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Systematic Pointing Error: 28 June 2004 - 09 March 2006

Between 28 June 2004 -- 09 March 2006 there was an indexing error in the Antenna Manager FITS file. The only parameters affected by this error were the observed commanded (Az,El) positions (OBSC_AZ, OBSC_EL). These columns of the Antenna Manager FITS file are typically only used during a Peak observation, where the offset coordinate mode is set to "Encoder". In this case the (OBSC_AZ, OBSC_EL) columns correspond to the (Az,El) coordinates of the J2000 pointing calibrator with time.

The error appears as an error in hour angle. The magnitude of the indexing error was 3 samples, or at a sampling rate of 10 Hz a timing error of 300 milliseconds. At the sideral rate of 15 arcsec per second this corresponds to 4.5 arcsec in hour angle. The direction of the timing error is such that the incorrectly recorded source position leads the true source position (e.g., in the southern sky the determined Az will be larger than the true Az as the sources moves from East to West with time or increasing azimuth).

The result of this error is that the telescope would track as expected, but the Local Pointing Corrections (LPCs) derived from Peak observations would be systematically incorrect, by typically a few arcseconds, due to the error in the recorded pointing calibrator tracking center positions. The functional form of the total error is simply 4.5*Cos(dec) arcsec. The relative distribution between azimuth and elevation depends upon the parallactic angle. Sample values for a range of source positions are given in the figures and tables below.

The errors are shown in the two figures below with the data plotted on an Aitoff projection from a northern and southern view. The red points are the correct (Az,El) position while the green lines indicate the magnitude and direction of the pointing error. (Note, the end of the green line is the incorrectly determined (Az,El) value.) The magnitude of the error has been multiplied by a factor of 3000 for clarity. The error goes to zero at the north celestial pole. As the source crosses the meridian the HA error goes into an Az pointing error, while as a source sets away from the meridian the HA error goes into an El pointing error. (Note, there are postscript figures attached below.)

The magnitude of this error is similar to the expected accuracy of a typical individual peak observation, and we do not expect it to be significant for observations made at K-band and below (frequencies below 26 GHz). The error may have had a marginal effect on observations made with the Ka and Q-band receivers (frequencies above 26 GHz); observers should feel free to contact Dana Balser or Richard Prestage for additional information.

-- DanaBalser - 14 Mar 2006

Attachment:	Action:	Size:	Date:	Who:	Comment:
northern.gif	action	19383	14 Mar 2006 - 21:41	DanaBalser
southern.gif	action	20414	14 Mar 2006 - 21:41	DanaBalser
northern.ps	action	2171606	14 Mar 2006 - 21:42	DanaBalser
southern.ps	action	1649500	14 Mar 2006 - 21:42	DanaBalser