Mathew's 430Mhz test feed after lowering it 3 feet

22Sep 2004

the cross scan data and the fits (.ps)  (.pdf)

 A 430 Mhz test feed was installed on carriage 1 for john mathew's to look into the 430 interferometer he wants to install.  4 cross scans on 3C286 were done on 12sep04 while the feed was 5 feet below the paraxial surface (the bottom of the carriage house was used as the reference).On 22 sep04 the feed was lowered about 3 feet and more tests were done. 4 cross scans were done on 3C286 using the test feed and 4 cross scans were done using the 430 Mhz linefeed (for comparison). The setup was;

    A 2-d gauss fit was done to each set of crosses to find the beam offsets, amplitude,  and fwhm. No cals were used so Tsys was not available (the receiver is not cooled). The plots show  the cross scan data and the fits (.ps)  (.pdf). The fit results for each measurement (12sep04 before the lowering, 22sep04 after the lowering , and 22sep04 linefeed) were averaged for the 4 crosses and are shown in the table below. The sefd is the System equivalent flux density. It is the strength of a source in Jy that you need to equal the system temperature (Tsys/Gain).
parameter 12sep04
srcDeflection [tsysUnits]  (24.3 Jy source) .05 .075 1.94
SEFD  [Jy] 486 Jy 324 Jy 12.5
(Tsys/Gain New)/(Tsys/Gain Ch) (486/8)
beamWidth Az (fwhm) 20.9 Amin 18.9 Amin 9.0 Amin
beam Width Za (fwhm) 23.2 Amin 20.0 Amin 9.7 Amin
pntErrAz  2.2 Amin -.32 Amin -.14
pntErrZa 1.9 Amin 1.9 Amin -.67
azOffset used (great circle:za=11) .5 deg .5 deg
zaOffset used  -1.6 deg -1.6deg
azimuth offset from line feed -3.66 ft
za offset from line feed 11.73 ft

Some comments on the results:

How good is the test feed.

    (See Rohlfs and Wilson tools for radio astronomy page 142 for the taper equations used below).
The measured beam width (fwhm = 20Amin) tells how much of the dish is illuminated by the feed. Since the horn is positioned close to Radius/2,  the half angle of the feed is close to twice the za measured from the center of curvature (see linefeed phase a=90-2*za). For the cos^2 taper this gives a horn beam width of
processing: x101/040922/