# Fitting Tsys for the alfa receiver.

09feb07

The system temperature for the Alfa receiver is computed by month using all of the spectral line cal on/offs (wapp data) taken  (see computing tsys from the archive for a description of how it is computed). A model is then fit to this data. The fitting procedure is iterative. It does the fit, and then throws out any points whose residuals are greater than 3 sigma.

#### The model used:

The model for the fit is:
Tsys=c0 + c1*za + c2*(za-14)^2 + c3*cos(az) + c4*sin(az) +c5*cos(2az) +c6*sin(2az) + c7*cos(2rotA) + c8*sin(2rotA) + c9*(freqGhz-1.36) + c10*(freqGhz-1.36)^2

Model notes:

• c2*(za-14)^2 is only used for za gt 14 degrees.
• rotA is the rotation angle of the alfa feed
• freqGhz is the frequency of the center of the band in GHz.
• Most receivers have seen a 2az Tsys term that is probably coming from the dish (elongated panels)

#### Model fits: 01jan05 thru 31jan07

There were between 15000 and 17000  good calon/offs in this date range (depending on pixel and polarization). The fit sigma's ranged from .47K to .67 There are 14 plots in all (one for each pixel/polarization). The plots contain:
• Page 1. The raw data is plotted in black. The fit is in red. Data points that were excluded from the fit are plotted in blue.
• The top plot is versus zenith angle. The purple line shows the za dependence of the fit (c0 + c1*za +c2*(za-14)^2.
• The 2nd plot is versus azimuth angle.
• The third plot is versus frequency. The purple line shows the frequency dependence of the fit: (c0 +c9*(freqGhz-1.36) + c10*(freqGhz-1.36)^2).
• The bottom plot is versus the rotation angle of alfa.
• Page 2. This has the fit residuals (data - fit). The black points were used in the fit. The blue points were excluded. The 4 plots are za, az, freq, and rotation angle.
The  plots for each pixel/polarization: (.ps files about 10 Mb, .pdf files about 3 Mb)
• pixel0A (.ps)  (.pdf)
• pixel0B (.ps)  (.pdf)
• pixel1A (.ps)  (.pdf)
• pixel1B (.ps)  (.pdf)
• pixel2A (.ps)  (.pdf)
• pixel2B (.ps)  (.pdf)
• pixel3A (.ps)  (.pdf)
• pixel3B (.ps)  (.pdf)
• pixel4A (.ps)  (.pdf)
• pixel4B (.ps)  (.pdf)
• pixel5A (.ps)  (.pdf)
• pixel5B (.ps)  (.pdf)
• pixel6A (.ps)  (.pdf)
• pixel6B (.ps)  (.pdf)
•  Beam SigPolA (K) SigPolB (K) 0 .49 .57 1 .50 .48 2 .60 .67 3 .60 .52 4 .48 .50 5 .60 .48 6 .50 .47

• Tsys contains the contribution from the sky. It will vary depending where the person was looking.
• The fit coefficients are supplied in the idl distribution of phil's routines (see downloading the idl routines). At AO this is located in /pkg/rsi/local/libao/phil/data/tsysdatR17.
• The idl routine alfatsysget() (in the ./gen directory) will evaluate tsys for a given freq, az,za, and rotation angle.
• #### History:

• 09feb07: redid fits using data from 01jan05 thru 31jan07 (about 17000 calon/offs).
• 01aug05: original fit using data 01jan05 thru 31jul05 (about 4000 calon/offs).
processing: x101/mbeam/tsys/archive/fit/doit.pro