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NRAO Forty Foot Telescope Calibration using 3C392 Measurement: 10/30/2014

This article discusses the calibration process using 3C392 measurement on the 40 Foot Telescope, estimating Tsys and fixing polarization issues. Assumptions, calculations, and measurements are reviewed.

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NRAO Forty Foot Telescope Calibration using 3C392 Measurement: 10/30/2014

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  1. NRAO Forty Foot Telescope Calibrationusing 3C392Measurement: 10/30/2014 Author: Skip Crilly skip.crilly@gmail.com GBO volunteer First rev 11/6/2014 Rev 10/29/2018 to fix polarization issue and estimate Tsys given aperture eff. and measurement

  2. Prediction of ΔTsys based on assumptions • Assumptions • 3C392 flux = 171 Jy at 1420 MHz in two polarizations; unpolarized source assumption • Ref: Table in Forty Foot observer’s guide • RA 18h 53m 37s; Decl. +1deg 18m (1950.0) – Ref: P.R.R. Leslie - Observatory 80,23, 1960 • Antenna aperture physical area calculated = 117 m^2 • Aperture Efficiency is assumed to be 60% -> Effective Aperture = 70 m^2 • Tsys is assumed to be 40 K • SNR 3C392 size is smaller (35mx27m) than antenna beamwidth FWHM (~1 deg) • Reference: Green’s Catalog - Cavendish Laboratory • Object Received Power: 171 Jy x 10^-26 (W/Hz m^2)/Jy * 70 m^2 = 1.20 x 10^-22 W/Hz • -189.2 dBm/Hz in two polarizations ; -192.2 dBm/Hz in each of two polarizations • Expected Receiver Noise Power @ Tsys = 40K: -174 dBm/Hz -10*LOGBASE10(290 K /40 K) = • -182.6 dBm/Hz • Single polarization Object Noise to Tsys Noise Power (dB) = -192.2 dBm/Hz –(-182.6 dBm/Hz) = -9.6 dB; • -9.6 dB -> 4.4 K increase in Antenna Sys. Temp is expected due to object measured in one polarization, assuming 40 K Tsys, and 70 m^2 effective area

  3. Measurement • +0.45 dB power measurement change when on and off object • Strip chart paper was left in the Forty Foot receiver room • Cross calibration of the +0.45 dB increase used a +-0.001 dB resolution HP power meter compared to measured strip chart recorder pen movement. • This calibration could be improved; error estimated = +-0.05 dB • +0.45 dB to linear power ratio -> 10^(0.45/10) = 1.109 ratio • 1.109 ratio at 40 K Tsys corresponds to 4.4 K increase due to object

  4. Conclusion • Estimate based on assumptions is that 3C392 will result in 4.4 K increase in assumed 40K dark sky antenna noise temp • 3C392 was measured at 4.4 K increase in 40K assumed dark sky antenna noise temp • Aperture Efficiency and Tsys are “reasonably” close to estimates • Calculation and measurement error can be estimated more accurately • Better Tsys estimate and Aperture Efficiency can be researched • Additional work: • Additional sources can be checked • Check assumptions about Tsys and Aperture Efficiency • Improve strip chart calibration accuracy, i.e. cross-calibration • The other polarization measured approx. +-10% compared to the measured polarization • A&B Channels were manually matched to ~ +-10% • This should be followed up with improved calibration • Pointing issues • To Do – verify assumptions, check calculations and measurements

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