Transcript The Millimeter Regime - Green Bank Telescope

The
Millimeter
Regime
SMT
Arizona
10m
GBT
West Virginia
100m
IRAM 30m
Spain
Crystal Brogan
(NRAO/NAASC)
ASTE
Chile
10m
MOPRA
Australia
22m
CSO
Hawaii
10.4m
Onsala
Sweden
20m
JCMT
Hawaii
15m
Nobeyama
Japan
45m
LMT
Mexico
50m
ARO 12m
Arizona
APEX
Chile
12m
The
Millimeter
Regime
Crystal Brogan
(NRAO/NAASC)
SMT
Arizona
10m
GBT
West Virginia
100m
Outline
IRAM 30m
Spain
• Effect of the Atmosphere at mm wavelengths
MOPRA System Temperature
• Effective
ASTE
Chile
10m
Australia
22m
CSO
Hawaii
10.4m
• Direct Method of mm calibration
Onsala
Sweden
20m
• Simplified formulationJCMT
of Chopper Wheel
Hawaii
method of mm calibration
15m
• More accurate approach
Nobeyama
Japan
45m
LMT
Mexico
50m
• Efficiencies and different ways of reporting
temperature
• Why is mm so interesting?
ARO 12m
Arizona
APEX
Chile
12m
Problems unique to the mm/sub-mm
• Atmospheric opacity is significant for λ<1cm: raises Tsys and
attenuates source
– Varies with frequency and altitude
– Changes as a function of time mostly due to H2O
– Causes refraction which leads to pointing errors
– Gain calibration must correct for these atmospheric effects
•Hardware
–Noise diodes such as those used to calibrate the temperature scale at
cm wavelengths are not available at mm to submm wavelengths
•Antennas
–Pointing accuracy measured as a fraction of the beam (PB ~ 1.22 l/D)
is more difficult to achieve
–Need more stringent requirements than at cm wavelengths for: surface
accuracy and optical alignment
Constituents of Atmospheric Opacity
Column Density as a Function of Altitude
• Due to the troposphere (lowest
layer of atmosphere): h < 10 km
• Temperature decreases with
altitude: clouds & convection can
be significant
• Dry Constituents of the
troposphere:, O2, O3, CO2, Ne,
He, Ar, Kr, CH4, N2, H2
• H2O: abundance is highly variable
but is < 1% in mass, mostly in the
form of water vapor
• “Hydrosols” (i.e. water droplets in
the form of clouds and fog) also
add a considerable contribution
when present
Stratosphere
Troposphere
Opacity as a Function of PWV
(PWV=Precipitable Water Vapor)
Optical Depth as a Function of Frequency
• At 1.3cm most opacity
comes from H2O vapor
• At 7mm biggest
contribution from dry
constituents
total
optical
depth
22 GHz
43 GHz
100 GHz
1.3cm
7mm
3mm
K band
Q band
MUSTANG
• At 3mm both
components are
significant
optical depth due
• “hydrosols” i.e. water
to H2O vapor
droplets (not shown)
can also add
optical depth
significantly to the
due to dry air
opacity
Effect of Atmosphere on Pointing
• Since the refractive index of the atmosphere >1, an electromagnetic wave
propagating through it will be bent which translates into a pointing offset
The index of refraction
n 1 
Pdry  Pwater
Tatm
The amount of refraction is strongly dependent on the elevation
-Pointing off-sets Δθ ≈ 2.5x10-4 x tan(i) (radians)
@ elevation 45o typical offset~1’
- GBT beam at 7mm is only 15”!
