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Comprehensive Guide to GBT Calibration Techniques and Data Analysis

This document provides a detailed step-by-step guide on calibrating the Green Bank Telescope (GBT) data, focusing on both the 8 GHz and 12 GHz receivers. It outlines the necessary preliminary steps, such as changing directories, accessing data files, and applying different observing techniques like 'OffOn' and 'NOD'. The guide explains how to process observations, calculate air mass, and determine calibration equations, including the use of weather models. It also covers advanced calculations for determining system temperature (Tcal) and signal processing with data containers.

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Comprehensive Guide to GBT Calibration Techniques and Data Analysis

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  1. Hands-On Calibration Ron Maddalena July 13, 2007

  2. Preliminaries • Change directory: cd /home/scratch/sdscal • Start GBTIDL • Access data • filein,’T_TCAL14MAR07.acs.raw.fits’ • summary • Note: For 8 GHz receiver, used observing technique of ‘OffOn’ with, for example, scan 6 an observation toward blank sky and scan 7 toward 3C286. Used 4 windows • Note: For 12 GHz, dual feed receiver, used observing technique of NOD. Source in feed 1 for scan 31, in feed 2 for scan 32. Used 2 windows. • getps,6,ifnum=0 • Try different windows to see what’s different • header • Record elevation, UT date and time • getnod,42,ifnum=0 • For the adventurous • .compile getscalquad.pro

  3. Typical Position-Switched Calibration Equation

  4. Putting it all together Remove AveragingSolve for Tcal

  5. What Do We Need? • η from graph, assume gain is elevation independent • Ap from dish diameter • Calculate Air Mass from elevation of observation • S from a catalog (e.g., Ott et al 1994, A&A, 284, 331) • Table: pp 333-334 • Functional fit: p. 335 • Note that S will vary significantly across wide bandwidths • τ from weather models • At Linux prompt, type: cleo forecasts

  6. What Do We Need? • τ from weather models • At Linux prompt, type: cleo forecasts • Select “Curves” tab • Enter date and UT of the observations • Enter frequency range for the receiver (e.g., 7-11 GHz, 11-16 GHz) • May want to select ‘Write Out Results’ to create an ASCII file of results • Click on ‘Process’ • Read opacities off of graph

  7. What again are we calculating?

  8. How to Calculate (RefOn-RefOff)/(Sig-Ref) • Use the commands ‘emptystack’, ‘select’ ‘avgstack’, ‘copy’, ‘subtract’, ‘divide’, and ‘scale’ • Emptystack • Clears anything that peviously has been done with the stack • Select,scan=6,cal=‘F’,ifnum=0,plnum=0,fdnum=0 • Finds all data that meet this selection criteria • Avgstack • Averages together the data found by ‘Select’ and places into Data Container (DC) zero • Copy,0,9 • Moves the results to another DC for later use • DC 9 will now contain RefOff • Repeat for scan=6, cal=‘T’, place into DC 8 to create RefOn • Repeat for scan=7, cal=‘F’, place into DC 7 to create SigOff • Repeat for scan=7, cal=‘T’, place into DC 6 to create SigOn

  9. How to Calculate (RefOn-RefOff)/(Sig-Ref) • Summary: • DC 9 contains RefOff • DC 8 contains RefOn • DC 7 contains SigOff • DC 6 contains SigOn • Create Sig and place into DC 10: • Add,7,6,10 • Scale,0.5,10 • Similarly create Ref and place into DC 11 • Create Sig-Ref and place into DC 12 • Subtract,10,11,12 • Similarly create RefOn-RefOffand place into DC 13 • Create (RefOn-RefOff)/(Sig-Ref) and place into DC 14 • Divide,13,12,14 • Show,14

  10. What again are we calculating? • Finally, scale DC 14 by η, S, … to determine Tcal. For example, using fictitious values: • scale, 0.5*1234/2*22, 14 • scale, 1/1.38e-16*exp(-0.12/sin(33*180/!pi)), 14 • etc. • show,14

  11. Check for non-linearity • If system is linear, than • (SigOn-SigOff) – (RefOn-RefOff ) = 0 • Model the response curve to 2nd order: • Pout = B * Pin + C * Pin2 • Our observations provide: • Pout = Refoff when Pin=Tsys • Pout = Refon when Pin=Tsys+Tcal • Pout = Sigoff when Pin=Tsys+TA • Pout = Sigon when Pin=Tsys+TA+Tcal • It’s easy to show that: • C = [(Sigon- Sigoff )-(Refon- Refoff)]/(2TATcal) • Try to estimate a value for C using ‘subtract’, ‘divide’ and ‘scale’ • Things are really a bit more complicated since we really measure Pout and want to determine Pin . Must invert 4 simultaneous linear equations.

  12. Now for the real easy way… • Getscal,7,6,ifnum=0,plnum=0,fdnum=0,tau=0.05, ap_eff=0.55,smth=1 • Show,13 (Tcal, assuming linearity) • Show,3 (Tcal, assuming non-linearity) • Show,15 (Tsys, assuming linearity) • Show,5 (Tsys, assuming non-linearity) • Show,11 (Source flux)

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