
	The GBT Flux Calibration Program (15A_486)

Introduction

The GBT Flux Calibration Program (GBT15A-486) was a filler project design to track the flux variability of the sources, to measure the stability of the GBT noise diodes, and to monitor the performance of the receivers, pointing model, and surface of the GBT. It is design to run within short 30 minute blocks time and can be carried out at any LST range using any one of the available receivers. A primary goal of the program is to make the GBT calibration consistent with the VLA calibration scale given by Perley & Butler (2013). All data taken from this program are publicly available. Contact David Frayer if you have questions or are interested in the data.

Measurements

The measurements from calibration program 15A_486 are tabulated here: (ascii).

Results

We have concentrated measurements on the VLA standard calibrators 3C286, 3C295, and 3C123 which are stable calibrator sources (Perley & Butler 2013). Below shows a figure from the currently available data that shows the comparison between GBT measurements and the adopted VLA flux densities (Figure [ps]) Figure Caption: GBT/VLA flux density ratios based on the calibrators 3C286, 3C295, and 3C123. The expected flux densities are based on the VLA calibration (Perley & Butler 2013), while the GBT values are based on the current adopted noise-diode values, many of which have not been updated in more than a decade. The plot justifies updating the GBT noise-diodes for every band, and potentially taking new SCAL observations. Only X-band is currently in agreement with the VLA calibration.

Band	Median GBT/VLA Flux Density Ratios
340	0.50
800	1.17
L	0.92
S	0.88
C	1.32
X	1.00
Ku	0.87
KFPA	0.90
Ka	1.01
Q	0.83

The above table shows median results from the GBT observations of 3c286 compared with the expected flux densities from the VLA calibration program. The results are based on measurements taken from 2015.07 to 2016.03. The frequencies observed within each band are the standard VLA calibration frequencies. The results for both polarizations for the GBT have been combined. Note that the results from Ka and Q-band are derived by on-going observing projects in good weather since the high-frequency data for the calibration program was impacted significantly by the weather, given the calibrators are weak at high-frequency (e.g., small Tsys differences on the sky between on/off yields large % scatter on source).

The L-band X-polarization noise diode is too low by ~20%.
The C-band noise diodes are overestimated.
The X-band noise diode values appear reasonable.
The KFPA has shown a wide range of results over the last two years and highlights the importance of checking that the cal-drain currents are near 5.0mA before your observations begin.
No data has been taken for Ka-band from this filler program.
Many of the Q-band data points are affected by weather.
The S-band noise diodes are close but slightly understimated.

Since most of the GBT noise diode TCAL values are out dated, we recommend observing a standard calibrator to calibrate your data.

GBT Calibration vs VLA Calibration

The GBT calibration scale is based on the absolute calibration from Ott et al. 1994 and Peng et al. 2000 while the VLA calibration is based on the Perley & Butler 2013 paper. The plot here shows the ratio between the current GBT calibration scale and the VLA calibration scale based on different calibrators as a function of frequency (Figure [ps]). The plot does not represent any observational data from the GBT or the VLA. It just shows the relative fluxes assumed for the calibrators as a function of frequency. The GBT SCAL programs should be updated to match the source coefficients given by Perley and Butler 2013 to place the GBT noise-diodes on the VLA calibration scale.

Calibration Method

The Ta temperature scale is derived from Ta = Tsys (ON-OFF)/OFF. The Ta' temperature scale has been corrected for atmospheric attenuation: Ta' = Ta * exp(tau_o*Airmass). The flux density scale for the GBT is Snu = Ta'/(2.84*eta_a), where eta_a is the aperture efficiency which is given by the Ruze equation with a long-wavelength efficiency of 0.71: eta_a= 0.71 exp(-1.*(pi4*esurf/lam)^2.). For default, we have assumed esurf errors of 250-microns. This will be checked with high frequency observations. With repeated measurements at low frequency where the effects of the surface are small, we can directly check the applicability of the current noise-diode Tcal values by measurements of the flux calibrators. If we find system discrepancies, we will suggest detailed SCAL observations across the full receiver band so that the values could be updated in the GBT calibration database.

Observing Method

The program runs for any LST range and for any available receiver on the GBT. The ASTRID script chooses the appropriate source based on the current LST and the system is configured based on the current receiver in the scan coordinator.

Operator Instructions:

(1) Put Rx of choice in the ScanCoordinator
(2) Run "run_GBTcal" in astrid under project code AGBT15A_486
(3) If time permits repeat run_GBTcal a second time
(4) As time permits, repeat steps (1)-(3) for different Rx's within the observing session