Sensitivity: System noise temperature
In addition to receiver noise, at millimeter wavelengths the
atmosphere has a significant brightness temperature:
Tsys ≈ Trx + Tsky
where Tsky =Tatm (1 – et)
(Tatm = temperature of the
atmosphere ~ 270 K)
so Tsys ≈ Trx +Tatm(1-et)
Receiver
temperature
Emission from
atmosphere
Before entering atmosphere the signal S= Tsource
After attenuation by atmosphere the signal becomes S=Tsource e-t
Consider the signal to noise ratio:
S / N = (Tsource e-t) / Tsys = Tsource / (Tsys et)
Effective System
Temperature
Tsys* = Tsys et ≈ Tatm(et 1) + Trxet
The system sensitivity (S/N) drops rapidly (exponentially) as opacity
increases
*
Atmospheric opacity, continued
Typical optical depth for 230 GHz observing at the CSO:
at zenith t225 = 0.15 = 3 mm PWV, at elevation = 30o  t225 = 0.3
Tsys*(DSB) = et(Tatm(1-e-t) + Trec)= 1.35(77 + 75) ~ 200 K
assuming Tatm = 300 K
 Atmosphere adds considerably to Tsys and since the opacity can change
rapidly, Tsys must be measured often
Many MM/Submm receivers are double sideband, thus the effective Tsys
for spectral lines (which are inherently single sideband) is doubled
Tsys*(SSB) = 2 Tsys (DSB) ~ 400 K
Direct Method of MM Calibration
Von  Voff
Voff
=
(TA ' e t  Tsky  Trx )  (Tsky  Trx )
Tsky  Trx
Von  Voff
Voff
=
TA ' e t
Tsky  Trx
Inverting this equation
TA ' =
TA * =
TA’ is the antenna
temperature of the
source corrected as if
it lay outside the
atmosphere
TA '
l
Von  Voff
Voff
Tsys et
t at the observing
frequency must be
obtained by a
tipping scan or
some other means
Where ηl accounts for ohmic losses, rear
spillover, and scattering and is < 1
This is the method used at the GBT
Direct Calibration of the Atmosphere
Tsys = TA
sky
= TRx  lTatm (1  et )  (1 l )Tspill  lTCBRet
e t = e t o / sin( el ) = e t o A
ηl accounts for ohmic losses, rear spillover,
and scattering and is < 1
With enough measurements at different
elevation, ηl and t can be derived as long as
reasonable numbers for the other
parameters are known
Trx: Receiver temp. from observatory
Tatm ~ 260 K
Tspill: Rear spillover temperature ~300 K
Tcmb = 2.7 K
Tsys * = Tsys et
Tipping scan
Down side of the Direct Method
• Atmosphere changes too rapidly to use
average values
•Tipping scans use considerable observing
time ~10min each time
Probably not done often enough
Assume a homogeneous, plane-parallel
atmosphere though the sky is lumpy
Done as a post-processing step so if
something went wrong you’re out of luck
e t = e t o / sin( el ) = e t o A
http://www.gb.nrao.edu/~rmaddale/Weather/index.html
For a forecast of current conditions
Determining the Trx and the Temperature Scale
In order to measure Trx, you need to make
measurements of two calibrated ‘loads’:
V
Vhot + V rx
Tcold = 77 K liquid nitrogen load
Thot = room temperature load
Then
Y=
VRX  VHOT
VRX  VCOLD
Treceiver
and
TRX =
Vcold +V rx
THOT  YTCOLD
Y 1
and the temperature conversion factor is
g[ Kelvins/ Volt] =
[VRX
THOT  TCOLD
 VHOT ]  [VRX  VCOLD ]
Tcold
Thot
• Trx is not a constant, especially for
mm/submm receivers which are more
difficult to tune to ideal performance.
• A significant improvement to the Tsys*
measurement can be made if Trx is
measured rather than assumed
• Currently the SMA and soon ALMA will
use a two temperature load system for
all calibration
T
Chopper Measurement of Tsys*
• So how do we measure Tsys* without constantly measuring Trx and the
opacity?
Tsys* ≈ Trxet  Tatm(et 1)
• At shorter mm λ, Tsys* is usually obtained by occasionally placing an
ambient temperature load (Thot) that has properties similar to a black body
in front of the receiver.
• We want to know the effective sensitivity, not
how much is due to the receiver vs. how much
is due to the sky. Therefore, we can use:
Tsys * = Thot 
Voff
Vload  Voff
)
Voff is the signal from the sky (but
not on source)
Vload is the signal from the hot load
• As long as Tatm is similar to Thot, this method
automatically compensates for rapid
changes in mean atmospheric absorption
IRAM 30m chopper
Blue stuff is called eccosorb
Simplified Load Calibration Theory
Let
Tcal = gVcal = Tload  Toff
= [TRx  Thot ]  [T RxlTatm (1 et )  (1l )Tspill ]
Note that the load totally blocks
the sky emission, which
changes the calibration
equations from cm result
Simplify by assuming that
Thot  Tspill  Tatm  Tamb i.e., all our loads are at ambient temp.
