GB . Data . HowToReduceTipData

How To Reduce Tipping Data

The goal of a tip reduction is to model the system temperature (which is equal to the sky temperature when you are not looking at a source) as a function of elevation. The system temperature equation on p. 7 of the 11/01 calibration memo is the relevant for this purpose. Basically, Tsys = (cal_off)/(cal_on-cal_off)*Tcal where cal_on, cal_off, and Tcal are all arrays.

Calibrate your total power data to produce antenna temperature as a function of elevation. Details on doing this can be found on page 7 of Specification of GBT Astronomical Intensity Calibration by Braatz et al.
Fit your data with the following equation :
- $\tau_0$ is always a free parameter, to be fit.
- The user should be given the option to make $T_{RX}$ and $T_{atm}$ free parameters in the fit, or fixed at a settable value.
- The user should be allowed to set the values of all other parameters

is antenna temperature in K
$\eta_l$ is rear spillover efficiency (~0.99)
$T_{atm}$ is mean temperature of the atmosphere in K
$\tau_0$ is zenith opacity
is the independent variable, in atmospheres (1/sin(elev))
$T_{spill}$ is effective temperature of rear spillover in K (~300.0 K)
$T_{fss}$ is forward spillover and scatter temperature in K (~1.0 K)
$T_{CBR}$ is temperature of the cosmic background (~2.7 K)
$T_{RX}$ is the receiver temperature at the given frequency, in K

A glish program to reduce tipping data is available from ~jbraatz/Glish/tip.g

-- JimBraatz - 10 Mar 2004

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