Spectral Line Calibration Techniques with Single Dish Telescopes K. O’Neil NRAO - GB

1. Spectral Line Calibration Techniques with Single Dish TelescopesK. O’NeilNRAO - GB

2. Determiningthe SourceTemperature

3. Determining Tsource • Tmeas (a,d,az,za) = • Tsrc(a,d,az,za)

4. Determining Tsource • Tmeas (a,d,az,za) = • Tsrc(a,d,az,za) Or at least, that is what we want…

5. Determining Tsource • Tmeas (a,d,az,za) = • Tsrc(a,d,az,za) • + Tsystem -> Rx, other hardware The noise that is picked up as the signal is received and passed down through the processing system

6. Tmeas (a,d,az,za) = • Tsrc(a,d,az,za) • + TRX, other hardware • + Tspillover (za,az) Determining Tsource

7. Tmeas (a,d,az,za) = • Tsrc(a,d,az,za) • + TRX, other hardware • + Tspillover (za,az) • + Tcelestial(a,d,t) Determining Tsource

8. Tmeas (a,d,az,za) = • Tsrc(a,d,az,za) • + TRX, other hardware • + Tspillover (za,az) • + Tcelestial(a,d,t) • + TCMB • + Tatm(za) Determining Tsource

9. Tmeas(a,d,az,za) = • Tsrc(a,d,az,za) • + TRX, other hardware • + Tspillover(za,az) • + Tcelestial(a,d,t) • + TCMB • + Tatm(za) • Tmeas =Tsource+Teverything else Determining Tsource

10. Tmeas(a,d,az,za) = • Tsrc(a,d,az,za) • + TRX, other hardware • + Tspillover(za,az) • + Tcelestial(a,d,t) • + TCMB • + Tatm(za) • Tmeas =Tsource+Teverything else Determining Tsource

11. Determining Tsource ON sourceOFF source Tsource+ Teverything else Teverything else Arbitrary Units

12. Determining Tsource ON-OFF (Tsource+ Teverything else)- (Teverything else) Arbitrary Counts

13. Determining Tsource (ON–OFF)/OFF [(Tsource + Teverything else)- (Teverything else)]/ Teverything else =(Source temperature)/(”System” temperature) % Tsys

14. Determining Tsource ??? (ON–OFF)/OFF [(Tsource + Teverything else)- (Teverything else)]/ Teverything else =(Source temperature)/(”System” temperature) % Tsys

15. Choosing the Best OFF

16. OffSource Observations Two basic concepts • Go off source in sky • Go off source in frequency/channels Like most things in science: Easy to state, complicated in practice

17. OffSource Observations Position Switching ON Source OFF Source

18. OffSource Observations Position Switching • Little a priori information needed • Typically gives very good results • Can be done through: • Moving the telescope (re-pointing onto blank sky) • Moving a secondary or tertiary dish • Beam switching (two receivers on the sky) • Disadvantages: • System must be stable over time of pointings • Sky position must be carefully chosen • Source must not be extended beyond positions • May take significant time (not true for beam switching)

19. OffSource Observations off source in frequency/channels • Typically gives very good results • Requires well understood line of interest • Can be done through: • Baseline fitting • Frequency switching • Some combination of the two

20. OffSource Observations Baseline Fitting • Simplest & most efficient method • Can be from average of multiple scans or single fit • Not feasible if: Line of interest is large compared with bandpass Standing waves in data Cannot readily fit bandpass Do not know frequency/width of line of interest • Errors are primarily from quality of fit

21. OffSource Observations Frequency Switching

22. OffSource Observations Frequency Switching • Allows for rapid switch between ON & OFF observations • Does not require motion of telescope • Can be very efficient • Disadvantages: Frequency of line of interest should be known* System must be stable in channel space Will not work with changing baselines, wide lines *But not always…

23. OffSource ObservationsVariations Mapping an Extended Source Possible alternative if frequency switching is not an option System must be very stable Off source must truly be off!

