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ALMA Bandpass Calibration: Standing Waves

ALMA Bandpass Calibration: Standing Waves. Aurore Bacmann (ESO) St é phane Guilloteau (IRAM). Standing Waves: background. Reflection of incident wave on feed system of standing waves between receiver and sub-reflector

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ALMA Bandpass Calibration: Standing Waves

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  1. ALMA Bandpass Calibration: Standing Waves Aurore Bacmann (ESO) Stéphane Guilloteau (IRAM) ALMA workshop - Leiden, December 18-20th

  2. Standing Waves: background • Reflection of incident wave on feedsystem of standing waves between receiver and sub-reflector • On single-dish telescopes: standing waves between receiver and calibration load ALMA antenna

  3. Frequency of standing waves Where D is the distance between the feed and the secondary Consequence: ripple of frequency Example: spectrum at the IRAM 30m D = 5 m  = 30 MHz ALMA workshop - Leiden, December 18-20th

  4. Reducing standing waves • Baseline ripple substantially degrades spectral observations • During observations: reflection receiver-secondary mirror“high frequency ripple” • During calibration scans: reflection receiver - calibration loadalso end up in observed spectrum • reducing the amplitude of the ripple  reducing the reflection coefficient • No control of the reflection on the feed • Need to reduce the reflection coefficient on the sub-reflector ALMA workshop - Leiden, December 18-20th

  5. Reflection coefficient F: gaussian illumination function ALMA workshop - Leiden, December 18-20th

  6. Asymptotical expression e: eccentricityL: distance to mirror vertex Due to gaussian tapering, the second term can be neglected Standing waves are most disturbing at long wavelengths If both terms are considered:  will oscillate with  around an increasing mean value ALMA workshop - Leiden, December 18-20th

  7. Representation of reflection coefficient in the complex plane ALMA workshop - Leiden, December 18-20th

  8. Considered Sub-reflector Geometries Study of the effect of various sub-reflector geometries on the reflection coefficient • Effect of an absorber on the blocage zone • Effect of a discontinuity (aperture in sub-reflector) • Effect of a scattering cone ALMA workshop - Leiden, December 18-20th

  9. Mirror alone Mirror with absorbing diskbetween 0 and qb Mirror with tangent cone of semi-angle a

  10. Sub-reflector with aperture Sub-reflector with aperture, cone within aperture ALMA workshop - Leiden, December 18-20th

  11. Possible design for sub-reflector Aperture in sub-reflector 45° mirrordeflecting light towards - load (calibration measurements)- cone (observations) Calibration: coupling of load to receiver ~ 1% Avoids saturation of receiver ALMA workshop - Leiden, December 18-20th

  12. Simple sub-reflector ALMA workshop - Leiden, December 18-20th

  13. Tangent cone on Sub-reflector 3 mm   < 87° But if a is small, the cone covers much of the sub-reflector ALMA workshop - Leiden, December 18-20th

  14. Sub-reflector with discontinuity at 3 mm • ~ 0.009 with discontinuity ~ 0.0045 with absorbing disk ALMA workshop - Leiden, December 18-20th

  15. Aperture with cone at 3 mm At angles < 85°,  ~ same value as without aperture (0.005) ALMA workshop - Leiden, December 18-20th

  16. Standing Wave Ratio • Incident amplitudeon the feed: In amplitude: In power: ALMA workshop - Leiden, December 18-20th

  17. Values for ALMA peak-to-peak ripple at 3mm For rm ~ 0.4 ALMA workshop - Leiden, December 18-20th

  18. Summary • At 3 mm, baseline ripple can reach ~1% • Aperture in sub-reflector increases ripple by a factor of 2 • Can be reduced with cone within aperture • Tangent cone is the best solution but choice of a = compromise ALMA workshop - Leiden, December 18-20th

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