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Previous lecture:
Fundamentals of radio astronomy
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Flux, brightness temperature...
Antennae, surface accuracy, antenna temperature...
Signal & noise.
Detecting a weak signal.
Some general considerations.
Merja Tornikoski
Metsähovi Radio Observatory
Blazar observing techniques
Word of warning:
It is possible that your friendly support staff
has no experience whatsoever
from blazar observations!
Special considerations:
continuum
sources of unknown fluxes (variable!)
faint sources
sources that may not be suitable for pointing (faint)
absolute calibration needs to be accurate
exercise special caution if looking for IDV!!!!!!
Merja Tornikoski
Metsähovi Radio Observatory
Receivers, problem areas
• Noise from the recei ver, gain fluctuations etc.:
large factor compared to the astronomical signal.
• Signal-to-noise ratio  radio astronomical measurements
arealso measuring ”noise”!
• Signal << background.
• Power levels are low (P ca. 10-15 - 10-20 W).
• Noise also from ground, atmosphere, etc.
Merja Tornikoski
Metsähovi Radio Observatory
We need:
• Good stability.
• Good sensitivity.
• Good measurement techniques
(elimination of background etc.).
Merja Tornikoski
Metsähovi Radio Observatory
Background noise
• Man-made noise
– Receiver itself, cables & other components,
other radio signals (GSM!), etc.
• Noise from the nature:
– Atmosphere, CMB, black body radiation from ground etc.,
Solar radiation, thunderstorms ...
Merja Tornikoski
Metsähovi Radio Observatory
Receivers
• Heterodyne receivers
– Defining technologies: HEMT, Schottky, SIS.
• Bolometers.
• Bolometer arrays.
• Especially in the receiver technology considerable differences
btw. microwave / millimetre / submm!
Merja Tornikoski
Metsähovi Radio Observatory
Heterodyne receiver
= Superheterodyne or super receiver, ”superhet”
• Uses a local oscillator at a freqeuency that is different from
the incoming signal, to obtain an itermediate frequency that is
processed using conventional microwave technology.
• Different technologies at different frequency ranges,
HEMT amplifiers vs. SIS mixers,
currently the limiting freqeuency ca. 100 GHz.
telescope
Dicke-switch
preamplifier
comparison
load
sideband
filter
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local
oscillator
Merja Tornikoski
Metsähovi Radio Observatory
mixer
IF
amplifier
detector
Preamplifier
• At lower freqeuencies, f <~ 120 GHz
(note: upper freq-limit changing with new technologies!)
• At higher frequencies not in use (too noisy, or nonexistent).
• Common abbreviations:
– LNA = Low Noise Amplifier.
– HEMT = High Electron Mobility Transistor.
Merja Tornikoski
Metsähovi Radio Observatory
Sideband filtering
• Not desirable/useful in continuum observations!
• Normally two sidebands, upper sideband (USB) and
lower sideband (LSB), get through the mixer.
• Used for sideband rejection, before the mixer,
to filter one of the sidebands --> single sideband, SSB.
• For continuum observations, wide bandwidth is desired
and both sidebands carry information: double sideband, DSB.
• If sideband rejection is normally in use, check if you can
get rid of it for continuum work!
Merja Tornikoski
Metsähovi Radio Observatory
Mixer
• The signal gets converted (”mixed”) to a lower frequency.
• Amplifying high radio frequencies (mm/submm) is difficult,
amplifiying low-freq signals is easy.
• (Especially earlier) no preamplifiers for high f exist(ed),
only amplifying after mixing possible.
• Signal frequency fs, local oscillator frequency fLO
 lower intermediate frequency fIF = | fs – fLO|
fIF much lower, much easier to handle (= amplify).
Merja Tornikoski
Metsähovi Radio Observatory
Mixer technologies
• Schottky diode
– Older technology, easy to build.
– Typically for lower frequencies & ambient temperatures.
• SIS junction mixer (Superconductor-Insulator Supercond.)
– Requires cooling, more challenging to build.
– For frequencies > 100 GHz
(at lower freqs no direct benefit & requires more maintenance),
widely in use at mm/submm telescopes especially for spectral
line work.
– Useful for detecting faint signals,
ie. when high sensitivity is needed.
– In practise often problems with stability!
– Bolometers much preferred for continuum work!
Merja Tornikoski
Metsähovi Radio Observatory
Local oscillator
• Typically a semiconductor oscillator that can be phaselocked
to an exact frequency.
• Most common: Gunn oscillator.
Difficult to make for n > 120 GHz: multipliers.
Gunn oscillator itself not very stable: phase locking to a more
stable oscillator.
Phase lock loop (PLL) keeps the Gunn LO stable (n & phase).
• Output signal: intermediate frequency, IF.
• Stability required especially for spectral line observations and
VLBI. At high frequencies stability may be a problem.
