alfa performance at OH (no 10db amps)

02apr05 links to plots:
comparison of the system performance for alfa and lbw at OH (.ps) (.pdf)

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 m performance was measured using:

1 minute on/off position switching
the source CTA21 (B0316+162. 7.2 Jy at 1666)
The data was centered at 1666 with a low side 1st lo.
The wapps were setup for 1 second dumps, 6.125 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.

The plots show the comparison of the system performance for alfa and lbw at OH (.ps) (.pdf)

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.
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.

The only question with these measurements are the low values for the off source power. Lbw is probably ok since 3 db below the optimum power level does not increase Tsys. I doubt that it should matter with alfa either.

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:

sefdLbw: (2.4+2.3)/2 = 2.35
sefdAlfa: The outer pixels have an sefd about 1 db higher than the center pixel (see sysperf page). The averaged sefd would then be (4.3+6.0)/2. * (1 +6*1.2)/7. = 6.0
For alfa to match the sensitivity of lbw alfa needs to integrate (6.0/2.35)^2 = 6.5 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: 6.5
alfa is more sensitive than lbw by sqrt(7.4/6.5) = 1.06

The comparisons are then:

The sensitivities are about equal (alfa 1.06 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 is available with lbw while it does not work for alfa.
Polarization information is available with lbw but probably not with alfa.

processing: x101/050401/doit.pro

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