Performance of the GBT at 5 GHz (6cm)

Measurements done March 8, 2004.
March 17, 2004;
Y. Kovalev, F. Ghigo

Introduction

GBT gain curve measurements in the 6cm band (C-band) were done March 8, 2004, 11:30 -- 16:30 EST. The gain versus elevation was measured by observing several strong calibration sources over a wide range of elevations using the peak procedure. The subreflector focus was adjusted about once per hour. The observations were done at a frequency of 4980 MHz with a bandwidth of 80 MHz. The frequency was chosen to match the VLBA C-band. A hybrid was used to produce LCP and RCP output from linearly polarized feed horns. Weather conditions were poor (cloudy, snow). The active surface (zero offset: ON, FEM model: OFF) and dynamical corrections were in use for this run.

Summary

Channel Maximum Gain Tsys (high elevation) Tcal
LCP 1.91 K/Jy 20 K 2.80 K
RCP 1.83 K/Jy 20 K 3.29 K

Observing Procedure

Observing of the calibration sources was done using the "peak" procedure and fitting gaussians with "GFM" . A typical peak sequence is shown in Figure 1. The red plot is the data, the green is the fitted gaussian, and the blue dashed line is the baseline. The procedure was to scan across the source forward and back in azimuth (scans 7 and 8), then up and down in elevation (scans 9 and 10). The telescope azimuth correction is updated after the first two scans, and the elevation correction is updated after the last two.

Figure 1: Typical peak data.

System Temperature Measurements

The Tsys values as calculated by "GFM" from each "peak" procedure, are plotted in Figure 1 for all data.

Figure 2: system temperature vs elevation

A simple model for the atmosphere was fitted by least-squares to the data:
Tsys = T0 + Tatm (1 - exp(-Tau*A)),
in which airmass A = 1/sin(Elev), T0 is extrapolated noise temperature for airmass = 0 (sum of the receiver temperature, antenna and groud contribution, etc.), tau is the atmospheric opacity.

The fitted parameters are as follows (assuming Tatm = 260 K):
Parameter Fit to LCP data Fit to RCP data
T0 17.81 K 17.55 K
Tau 0.0077 0.0074

Tipping scan

More study of Tsys versus elevation was done by a tipping scan, driving the antenna from 86 to 8 degrees elevation at 106 degrees azimuth, and recording the system temperature five times per second. Figure 3 shows these scans and the model fits using a similar model as above. There is clearly some irregularity in the Tsys data at higher elevations, but the opacities, which depend more on the low elevation data, agree well with those derived from the "peak" data.

Figure 3: Tipping Scans.

Gain Curve Measurements

The Tant values for observed sources were calculated by "GFM" from each "peak" procedure. The following sources forming standard scale were observed: 3C48 (0134+329), 3C123 (0433+295), 3C147 (0538+498), 3C295 (1409+524), DR21 (2037+422), NGC7027 (2105+420). We have used Baars et al. (1977) parameters for the spectra of the sources 3C48, 3C295. Significant change in flux of 3C123 and 3C147 encouradged us to use Ott et al. (1994) spectra parameters for them. Flux density for DR21 and NGC 7027 was interpolated by us to the frequency of observations by using Ott et al. (1994) data. The size correction factor taken from Ott et al. (1994) was applied.
Actual values (size correction factor applied) of calibrators flux density in use are as follows:
Calibrator Flux density, Jy
3C48 5.26
3C123 15.54
3C147 7.57
3C295 6.39
DR21 18.86
NGC7027 5.58

Gain = Tant/Sstandard is plotted on the Figure 4.

Figure 4: noncorrected gain curve

A second order polynomial was least-square-fitted to the data. The maximum value of the gain was taken as the DPFU. Resulted gain curve as a function of the zenith distance (ZD=90-Elev, deg.) has the following form:
Gain[LCP,RCP] = DPFU[LCP,RCP] * (A0 + A1 * ZD + A2 * ZD^2).
The fitted parameters for not corrected gain are as follows:
A0, A1, A2 = 0.93999, 3.4857e-03, -5.0846e-05
Parameter Fit to LCP data Fit to RCP data
DPFU 1.91 K/Jy 1.82 K/Jy
Mean gain for elev. 20-87 deg. 1.87 K/Jy 1.79 K/Jy

Figure 5: corrected for atmospheric absorption gain curve

A second order polinomial was least-square-fitted to the data. The maximum value of the corrected gain was taken as the DPFU_corr. Resulted gain curve as a function of the zenith distance (ZD=90-Elev, deg.) has the following form:
Gain_corr[LCP,RCP] = DPFU_corr[LCP,RCP] * (A0_corr + A1_corr * ZD + A2_corr * ZD^2).
The fitted parameters for corrected gain are as follows:
(A0, A1, A2)_corr = 0.93711, 3.6330e-03, -5.1134e-05
Parameter Fit to LCP data Fit to RCP data
DPFU_corr 1.91 K/Jy 1.83 K/Jy
Mean gain_corr for elev. 20-87 deg. 1.88 K/Jy 1.80 K/Jy

The luck of low elevation data in this run calls for additional gain curve measurements, when appropriate period of GBT time will be available.

Effeciency Estimation

We have estimated the GBT effeciency, assuming that projected geometrical GBT area is equal to 7853.98 m^2. Error in the effeciency estimation is about 10% and comes from the uncertanty of Tcal value.

Effeciency is plotted on the Figure 4.

Figure 4: effeciency