considerations for millimeter-wave observations amy lovell, agnes scott college fifth naic/nrao...
Post on 18-Dec-2015
215 Views
Preview:
TRANSCRIPT
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
top related