Considerations for Millimeter-wave Observations

Amy Lovell, Agnes Scott College

Fifth NAIC/NRAO Single-Dish Summer SchoolJuly 2009

Key Issues

mm-wave frequency allocation & science motivation

Atmosphere & system requirements

mm-wave spectrum

mm-wave spectrum

Probing cool gas & dust

Rayleigh-Jeans limit h « kT

Probing our galaxy

UPPER: HI image (1.4 GHz, 21cm)

MIDDLE: Continuum (2.5 GHz, 12cm)

LOWER: CO image (115 GHz, 3mm, tracing H2)

http://mwmw.gsfc.nasa.gov/

Probing star forming regions

•Temperature

•Star formation efficiency

•Morphology of cores

•Dynamics

•Dust masses

CO Image from NRAO 12m http://www.cv.nrao.edu/~awootten/research.html

(R) Image from http://www.mpifr-bonn.mpg.de/div/mm/fachbeirat-02/node20.html

(L) Jørgensen et al. 2006 and Kirk et al. 2006

NGC1333 Spitzer image with SCUBA 850 µm contours

1.2 mm continuum with velocity contours

Probing high-redshift galaxies

High Redshifts place high frequency lines and the Spectral Energy Distribution (SED) of cool dust into the millimeter region

Observing molecules

•Density & Dynamics from tracers of H2

•Organics and Building Blocks for Life

•Rotational transitions visible at mm-wavelengths

•Chemistry in the ISM, Galaxies, atmospheres

Some Detected MoleculesH2 HD H3+ H2D+ CH CH+ C2 CH2 C2H C3

CH3 C2H2 C3H(lin) c-C3H CH4 C4

c-C3H2 H2CCC(lin) C4H C5 C2H4 C5HH2C4(lin) HC4H CH3C2H C6H HC6H H2C6

C7H CH3C4H C8H C6H6

OH CO CO+ H2O HCO HCO+ HOC+ C2O CO2 H3O+ HOCO+ H2CO C3O CH2CO HCOOH H2COH+ CH3OH CH2CHOCH2CHOH CH2CHCHO HC2CHO C5O CH3CHO c-C2H4O CH3OCHO CH2OHCHO CH3COOH CH3OCH3 CH3CH2OH CH3CH2CHO C2H5OCHO C3H7CN (CH3)2CO HOCH2CH2OH C2H5OCH3 NH CN N2 NH2 HCN HNC N2H+ NH3 HCNH+ H2CN HCCN C3NCH2CN CH2NH HC2CN HC2NC NH2CN C3NHCH3CN CH3NC HC3NH+ HC4N C5N CH3NH2

CH2CHCN HC5N CH3C3N CH3CH2CN HC7N CH3C5N HC9N HC11NNO HNO N2O HNCO NH2CHO SH CS SO SO+ NS SiHSiC SiN SiO SiS HCl NaClAlCl KCl HF AlF CP PNH2S C2S SO2 OCS HCS+ c-SiC2

SiCN SiNC NaCN MgCN MgNC AlNCH2CS HNCS C3S c-SiC3 SiH4 SiC4 CH3SH C5S FeO

Observing molecules

www.splatalogue.net

Challenges at higher frequencies

• Atmospheric opacity : not all photons get through– Varies with frequency– Varies with altitude– Varies with time (mostly humidity)– Gain calibration needs to account for these effects

• Antennas and Receivers–Surface accuracy must be better for short wavelengths–Pointing constraints are more difficult for smaller beams–Calibration of system temperature is done differently

Surface, Resolution & Pointing

• Surface accuracy affects efficiency, expressed as 16Example: 1mm observations <62 m accuracy

With the aperture efficiency of a perfect reflector

and RMS of surface deviations , then aperture efficiency

Surface, Resolution & Pointing

• Beam resolution = 1.22 /D is wavelength, D is diameter of the telescope

Example: 3mm with GBT (100m), ~ 7”

• Pointing as a fraction of is then harder ( ≤ 1” )

• Some telescopes require 30-40° sun avoidance

• Air has refractive index n >1 and changes with air density, so can influence pointing, described by Olmi p.413

Pointing and Calibration

Good flux calibration depends on well-known flux standardsStable or predictable fluxSmall fraction of the beamNear source on sky

At millimeter wavelengths, planets are often used for calibrationMercury and Venus near the sunVenus and Jupiter can be quite large or too brightSaturn’s ring opening angle is an influenceNeptune may be too faint

Pointing and Calibration

Venus 10” to 66” Mars 4” to 25”Jupiter 30” to 49” Saturn 15” to 20”

Uranus 3” to 4” Neptune 2”

Negligibly small in arcminute-scale beams, but not so at high frequencies!

