Field of view at the focal plane using lbn


      The multi beam system will have off axis feeds. The size of the horn selected determines the minimum spacing and places some constraints on the optics. To check the current performance, the field of view was measured using the current lband narrow horn (kildall horn). Azimuth (great circle) and zenith angle strips were done on the source 3C48 (J0137+331). The procedure did an az, za pair of strips with the horn centered on the tertiary and then a pair of strips was done with the turret rotated to move the horn off of the paraxial ray. The azimuth position was updated to correct for the pointing error caused by the horn motion. The on/off pairs were separated by 3 minutes of time. Data was taken on 07jun02 (19 cm offset)  and 10jun01 (25 cm offset) from 8am to 9 am. To compute the motion I used:
  • radius focal circle on turret = 70.48" + 18.75*sin(8.11) = 73.125 inches.
  • 1 degree turret motion = 73.125*2*pi/360.*2.54=3.242 cm
  • pointing correction -45 asecs/turret degree. I then played with this offset to maximize the source deflection.

  • The parameters used were:
    frequency: 1405 Mhz 19 cm offset 25 cm offset
    turret pos (310.14 nominal) 315.92 (+5.78 tur degrees) 317.84 (+7.7 tur degrees)
    pointing correction  4.4 Amin (264 Asec) 5.2 Amin (312 Asec)
    az, za strip length 30 arc minutes 30 arc minutes

        The plots show the results on the tertiary and with 19 cm and 25 cm horn offsets.


            The za strips are not affected by the turret offsets. These results are for the lband narrow horn (kildall horn) which is out of focus by 3 or more inches. There is also a rail focus error that varies across the track by about an inch or two. The roll error at high za is causing a coma lobe for the on axis measurements.
        Horn spacings currently under consideration for the multi beam system are 26 cm and 22 cm. They will not have the same horn illumination as the kildall horn used in this test.
    processing: x101/020610/