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)
||4.4 Amin (264 Asec)
||5.2 Amin (312 Asec)
|az, za strip length
||30 arc minutes
||30 arc minutes
show the results on the tertiary and with 19 cm and 25 cm horn offsets.
Fig 1 plots the raw data by strip for the on and off positions. The
azimuth strips are black and the zenith angle strips are green. The top
plot is for the 25 cm offset while the bottom plot is the 19 cm offset.
The 25 cm data was taken before transit and the 19 cm data was taken
after transit. A 2D gauss fit was done to the data to get the amplitude,
offsets, beam widths, and rotation angle of ellipse. The dotted lines show
the data that was used for the fit (the sidelobes were excluded).
Fig 2 first two plots: show the results of the 2D gauss fit. The top 2
plots show the Amplitude of the gaussian (src deflection) divided by the
system temperature. The black lines have the horn on the optic axis. The
top plot versus azimuth angle has a fall off in gain from 180 -> 140 degrees
azimuth. It is caused by the focus error in the rails (which increases
by an inch or two over this region). There should be no gain variation
at lband for this error if the horn was placed correctly. The 3.3
inch horn focus offset moves the horn far enough off of the focal plane
that the extra inch or two rail error affects the gain. The red and green
plots are for the 25 cm and 19 cm offsets.
Fig 2 the 3rd plot: shows the ratio of SrcDeflection/Tsys for
the offset and on focus positions. Since the flux is common for on focus
and offset, the ratio is Gain/Tsys(offsetfocus) / (gain/Tsys(onfocus)).
For 19 cm it is .2db down. The 25 cm offset is about .7 db down. The system
temperature is constant over these turret motions (see
tsys for various turret motions page 2 of plot).
Fig 2 bottom plot: The beam widths for the major (*) and minor (+)
axis. The 25 cm offsets cause the major and minor axis beam widths to approach
Fig 3 plots the srcDeflection/(PeakSrcDeflection from fit) in db's. The
black plots have the horn on the optic axis while the red curves are for
the offset strips. There is a coma lobe present for the on axis azimuth
strips that is caused by the roll error in the rails. The offset position
increases the coma lobe on the opposite side. The table below lists the
worst case sidelobes.
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.
||.2 db loss (19 cm)
||.7 db loss (25 cm)
|az 25 cm
|az 19 cm
|za 25 cm
|za 19 cm
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.