Then most everything cancels out and we are left with
Recall from
cm signal
processing
and
Ton off
Tcal
= (Ton  Toff )
Tcal
Ton off
Tcal = lTamb et
But instead of
diode we have
a BB load so
1
TA
= (Ton  Toff ) t =
 TA *
t
l e
l e
Tcal  Thot  Tamb
So How Does This Help?
Ton off = (Ton  Toff )
1
TA
=
 TA *
t
t
l e
l e
Relating things back to measured quantities:
Thot
TA * = (Ton  Toff )
Tload  Toff
To first order, ambient absorber (chopper wheel) calibration corrects
for atmospheric attenuation!
So all you have to do is alternate between Ton and Toff and occasionally
throw in a reading of Thot (i.e. a thermometer near your hot load) and a
brief observation with Tload in the beam
The poorer the weather, the more often you should observe Tload . This
typically only takes a few seconds compared to ~10min for a tipping scan
Millimeter-wave Calibration Formalism
Corrections we must make:
1. At millimeter wavelengths, we
are no longer in the R-J part of
the Planck curve, so define a
Rayleigh-Jeans equivalent
radiation temperature of a Planck
blackbody at temperature T.
Once the function
starts to curve, the
assumption breaks
down
Linear part
is in R-J limit
h / k
J ( , T ) =
exp(h / kT )  1
2. Let all temperatures be different:
Tatm  Tspill  Tchop  Tamb
3. Most millimeter wave receivers using SIS mixers have
some response to the image sideband, even if they are
nominally “single sideband”. (By comparison, HEMT
amplifiers probably have negligible response to the
image sideband.)
•
•
The atmosphere often has different opacity in the signal &
image sidebands
Receiver gain must be known in the signal sideband
Gi  Gs = 1
Gs = signal sideband gain, Gi image sideband gain.
 Gi 
1  G  (TRx  Tsky )
*
s
Tsys = 
 exp(t )
Commonly used TR* scale definition (recommended
by Kutner and Ulich):
TR * 
TA *
 fss
• TR* includes all telescope losses except direct source coupling of the
forward beam in d
• The disadvantage is that fss is not a natural part of chopper wheel
calibration and must be included as an extra factor
• TA* is quoted most often. Either convention is OK, but know which
one the observatory is using
Main Beam Brightness Temperature
If the source angular extent is comparable to or smaller than the main
beam, we can define a Main Beam Brightness Temperature as:
TMB 
TA *
 fss M *
fss the forward spillover and scattering can be
measured from observations of the Moon, if
moon = diffraction region
 fss
TA * (m oon)
=
TB (m oon)
M* -- corrected main beam efficiency – can measure from
observations of planets which have mm Tb ~ few hundred K
*
TA [ Planet]
*
M =
 fssc (disk)TB [ Planet]
2

  disk  


 
c (disk) = 1  exp ln 2

 beam  


Flux conversion factors (Jy/K)
2kTA
Sv =
 A Ap
l TA * 2k
Sv =
 A Ap
Conventional
TA* definition
Why do we care about mm/submm?
mm/submm photons are
the most abundant
photons in the spectrum of
most spiral galaxies – 40%
of the Milky Way Galaxy
•
After the 3K cosmic
background radiation,
mm/submm photons carry
most of the radiative
energy in the Universe
•
Probe of cool gas and
dust
•
Science at mm/submm wavelengths:
dust emission
In the Rayleigh-Jeans regime, h « kT,
S = 2kT2t Wm-2 Hz-1
c2
and dust opacity, t 2
so for optically-thin emission, flux density
S  4
 emission is brighter at higher frequencies
Galactic star forming
region NGC1333
Spitzer/IRAC image from
c2d with yellow SCUBA
850 µm contours
• Dust mass
• Temperature
• Star formation
efficiency
• Fragmentation
• Clustering
Jørgensen et al. 2006 and Kirk et al. 2006
Unique mm/submm access to highest z
Redshifting the steep
FIR dust SED peak
counteracts inverse
square law dimming
SED of Arp 220 at z=0.02
Increasing z
redshifts peak
SED peaks at
~100 GHz for
z~10!
