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Hands-On Calibration

Hands-On Calibration. Ron Maddalena July 13, 2007. Preliminaries. Change directory: cd /home/scratch/sdscal Start GBTIDL Access data filein,’T_TCAL14MAR07.acs.raw.fits’ summary

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Hands-On Calibration

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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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