Gain Curve for 22.236 GHz, December 28, 2005

by F. Ghigo, Y. Kovalev

Observations were made of several calibration sources at 22.236 GHz using the 18-22 GHz receiver. Peak and focus scans were done, as well as tipping scans. The weather was mostly clear with temperatures near 20 F. The observing session was from 05:00 to 14:00 UT ( or 0h to 9h local time). The K-band receiver has 4 feeds; but for these observations we only used feed number one.

Pointing model 4c was in use, and the active surface corrections used the FEM model.

The following sources were observed:
Source num peak seqs elevation range
3C48 7 9 - 15
3C147 4 29 - 31
3C286 45 9 - 81
3C295 24 21 - 76
3C309.1 3 55 - 55
0403+768 4 27 - 27
0858-279 4 23 - 23
1151-348 4 16 - 16
1345+125 4 63 - 63
1358+624 3 53 - 53

Methods and Calibration

Data was recorded in continuum mode with the DCR (digital continuum receiver). Observations were centered at 22.236 GHz with a bandwidth of 320 MHz.

Peak sequences consist of 4 scans: forward and reverse in azimuth, then forward and reverse in elevation, centered on the source position. After the azimuth scans, a correction is made to the local pointing correction (LPC), so that the elevation scans go through the true source position. Focus scans were done several times during the observing session to touch up the focus offset. Amplitude calibration was done by injecting a calibrated noise signal (Tcals) at a 10 Hz rate. Gaussians were fit to each cross scan, and peak values and system temperatures calculated.

The program "prepoint" was used to extract the calibrated peak data.

Figure 1 shows the coverage of azimuth and elevation.

Figure 2 shows the system temperatures (Tsys in Kelvin) plotted versus elevation. LCP data plotted as plusses, and RCP data as triangles. In Figure 2, we also plot the fitted curves in Tsys vs opacity of the form: The Tcals and fitted parameters are listed in the following table:

Corrections for opacity

Antenna temperatures (Tant) for each source are the peaks of the fitted gaussians. We used only the peaks of the elevation scans because these scans were done after the azimuth pointing correction had been updated, and thus they are centered optimally on the true position of the source.

In the upper panel of Figure 3 we plot Tant for all sources. In the lower panel Tant is shown after correction for opacity:

Gain and Efficiency

The gain, in terms of antenna temperature per flux density, is found by dividing the corrected antenna temperatures (Ta') by the flux densities (S) of the calibration sources. The gain relates to the aperture efficiency (&eta ) as follows, for the GBT. k is the Boltzmann constant, and A is the geometric area of the telescope. Calibrators with very well known flux densities over a wide range of frequencies are described by Baars in the web page "http://www.vla.nrao.edu/astro/calib/manual/baars.html" in which analytic expressions are given for calculating the flux density at any frequency between 300 MHz and 50 GHz. For determinations of gains we used only 3C48, 3C147, 3C286, and 3C295, and the flux densities at 22.235 GHz are listed: The resulting gains are plotted in Figure 4 for both LCP and RCP polarizations. A quadratic function has been fit to the data with the same constants (A0, A1, A2) for both polarizations and scaling by different factors Gmax(L) and Gmax(R), as follows: Corresponding efficiency values are given on the right hand axis.

The parameters of the fitted curves are as follows; the rms error is the rms of the data with respect to the fitted curves.

Comments

The gain curve is primarily determined by the data on 3C286 and 3C295 which accounted for the great majority of the observations. One may note that the gain for 3C147 (at elevation = 30 deg) is somewhat discrepant - perhaps the flux scale is not correct for this source.

Note that we do not believe that the efficiencies should be different for the two polarizations. This disrepancy must be due to uncertainties in the calibration temperatures (Tcals).