Mars, when not too large, and Uranus are suitable

Large asteroids have been considered for sub-mm, but they, like Mars, vary in flux as they rotate

mm Calibration References

Cogdell, J.R., Davis, J.H., Ulrich, B.T., & Wills, B.J. 1975, ApJ, 196, 363

Dent, W.A. 1972, ApJ, 177, 93

Greve, A., Steppe, H., Graham, D., & Schalinski, C.J. 1994, A&A 286, 654

Griffin, M.J., & Orton, G.S. 1993, Icarus, 105, 537

Rowan-Robinson, M., Ade, P.A., Robson, E.I., & Clegg, P.E. 1978, A&A, 62, 249

Sandell, G. 1994, MNRAS 271, 75

Su, Y.-N., et al. 2004, ApJL 616, L39

Ulich, B.L. 1981, Astron.J., 86, 1619

Wood, D.O.S., Churchwell, E., & Salter, C.J. 1988, ApJ, 325, 694

Wood, D.O.S., Handa, T., Fukui, Y., Churchwell, E., Sofue, Y., & Iwata, T. 1988, ApJ, 326, 884

Atmospheric Windows

Millimeter “window”

• Attenuation (previous) e- Transmission 1 – e-

• Airmass = sec(z) = 1/sin(el) zenith opacity 0 at z=0

Typical optical depth for 230 GHz at CSO, 3mm H2O

at zenith 0.15, at 30o elevation 0.3

Ael oo eee )sin(/

http://www.gb.nrao.edu/~rmaddale/Weather/index.html

Blue = 2mm PWV Red = 5mm PWV

3mm Band is truncated by Oxygen lines

Opacity and water vapor

PWV=Precipitable Water Vapor

System Temperatures

Original source is attenuated passing through atmosphere

Tsource above the atmosphere

Tsource e- below the atmosphere

EXPONENTIAL decrease in the signal

Tsys = Trx + Tatm (1 - e- )

Trx receiver temperature (cooled)

Tatm atmosphere temperature (270-300 K)

Skydip to estimate zenith opacity 0

Requires time every 1-2 hours

Assumes homogeneity

Processed after observations

Chopper Wheels and loads

Tsys = Trx + Tatm (1 - e- )

offload

off

hotsys VV

VTT

*

Voff is sky (no source)

Vload is the hot load (no sky)

eTT syssys *

ambient temperature load (Thot)

Frequency Switching

For spectral line observations, instead of switching to a “blank” sky position, you can switch to a close-by frequency

Assumptions:

the atmosphere is the same across your band

there is no line (or RFI) where you are switching

the observed line is contained in ¼ of the band

System Measurements

Ton & Toff

Thot occasional

Tload More often in poorer weather

Chopper measurements are rapid, no need to skydip

Some mm subreflectors are small enough to nod and “throw” the beam without having to move the primary

One sideband contains the spectral line signal, the other just sky noise (maybe different ) that must be filtered

SIS mixers have some response even if they are nominally “single sideband”

Double sideband systems double system noise for spectra

See Payne Fig. 9, p. 109 for a diagram

Double sideband receivers

Words of Caution

Atmosphere Attenuates source and adds noise

Choose your flux calibration sources carefully

Know your Temperature scales

Words of Inspiration

Lots of photons and high angular resolution

Lower receiver noise and lower RFI

Millimeter Telescopes

MOPRA Australia

22m

LMT Mexico

50m

APEX Chile 12m

IRAM 30m Spain

Nobeyama Japan

45m

CSO Hawaii 10.4m

JCMT Hawaii

15m

SMT Arizona

10m

Onsala Sweden

20m

GBT West Virginia

100m

ASTE Chile 10m

ARO 12m Arizona