---+ Notes for gaining intuition on Atmospheric Subtraction

Gases in the atmosphere absorb mm light, attenuating the magnitude of sky signals. This absorbed power will later be thermally radiated, adding noise to our measurements. This is further complicated because time and spatially varying inhomogeneities in the water vapour distribution can cause fluctuations in the atmospheric thermal radiation, called sky noise. Slow variations in attenutation will also complicate calibrations from astronomical signals such as planets.

Transmission of the Atmosphere (Opacity)

Because of the proximity of water lines to our frequency bands, the integrated precipital water vapour (PWV) contributes most to opacity. There is also evidence of a dry air component (O2, N2) to opacity, but it is very well mixed and thus much more stable - forming a uniform background of radiation that contributes only to the experiment's photon noise.

so even if the magnitude is measurable, the fluctuations are so low as to be negligible. (source Peterson)

Location Median Zenith Optical Depth at 225 GHz
S. Pole 0.053
Chanjnator 0.061
Mauna Kea 0.091
(source astro-ph/0211134 Peterson)

Bolocam (talk by Jack Seyers) Bolocam measures atmospheric opacity:

Frequency Band (GHz) 150 270
zenith opacity (tau) 0.05 0.13
  • median PWV 1.75mm

ACBAR astro-ph/0303515 ACBAR measures atmospheric opacity using sky-dips:

Frequency Band (GHz) 150 220 280 350
zenith opacity (tau) 0.033 0.052 0.10 0.18
(sect 7.2)

Mean PWV at the pole during winter is 0.26 +- 0.2mm (Mauna Kea 1.65mm, Chanjnator 1.00mm).

The contribution of the atmosphere to the total temperature measured by a bolometer is given by . (sect 6.4)

where are the physical temperature of the atmosphere, average in-band zenith opacity, and the zenith angle of the observation. They measure a temperature loading from the atmosphere of 8K for EL=60 degrees at 150GHz for a typical day (total loading at EL=60degrees is about 39+-6K at 150 GHz).

Note: we can measure with our skydip data. This is a very simple analysis, and we can do it with each bolometer independently.

near field / far field

Bolocam

  • far field distance is 50 km.
  • in the near field, the diameter of the beams is nearly constant, about 10m.

Modelling the Atmosphere

(Lay and Halverson)

  • 3-D Kolmogorov spectrum,
  • at a height h_ave. If is small, then we can approximate it as a 2D spectrum (see Bussman paper)

Scan speed

If is the measured atmospheric angular velocity for height h, then the linear velocity is given by . For small angular velocities, .

The angular velocity is , where the sample shift is given in time samples per arcsecond.

Sample Shift scan speed wind speed
200 Hz samples per arc second deg/sec 0.5km 2km
1 TS/" 0.056 deg/sec 0.49 m/s = 1.73 km/hr 1.95 m/s = 7.02 km/hr
0.1 TS/" 0.56 deg/sec 4.88 m/s = 17.3 km/hr 20 m/s = 70.2 km/hr
0.01 TS/" 5.6 deg/sec 48.8 m/s = 173 km/hr 200 m/s = 702 km/hr

Bolocam:

  • 3000" / 12.5 seconds = 1/15 deg/sec

-- MattDobbs - 04 Aug 2006

This topic: APEX_SZ > WebHome > 2006_AtmosphereSubtraction > 2006_AtmosphereSubtractionIntuition Topic revision: r2 - 2006-08-06 - MattDobbs
© 2020 Winterland Cosmology Lab, McGill University, Montréal, Québec, Canada

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