Alfa spider scans taken at 1665 Mhz

    100 Mhz filters centered at 1665 Mhz were installed after the dewar on  alfa. Heiles calibration scans (spider scans) were then taken centering each alfa beam on the source 3C43 (the flux used was 2.46 Jy at 1665 Mhz). 24 spider scan patterns were taken but only 13 of these were analyzed (the other 11 patterns spanned multiple files and were not analyzed). An attempt had been made to measure the alfa cal values at 1665 Mhz but the cal coupling into the horn at 1665 is very poor. The program ended up using the cal values at 1520 Mhz so the gain and Tsys were not accurately measured.
    Each pattern contains 4 strips of 6 beam widths each (about 3.5 Amin per beam width). While tracking the source, the four strips drove :

• strip 1: -az to + az (no za offset)
• strip 2: -za to +za (no az offset)
• strip 3: (-az,+za) to (+az,-za)
• strip 4: (-az,-za) to (+az,+za)

•  Fig 1 thru 7 Top plot:  The 4 strips (separated by dashed green lines) are plotted versus sample using  a linear power scale. The dashed black and red lines are the baselines that were used to compute the Db plots on the bottom of each page. There are two sets of black, red lines since each pixel was measured twice (except for pixel 5 which was only measured once).  Some of the pixels (3,4,6) have offsets between the two measurements (mainly for the first strip). This may be a real pointing error, but I think it is because of the uncertainty in the synchronization of the antenna drive and the data sampling.
• Fig 1 thru 7 bottom plot: The bottom plot  y axis has power measured in db below the peak value for each strip (note that the strips do not necessarily go through the source peak).  To remove the system temperature, a linear fit was done using the 4 lowest points on each side of the middle (the dashed lines on the top plot are the baselines used).

    The second set of plots show the system performance. They were generated after passing the measured data thru the heiles calibration software. This fits individual gaussians for each strip (the main beam and two sidelobes). It then does a 2d fit for the beam. The values for each pixel are plotted in a separate color.

• Fig 1 top plot: This is the SEFD measured for each beam using 2.46 Jy as the source flux.
• Fig 1 2nd plot: the average beam width in arc seconds for each measurement.
• Fig 1 3rd plot: The coma parameter for each pattern. The coma parameter is defined as : exp((x-x0)^2*(1-coma)/sigma^2). I think it is constrained be by less than  .2 .
• Fig 1 bottom plot: The average sidelobe height. This value is 3 to 6 db less than the single strip plots. The pointing error (next plot) would account for about a 1 db difference. It is also caused because this is the average sidelobe height. The large bumps that we seen in some of the total power strips are only on 1 side (coma) and not necessarily in the azimuth and za directions.
• Fig 2: This is the azimuth (top) and zenith angle (bottom) pointing error in arcseconds derived from the fits. It assumes that the az, za positions that are used for the strip positions are correct. Most pixels have an azimuth and za error of about 40 Asecs each. Pixel 4 differs with a zenith angle pointing error of -15 and 5 asecs. One of these patterns had lots of interference but the 2nd pattern was relatively clean.

Conclusions:

• The sefd at 1665 is 4.5 to 6.5 Jy (this depends on the accuracy of the flux for 3C43).
• The individual strip plots show sidelobes as large as -6 Db with a large coma lobe. The average sidelobe fits are -15 to -10db below the peak.
• The  fitted pointing errors are about 40 asecs in both az an za.  These offsets may be from real pointing errors, or from inaccuracies in aligning the az, za offsets of the strips with the data samples. The jumps (in az) between the two measurements of the same horn are probably alignment problems.