24. OffSource ObservationsVariations • There are many other variations to the main themes given: • Position switching by moving subreflector • Using average offs from a variety of observations • Frequency switching with many baseline scans • Etc, etc • The ideal off: • Is well understood • Has minimal systematic effects • Adds little noise to the results • Maximizes the on-source telescope time

25. Tsource Tsystem Result = Determining Tsource (ON–OFF)/OFF [(Tsource + Teverything else)- (Teverything else)]/ Teverything else Units are % System Temperature Need to determine system temperature to calibrate data

26. DeterminingSystemTemperature

27. Determining Tsystem Theory Measurevarious components of Tsys: TCMB ― Well known (2.7 K) TRX, hardware ― Can be measured/monitored Tcel(a,d,t) ― Can be determined from other measurements Tatm(za) ― Can be determined from other measurements Tgr(za,az) ― Can be calculated Decreasing Confidence

28. Determining Tsys Noise Diodes

29. Determining Tsys Noise Diodes Tsrc/Tsys = (ON – OFF)/OFF Tdiode/Tsys = (ON – OFF)/OFF Tsys = Tdiode * OFF/(ON – OFF)

30. Determining Tsys Noise Diodes - Considerations • Frequency dependence Lab measurements of the GBT L-Band calibration diode, taken from work of M. Stennes & T. Dunbrack - February 14, 2002

31. Determining Tsys Noise Diodes - Considerations • Frequency dependence • Time stability Lab measurements of the GBT L-Band calibration diode, taken from work of M. Stennes & T. Dunbrack - February 14, 2002

32. Determining Tsys • Noise Diodes - Considerations • Frequency dependence Noise Diodes - Considerations • Time stability • Accuracy of measurements Typically measured against another diode or other calibrator Errors inherent in instruments used to measure both diodes Measurements often done in lab -> numerous losses through path from diode injection to back ends s2 measured value = s2 standard cal + s2 instrumental error + s2 loss uncertainties

33. Determining Tsys Noise Diodes - Considerations • Frequency dependence • Time stability • Accuracy of measurements s2total = s2freq. dependence + s2stability + s2measured value + s2conversion error

34. Determining Tsys Hot & Cold Loads • Takes antenna into account • True temperature measurement Cooling System Tcold Hot Load Thot

35. Determining Tsys Hot & Cold Loads • Takes antenna into account • True temperature measurement • Requires: • Reliable loads able to encompass the receiver • Response fast enough for on-the-fly measurements

36. Determining Tsys Theory: Needs detailed understanding of telescope & structure Atmosphere & ground scatter must be stable and understood Noise Diodes: Can be fired rapidly to monitor temperature Requires no ‘lost’ time Depends on accurate measurements of diodes Hot/Cold Loads: Can be very accurate Observations not possible when load on Must be in mm range for on-the-fly measurements In reality, all three methods could be combined for best accuracy

37. From diodes, Hot/Cold loads, etc. Determining Tsource Tsource = (ON–OFF)Tsystem OFF Blank Sky or other Telescope response has not been accounted for!

38. DeterminingTelescope Response

39. Telescope Response Ideal Telescope: • Accurate gain, telescope response can be modeled • Can be used to determine the flux density of ‘standard’ continuum sources • Not practical in cases where telescope is non-ideal (blocked aperture, cabling/electronics losses, ground reflection, etc)

40. Telescope Response • Ideal Telescope:

41. Tsource = (ON –OFF)Tsystem1 OFFGAIN GAIN = (ON – OFF)Tsystem OFF Tsource Telescope Response • ‘Bootstrapping’: Observe source with pre-determined fluxes Determine telescope gain

42. Telescope Response • ‘Bootstrapping’: Useful when gain is not readily modeled Offers ready means for determining telescope gain Requires calibrator flux to be well known in advance Not practical if gain changes rapidly with position

43. Telescope Response Pre-determined Gain curves: Allows for accurate gain at all positions Saves observing time Can be only practical solution (complex telescopes) Caveat: Observers should always check the predicted gain during observations against a number of calibrators!

44. Determining Tsource Tsource = (ON –OFF)Tsystem 1 OFF GAIN Theoretical, or Observational Blank Sky or other From diodes, Hot/Cold loads, etc. Great, you’re done! Great, you’re done?

45. A FewOther Issues

46. Other Issues:Pointing Results in reduction of telescope gain Always check telescope pointing!

47. Other Issues:Focus Results in reduction of telescope gain Corrected mechanically Always check focus!!!

48. Other Issues:Side Lobes* • Allows in extraneous or unexpected radiation • Can result in false detections, over-estimates of flux, incorrect gain determination • Solution is to fully understand side lobes Beam *Covered more fully in talk by Lockman

49. Other Issues:Coma & Astigmatism Comatic Error: • sub-reflector shifted perpendicular from main beam • results in an offset between the beam and sky pointing Image from ATOM 99-02, Heiles

50. Image from ATOM 99-02, Heiles Other Issues:Coma & Astigmatism Astigmatism:deformities in the reflectors Can result in false detections, over-estimates of flux, incorrect gain determination Solution is to fully understand beam shape