# Usage of estimateOpacity

Casa contains Juan Pardo's atmospheric model. This function is a convenient wrapper for it if you just want to compute the opacity at a single frequency. To try to compute the opacity for your specific observations, see listconditions. You can also supply a Trx (e.g. from a SYSCAL table) and it will estimate the system temperature at any requested elevation. It uses the following formula: Tsys = (Trx + etaTelescope*T*(1-etaSky) + T*(1-etaTelescope)) / (etaTelescope*etaSky) where etaSky is the transmission through the atmosphere at the requested elevation and etaTelescope is the telescope efficiency.

```CASA <6>: au.estimateOpacity(help=True)
Usage: estimateOpacity(pwvmean=1, reffreq=230, conditions=None, verbose=True,
elevation=90, altitude=5059., P=563., H=20., T=273., Trx=0,
etaTelescope=0.75, telescope=None, help=False)
units: pwv(mm), reffreq(GHz), elevation(deg), altitude(m), pressure(mb), temp(K)
The default values are nominal conditions at ALMA.  To change them, you may
either specify the weather and location variables, or use the shortcuts:
telescope: 'ALMA', 'SMA', 'EVLA'
conditions: a dictionary containing:
pressure,solarangle,temperature,humidity,elevation

CASA <3>: au.estimateOpacity(1,690)
Using pressure=563.0mb, temperature=273.0K and humidity=20% at ALMA.
690.0 GHz tau at zenith = 1.207 (dry=0.075, wet=1.133), trans=29.9%, Tsky=190.98
Out: [1.2073223221182718, 1.2073223221182718]

CASA <4>: au.estimateOpacity(1,690,telescope='SMA',elevation=45)
Using pressure=629.5mb, temperature=273.0K and humidity=20% at SMA.
690.0 GHz tau at zenith = 1.356 (dry=0.091, wet=1.265), trans=25.8%, Tsky=201.39
690.0 GHz tau toward source (elevation=45.0) = 1.917, trans=14.7%, Tsky=231.44
Out: [1.3557141565856619, 1.9172693469446449]

CASA <5>: au.estimateOpacity(10,22.235,telescope='VLA',T=290)
Using pressure=785.0mb, temperature=290.0K and humidity=20% at VLA.
22.2 GHz tau at zenith = 0.076 (dry=0.009, wet=0.066), trans=92.7%, Tsky=22.84
Out: [0.075975022445379875, 0.075975022445379875]

CASA <6>: au.estimateOpacity(pwvmean=.42,reffreq=690,elevation=56,Trx=80)
Using pressure=563.0mb, temperature=273.0K, humidity=20%, etaTelescope=0.75 at ALMA.
690.0 GHz tau at zenith= 0.550 (dry=0.075,wet=0.476), trans=57.7%, Tsky=120.52
690.0 GHz tau toward source (elevation=56.0) = 0.664, trans=51.5%, Tsky=138.14
Expected Tsys at zenith = 543.1K,   toward source = 641.2K

-- Main.ToddHunter - 2012-02-17

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Topic revision: r3 - 2012-05-23, ToddHunter
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