Andrew Blain
Science at mm/sub-mm wavelengths:
molecular line emission
• Most of the dense ISM is H2, but H2 has no permanent dipole moment
 use trace molecules
Plus: many more complex molecules (e.g. N2H+, CH3OH, CH3CN, etc)
– Probe kinematics, density, temperature
– Abundances, interstellar chemistry, etc…
– For an optically-thin line S  4; TB  2 (cf. dust)
List of Currently Known Interstellar Molecules
H2
CH
CH3
c-C3H2
H2C4(lin)
*C7H
OH
HOC+
C3O
CH2CHOH
CH3OCHO
(CH3)2CO
NH
N2H+
CH2CN
CH3CN
CH2CHCN
NO
SH
*SiC
*AlCl
H2S
*SiCN
H2CS
CH3SH
HD
H3+
CH+
C2
C2H2
C3H(lin)
H2CCC(lin)
*HC4H
CH3C2H
CH3C4H
C8H
CO
CO+
C2O
CO2
CH2CO
HCOOH
CH2CHCHO
CH2OHCHO
HOCH2CH2OH
CN
N2
NH3
HCNH+
CH2NH
HC2CN
CH3NC
HC3NH+
HC5N
CH3C3N
HNO
N2O
CS
SO
SiN
SiO
*KCl
HF
C2S
SO2
*SiNC
*NaCN
HNCS
C3S
C5S
FeO
H2D+
CH2
C2H
c-C3H
*CH4
C4H
*C5
C6H
*HC6H
*C6H6
H2O
HCO
H3O+
HOCO+
H2COH+ CH3OH
HC2CHO C5O
CH3COOH CH3OCH3
C2H5OCH3
NH2
HCN
H2CN
HCCN
HC2NC
NH2CN
*HC4N
C5N
CH3CH2CN HC7N
HNCO
NH2CHO
SO+
NS
SiS
HCl
*AlF
*CP
OCS
HCS+
*MgCN
*MgNC
c-SiC3
*SiH4
*C3
C4
*C2H4
H2C6
C5H
HCO+
H2CO
CH2CHO
CH3CHO
c-C2H4O
CH3CH2OH CH3CH2CHO
HNC
C3N
C3NH
CH3NH2
CH3C5N
SiH
*NaCl
PN
c-SiC2
*AlNC
*SiC4
HC9N
HC11N
The GBT PRIMOS Project:
Searching for our Molecular Origins
Many of these lines are
currently unidentified!
Hollis, Remijan, Jewell, Lovas
Detection of Acetamide (CH3CONH2):
The Largest Molecule with a Peptide Bond
(Hollis et al. 2006, ApJ, 643, L25)
Detected in emission and absorption toward Sagittarius B2(N) using four A-species and
four E-species rotational transitions. All transitions have energy levels less than 10 K.
This molecule is interesting
because it is one of only two
known interstellar molecules
containing a peptide bond.
Thus it could provide a link to
the polymerization of amino
acids, an essential ingredient
for life.
GBT at 7mm
Comparison of GBT with mm Arrays
All other things being equal, the effective collecting area (A) of a
telescope is good measure of its sensitivity
Telescope
GBT
SMA
CARMA
IRAM PdBI
ALMA
range
altitude
(feet)
diam.
(m)
No.
dishes
A
(m2)
2,650
100
1
1122*
80 - 115
13,600
7,300
8,000
16,400
6
3.5/6/10
15
12
8
23
6
50
230
800
1060
5700
220 - 690
80 - 230
80 - 345
80 - 690
* Effective GBT collecting area at 3mm is ~10%
compared to ~70% for others so GBT A/7 is listed
(GHz)
The
Millimeter
Regime
Summary
SMT
•
Arizona
Effect ofGBT
the Atmosphere10m
at mm wavelengths
IRAM 30m
West Virginia
 Attenuates
source and adds noise Spain
100m
• Effective System Temperature
• Direct Method of mm calibration
MOPRA
Requires measurement of t, used at GBT
ASTE
Chile
10m
•
Australia
22m
Simplified
CSO
Onsala
formulation
of Chopper Wheel
method of
Hawaii
Sweden
10.4m
mm calibration
20m
JCMTmethod at shorter λs
 Needed to replace diode
• More accurate
Hawaii
15m
approach
 Planck, Different temperatures, Sidebands
LMT
Mexico
50m
• Efficiencies and
temperature
Nobeyama
Japan
different
45m
ways of reporting
 Know what scale you are using
• Why is mm so interesting?
ARO 12m
Arizona
 Traces
cool universe
APEX
Chile
12m