Tsys= Tomt*alpha + (1-alpha)*Tsky + Tamp.Where Tsky includes the sky and scattered radiation, Tomt is the physical temperature of the OMT and Tamp is the amplifier temperature referenced to the input of the amplifier. alpha is the fractional loss of the sky signal caused by these resonances. Rewriting the equation:
Tsys= Tsky + Tamp + alpha*(Tomt-Tsky)This shows that the spectral density at the resonance will increase or decrease (relative to the adjacent frequencies) depending on whether or not (Tomt-Tsky) is positive or negative. If you are on cold sky then Tsky may be 6+ 14(scattered)=20 K while the OMT temperature is about 85 K. If you look at a 10 jansky source then Tsky= 20+12*11k/Jy= 152K is greater than Tomt so Tsys will have a negative going bump at the resonance frequency.
On 17apr03 1 minute on off position switching was done on the source 3C138 (J0521+166). This source is 9.2 Jy at 1400 Mhz. The correlator was setup to run 4 by 25 Mhz bands. After each on/off pair, the frequency was stepped by 100 Mhz covering 1100 to 1700 Mhz. The entire range was covered in 6 steps. The frequency range was repeated with a 10 Mhz offset at each frequency setting to cover the frequency where the subcorrelators overlapped. The lbw receiver was set to native linear polarization (the hybrid was not inserted).
The ons and offs were searched for patterns that had positive bumps in the off and negative bumps in the on. The rms for each 60 sec off was also computed to see if there was any rfi in a channel. Three resonances were found (there is a fourth resonance above 1720 which wasn't covered). Since 3C138 is about 7.5% polarized at 1400 Mhz, polA and polB were averaged. This gave a consistant source strength versus parallactic angle.
The cals were used to convert the data to Kelvins.
- Black lines. The off source. The bump should be positive going.
- Red lines: The bump should be negative going.
- Blue lines: On/off -1 The bump should be negative going.
Plots were made of on/off-1, rms by channel, and the individual spectra of the offs where the resonances occur. The resonance are flagged with a green vertical line.
- Black lines: on/off-1 polA. The gaps are where each 25 Mhz sbc ended. There are two measurements with a 10 Mhz offset between them
Blue lines: the rms of each 60 second off scan. Spikes in the rms are caused by rfi.
- Black lines: Offsrc
- Red lines: onSrc
- Blue lines: rms/mean for the off source scan. Spikes are caused by rfi.