Transcript The Millimeter Regime

Considerations for
Millimeter-wave
Observations
Amy Lovell, Agnes Scott College
Fifth NAIC/NRAO Single-Dish Summer School
July 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
hn « 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
NGC1333 Spitzer
1.2 mm continuum with
velocity contours
image with
SCUBA 850 µm
contours
(L) Jørgensen et al. 2006 and Kirk et al. 2006
(R) Image from http://www.mpifr-bonn.mpg.de/div/mm/fachbeirat-02/node20.html
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 Molecules
H2
HD
H3+
H2D+
CH
CH+
C2
CH2
CH3
C2H2
C3H(lin) c-C3H
c-C3H2 H2CCC(lin)
C4H
H2C4(lin) HC4H
CH3C2H C6H
C7H
CH3C4H C8H
C6H6
OH
CO
CO+
H2O
HOCO+ H2CO
C3O
CH2CO
CH2CHOH
CH2CHCHO
CH3OCHO
CH2OHCHO
CH3CH2CHO
C2H5OCHO
NH
CN
N2
NH2
N2H+
NH3
HCNH+ H2CN
CH2CN CH2NH HC2CN HC2NC
CH3CN CH3NC HC3NH+ HC4N
CH2CHCN
HC5N
CH3C3N
NO
HNO
N2O
HNCO
SH
CS
SO
SO+
SiC
SiN
SiO
SiS
AlCl
KCl
HF
AlF
H2S
C2S
SO2
OCS
SiCN
SiNC
NaCN
MgCN
H2CS
HNCS
C3S
c-SiC3
C2H
CH4
C5
HC6H
C3
C4
C2H4
H2C6
C5H
HCO
HCO+
HOC+
C2O
CO2
H3O+
HCOOH H2COH+ CH3OH CH2CHO
HC2CHO C5O
CH3CHO
c-C2H4O
CH3COOH
CH3OCH3
CH3CH2OH
C3H7CN (CH3)2CO HOCH2CH2OH
C2H5OCH3
HCN
HNC
HCCN
C3N
NH2CN C3NH
C5N
CH3NH2
CH3CH2CN HC7N CH3C5N
HC9N
HC11N
NH2CHO
NS
SiH
HCl
NaCl
CP
PN
HCS+
c-SiC2
MgNC
AlNC
SiH4
SiC4
CH3SH C5S
FeO
Observing molecules
www.splatalogue.net
Challenges at higher frequencies
• 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
• 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
Surface, Resolution & Pointing
• Surface accuracy affects efficiency, expressed as l/16
Example: 1mm observations <62 mm accuracy
With the aperture efficiency of a perfect reflector h0
and RMS of surface deviations
s, then aperture efficiency
Surface, Resolution & Pointing
• Beam resolution q = 1.22 l/D
l is wavelength, D is diameter of the telescope
Example: 3mm with GBT (100m), q ~ 7”
• Pointing as a fraction of q 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 standards
Stable or predictable flux
Small fraction of the beam
Near source on sky
At millimeter wavelengths, planets are often used for calibration
Mercury and Venus near the sun
Venus and Jupiter can be quite large or too bright
Saturn’s ring opening angle is an influence
Neptune may be too faint
Pointing and Calibration
Venus 10” to 66”
Jupiter 30” to 49”
Uranus 3” to 4”
Mars
4” to 25”
Saturn 15” to 20”
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
e

e
 o / sin( el )
e
 o A
Typical optical depth for 230 GHz at CSO, 3mm H2O
at zenith 0.15, at 30o elevation 0.3
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 )

Tsys *  Tsys e
Tsys *  Thot 
Voff
Vload  Voff
Voff is sky (no source)

Vload is the hot load (no sky)
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
Double sideband receivers
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
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
ASTE
Chile
10m
SMT
Arizona
10m
GBT
West Virginia
100m
MOPRA
Australia
22m
IRAM 30m
Spain
CSO
Hawaii
10.4m
Onsala
Sweden
20m
JCMT
Hawaii
15m
Nobeyama
Japan
45m
LMT
Mexico
50m
ARO 12m
Arizona
APEX
Chile
12m