alfa performance at OH with 10db amps in
06may05
links to plots:
comparison
of the system performance for alfa and lbw at OH (.ps) (.pdf)
(no amps in)
comparison
of the system performance for alfa and lbw at OH (.ps) (.pdf)
(with amps in)
A filter bank was installed in the alfa receiver
in mar05 that allowed switching between the 300 Mhz filters and filters
that let you look at the OH line. On 01apr05 the system performance
was measured using the continuum source CTA21 (B0316+162. 7.2 Jy at
1666 Mhz). After these measurements amplifiers (about 10db)
were installed before the filter bank to improve the noise figure of the
system. The system performance was remeasured on 06may05 using the continuum
source B0428+205 (3.8 Jy at 1666 Mhz). For each of these dates the
procedure was:
-
do 1 minute on/off position switching on the continuum source
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The data was centered at 1666 with a low side 1st lo.
-
The wapps were setup for 1 second dumps, 6.25 (apr) and 3.125 (may) Mhz
bandwidths.
-
on/offs were also done with lbw using the 1500 to 1800 Mhz filter.
Since the alfa
cals do not work at 1666 Mhz the metrics used were srcDeflection/Tsys
(G/T) and the ratio of G/T for alfa and lbw.
01apr05 with no amplifiers installed:
The plots show the comparison
of the system performance for alfa and lbw at OH (.ps) (.pdf)
(no amps installed).
-
Fig 1: Average on source and off source. Black is polA, red is polB.
The top plot is alfa while the bottom plot is lband wide. These plots include
the system temperature. The za's for the measurements were alfa:7.1, lbw:11.5
degrees. The power levels were adjusted to the optimum levels (1.) on source.
The off source levels dropped to about .4 of optimum.
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Fig 2: This shows srcDeflection/Offsrc. This is G/T. The two polarization's
of alfa differ by about 40%. This source is not polarized at lband. Using
the fluxes of 7.2 Jy, the sefd for the measurements are: Alfa: 4.3,6.0
Lbw: 2.4,2.3 Jy.
-
Fig 3: This shows (G/T alfa)/(G/T lbw) The average values are: .55
PolA, .37 polB.
06may05 with the amplifiers installed:
This used the source B0428+205 (3.8 Jy at 1666 Mhz).
The power levels were adjusted off source.
The plots show the comparison
of the system performance for alfa and lbw at OH (.ps) (.pdf)
(with amps installed).
Fig 1: This shows srcDeflection/Offsrc (G/T). Black is polA and red is
polB. The top plot is pixel 0, the middle plot is pixel 1, and the bottom
plot is lband wide. The two polarization's of alfa differ now differ by
about 30%. This source is not polarized at lband. Using the fluxes of 3.78
Jy, the sefds for the measurements are: Alfapix0: 4.4,5.7 alfaPix1:6.2,9.9
Lbw: 2.5,2.4 Jy. By comparing the lbw sefd's between 01apr05 an 06may05,
the sefd's (source fluxes) probably have an error of 5%.
Fig 3: This shows (G/T alfa)/(G/T lbw) for pixel0 (top) and pixel 1 bottom.
The average values are: .56,.42 pixel 0 and .5,.25 pixel1.
Summary:
The results of the measurements are:
Measured Sefd
measurement |
lbw
(PolA,B) |
Pixel0
(PolA,B) |
Pixel1
(PolA,B) |
noAmps (01apr05) |
(2.4 ,2.3) |
(4.3,6.0) |
|
with Amps (06may05) |
(2.5 ,2.4) |
(4.4,5.7) |
(6.2,9.9) |
(G/T alfa)/(G/T lbw)
|
Pixel 0
(PolA,B) |
Pixel1
(PolA,B) |
no Amps 01apr05 |
(.55,.37) |
|
with Amps (06may05) |
(.56,.42) |
(.40,.25) |
Adding the amplifiers may a small improvement to
polA and a larger improvement to polB.
The polB/polA performance ratios are:
-
noAmps pix0: .67
-
with amps Pix0: .75
-
with amps Pix1: .63
My guess is that polB pixel 1 could use a higher power level (and
maybe some of the other outer pixels).
The original reason for the oh filters were for comet
observations that do a hexmap about the comet nucleus. With alfa
this could be done 7 times faster (if the outer beams were in the correct
position). The effectiveness is determined by the square of the ratios
of the sefd's. The numbers to use are (using the data with the amps in):
-
sefdLbw: (2.5+2.4)/2 = 2.45
-
sefdAlfa: Assuming pixels 2-6 have the same performance as pixel 1 the
average for alfa is: ((4.4+5.7) + 6*(6.2+9.9))/(2*7)=7.6
-
For alfa to match the sensitivity of lbw alfa needs to integrate (7.6/2.45)^2
= 9.6 times longer.
Lbw must also move between positions. For a1851 this was taking about 5%
longer. The relative sensitivity is:
-
lbw time: 7*1.05=7.4
-
Alfa time: 9.6
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lbw is more sensitive than alfa by sqrt(7.4/6.5) = 1.14
The comparisons are then:
-
The sensitivities are about equal (lbw is 1.14 time more sensitive).
-
Alfa allows simultaneous measurements. This is an advantage if things are
changing on the time frame of a hexmap pattern.
-
Lbw has the advantage that you can position the off positions anywhere
you want, while alfa restricts you to rotations at the beam spacing.
-
Calibration using the cals are available with lbw while it does not work
for alfa. This has meant that some of the alfa observation time, lbw has
to be used to get an idea of the calibration.
-
Polarization information is available with lbw but probably not with alfa.
-
If the source is extended then the comparison should be with Tsys and not
G/T
-
polB of alfa performs worse than polA by 30 to 40%. Pixel1 polB could probably
use a higher power level.
processing: x101/050401/doit.pro, x101/050606/doit.pro
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