2*sin(az) + C3*sin(za)

The second plot shows the 1 az fit to the azimuth errors. Both the data and the fit have had the sin(za) portion of the fit removed. The amplitude is 7.1 Asecs and the phase angle (of max) is -59.8. This azimuth fit has an rms of 4.9 Asecs. A platform tilt should have the maximum error for azimuth and za out of phase by 90 Degrees. The measured difference is 105 degrees.
• Sin(za) portion of az error fit.

The bottom plot has the sin(za) fit to the azimuth errors (after removing the 1 az term from the fit and the data). The amplitude is 45.9 asecs (at the azimuth encoder). The radius of the encoder rack gear is about 60 feet. This translates to an azimuth error in the positioning of about .19 inches. At 10 deg za this would be a 7.8 Asec error on the sky (about equal to the azimuth offset error of 7.08 Asecs).

Conclusions  top

• There is a 1 azimuth term in the az/za pointing error. It's amplitude is 9.5 Asecs with a peak at az=45.4 degrees (I'm using the za error values since the fit was much better). It is probably coming from the tiedown cable lengths changing from the values when the model was made.
• A sin(za) term in the az errors of 45.9 Asecs exists. This would be an error at the encoder of about .19 inches. This error is more prominent in the lbw data than the sbn data. At 10 degrees za this is an error of 7.8 Asecs on the sky. The measured average az offset was 7.08 Asecs.
• The measured pointing error needs to be subtracted from the computed az, za value to point at the source. A positive za Error means the dome needs to be brought down. This can be done by pulling on a tiedown. Using the za Error of 9.5 Asecs and azimuth of 45.4 for the peak Za error, the tiedowns need to be moved by [.094, .027,-.122] inches (td12,4,8). Positive numbers mean the tiedowns need to pull the platform down. The reference position for the tiedowns at 70 deg F:
• 14.875, 14.616, 13.243 inches (old reference position)
• 14.969, 14.643, 13.122 inches (new reference position)
• I need to do azimuth spins with the tilt sensors to measure the tilt of the platform. From those values we can compute the new values for the tiedown cables to level the platform and compare them with the pointing errors.

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