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)

**Fit residuals (degK)**
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

Comments:

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

home_~phil