The 610 reciever is an uncooled reciever that
uses a cross dipole antenna. This page contains specific issues that relate
to the 610 receiver. Performance off the system (gain,tsys,sefd,pointing
..) can be found on the system
performance page under the 610 receiver.
system performance measurements
monitorying of Tsys
14jan04: 610 receiver moved to new turret floor position.
Gain,Tsys, beam width, and pointing Error.
610 receiver moved to new turret floor position.
14jan04: 610 receiver moved to new turret floor position. (top)
The 610 receiver was moved from
turret position 330.55 to make room for the alfa receiver. The following
steps were taken to find the new turret position.:
16jan04: The turret was swung +/- 20 turret degrees while tracking a source.
The center of these swings was 183.16 turret degrees. This was the starting
position for the calibration scans below.
16jan04: Calibration scans were done on 3 sources and the pointing
error and system performace was measured. The measured error was az:-200",
23jan04: The pointing model offsets for the 610 receiver were changed to
make them as close as possible to the average receiver model offsets. The
extra az error was then used to move the turret. Calibration
runs were rerun with the new pointing model offsets and turret position.
The mean offsets after these changes was 7 asecs in za and -1.4 asecs in
How the new model offsets/turret position was computed:
The telescope model contains an offset in az and za
(great circle) for each feed. If the feeds are positioned correctly, then
these offsets should be the same for each feed (they may not be zero since
the tilt of the platform, offset of the main bearing from the center of
the dish, or the lattitude of the observatory may not be exactly correct).
I decided to correct the 610 pointing error by trying to make the
pointing model offset for 610 the same as the other receivers. After doing
this, i moved the turret to make up for any remaining azimuth error.
To compute the new positions:
The pointing model values are added to the computed position of a source.
The pointing errors from the calibration scan need to be subtracted from
the computed position of the source so that you point at the source.
The table below shows the relevant numbers (all in arc seconds).
|std model offsets (these are added to the computed position)
|610 modelOffsets when data taken
|pnt Errors measured with above 610 model offsets:
(these are subtracted from the computed position)
|new 610 model correction
|error to be aborbed by turret move
With the 610 az offset at -42 asecs, there is -20 asecs of error that
needs to be subtracted from the computed position (so that we peak up on
the source). This means we need to move the azimuth or turret by
+ 20 asecs on the sky. The turret coordinate is opposite from the
azimuth (increasing turret position is counter clockwise when viewed from
above). 1 turret degree moves by about 45 asecs on the sky. The +20 asec
move would be -20/45 turret degrees. The new turret position is 183.16
- 20/45=182.71 turret degrees.
The new za model offset for 610 is 116 asecs larger that the standard
za offset for a feed. If this is from the positioning of the feed, then
the feed is positioned too low in the za direction (we add 116 extra arcseconds
to get on source). Using a radius of 435 feet gives :116/3600*!degToRad*435.*12=2.9
inches (using plate scale of 17.04 asec/cm would give 2.7 inches). So the
feed may need to be moved uphill by about 2.8 inches. For the 610 feed
with a wavelength of 20 inches, it is probably not necessary to move it
(although it probably wouldn't hurt to survey the horn in position ).