How To Measure Gain and Efficiency on the GBT
This page describes how GBT users can measure the gain and efficiency of the telescope. Observers may wish to measure these parameters to check telescope performance, as when snow or ice have accumulated on the dish. Observers may also measure these parameters to test or improve calibration accuracy.
To measure efficiency, run an AutoPeak or AutoPeakFocus on a flux calibrator, record the antenna temperature from the elevation scans as reported by GFM, and run the IDL procedure "gbtgain" (described below) to calculate the gain and efficiency. The gbtgain procedure solves equation (4) from the GBTIDL calibration document in order to determine aperture efficiency (eta_A). The gain as calculated by this program is simply the ratio of the corrected antenna temperature (Ta') to flux density.
Calibrators can be selected from Ott et al., 1994. They include:
| Source | RA (J2000) | Dec | Freq range (MHz) | Angular Size (arcsec) |
|---|
| 3C48 | 01:37:41.30 | +33:09:35.13 | [1408, 43780] | 1.5 |
| 3C123 | 04:37:04.40 | +29:40:15.0 | [1408, 23780] | 23 |
| 3C147 | 05:42:36.127 | +49:51:07.23 | [1408, 50000] | 1 |
| 3C161 | 06:27:10.10 | -05:53:04.8 | [1408, 10550] | 3 |
| 3C218 | 09:18:06.00 | -12:05:45.0 | [1408, 10550] | 47 |
| 3C227 | 09:47:46.4 | +07:25:12.0 | [1408, 4750] | 200 |
| 3C249.1 | 11:04:11.50 | +76:59:01.0 | [1408, 4750] | 15 |
| VirA | 12:30:49.60 | +12:23:21.0 | [1408, 10550] | 250 |
| 3C286 | 13:31:08.284 | +30:30:32.94 | [1408, 43200] | 1.5 |
| 3C295 | 14:11:20.70 | +52:12:09.0 | [1408, 32000] | 5 |
| 3C309.1 | 14:59:07.584 | +71:40:19.87 | [1408, 32000] | 1.5 |
| 3C348 | 16:51:08.3 | +04:59:26.0 | [1408, 10550] | 170 |
| 3C353 | 17:20:29.5 | -00:58:52.0 | [1408, 10550] | 210 |
| CygA | 19:59:28.356 | +40:44:02.10 | [1408, 10550] | 170 |
| NGC7027 | 21:07:01.60 | +42:14:10.0 | [1408, 43200] | 10 |
3C286 and 3C48 are often the best choices. Choosing an extended source is not usually a good idea. Sources with an angular size > 10% of the beam are not good choices. 3C286 and 3C295 have shown no variability. The frequency range listed above reflect the ranges given in Ott et al. for which the flux density calculations are accurate.
Once you have run a peak scan on one of the flux calibrator sources, run the IDL program ~jbraatz/gbtidlpro/gbtgain.pro. in IDL or GBTIDL, type "gbtgain" to see the parameters required:
GBTIDL -> gbtgain
Usage: gbtgain, source, freq, Ta, tau, elev
The tau value can be read from weather information here.
Example:
GBTIDL -> gbtgain, '3C286', 14000.0, 4.35, 0.04, 20.0
Source : 3C286
Ta : 4.35000 K
Ta' : 4.40992 K
Snu : 3.55828 Jy
Gain : 1.23934 K/Jy
ApEff : 0.439248
-- JimBraatz - 24 Jan 2008
Revision r1.1 - 24 Jan 2008 - 15:30 GMT - JimBraatz
Parents: TWikiUsers > JimBraatz
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