Merja Tornikoski
Metsähovi Radio Observatory
Dicke-switching
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Gain fluctuations from mixers.
Varying background noise.
Observed signal e.g. 1/10000 of the overall system noise.
Amplifications by a factor of ca. 1012 may be desired.
 comparison measurement by Dicke-switching
(R.H. Dicke 1946).
Merja Tornikoski
Metsähovi Radio Observatory
... Dicke switching
• Swithching rate e.g. 5 – 100 Hz.
• Reference source:
noise diode; attenuator; signal from the blank sky
(e.g., two-feedhorn method in Metsähovi).
• Two beam-method for beam switching:
two feedhorns or one beam + chopper wheel.
Merja Tornikoski
Metsähovi Radio Observatory
Millimetre-domain:
special considerations
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Components are small.
Tolerances are small.
Components are often expensive.
Circuit losses larger than in the microwave region.
Amplifier technology still in development.
Less power.
Atmospheric attenuation is large.
• Millimetre domain technology:
– Quasioptics
(Mirrors, lenses; diffraction must be taken into account!)
– Cooling is necesssary.
Merja Tornikoski
Metsähovi Radio Observatory
Quasioptics
• In the (sub)mm domain the interface from the telescope
to the detector.
• Mirrors, lenses, grids are small
 geometrical optics can not be used (diffraction)
Gaussian optics = quasioptics: optimizes the feeds of
the receiver to the antenna beam pattern.
• Can include a polarizer plate.
Merja Tornikoski
Metsähovi Radio Observatory
Cryostat
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SIS requires very low temperatures.
Helium cooling.
Cooled parts of the receiver in vacuum container, dewar.
Dewar enclosed in a radiation shield.
Radiation shield cooled by a closed-cycle refrigerator.
Merja Tornikoski
Metsähovi Radio Observatory
Bolometer
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A total power detector.
High sensitivity from a cooled device.
Wide bandwidth.
Relatively easy to construct & operate.
Classical semiconductor bolometers
Superconducting TES devices
Silicon pop-up detectors (PUDs)
Merja Tornikoski
Metsähovi Radio Observatory
... Bolometer
• Classical Germanium bolometer.
• The temperature rise causes a change in the resistance of the
bolometer and consequently in the voltage across it.
V is amplified and measured.
Merja Tornikoski
Metsähovi Radio Observatory
Bolometer characteristics
• Noise Equivalent Power, NEP
= The power absorbed that prduces a S/N of unity
at the bolometer output.
NEP2 = NEP2(detector) + NEP2(background)
• Thermal time constant t
= a measure of the response time of the bolometer
to incoming radiation
= C/G
• Compromise btw response time and NEP!
• In practise, with wide bandwith the bolometer performance
can degrade due to power loading from the background
 “background” = loading + photon noise.
Merja Tornikoski
Metsähovi Radio Observatory
Bolometer arrays
• Until mid-1990’s bolometers were mainly single-channel devices.
• Main advantage: mapping of extended sources,
i.e. not directly applicable for blazar studies.
• Can eliminate the need for separate pointing observations
(depending on the obs. configuration).
Merja Tornikoski
Metsähovi Radio Observatory
Bolometer arrays currently in use
• SCUBA @JCMT
– 450 / 850 mm, 91/37 pixels, 300/65 mJy/sqrt(Hz).
• MAMBO-2 @IRAM
– 350 mm, 384 pixels, 500 mJy/sqrt(Hz).
Future:
• Many hundreds or even thousands of pixels.
• Silicon-micro machining, thin-film deposition and hybridization
techniques.
• Integrated SQUID multiplexers in the same plane as the detector chip.
• E.g., SCUBA-2: 10000 pixels, for JCMT.
SPIRE for HERSCHEL: 200-700 microns, 3 arrays of 43, 88 and 139
pixels.
Merja Tornikoski
Metsähovi Radio Observatory
Observing techniques: beam switching
• Source size, line/continuum, etc: --> Observing method
(Position switching, Frequency swithching, Load Switching)
• Continuum observations of point sources: beam switching.
• Single beam switching: The source is in the signal beam, the
sky is observed in the reference beam.
• Dual beam switching: Alternates the source/sky in the signal
and reference beams.
• Dual beam switching produces good results when sky noise
is the problem = most of the time 
• Note: ON/ON, ON/OFF terminology not fully standard!
• Technology: dual horn setup or chopper; nodding secondary;
telescope movement.
Merja Tornikoski
Metsähovi Radio Observatory
... beam switching, things to remember
• Relatively large overheads: do not use too short
integration times per beam.
• Data reduction: make sure to know whether your
intensities are to be divided by 2 or not!
Merja Tornikoski
Metsähovi Radio Observatory
Dual beam switching
• 2 feedhorns / beams, A & B
• Dicke-swithcing: measures signal A – signal B
(integrated in e.g. 1 s chunks).
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A
B
I A: background (b)
B: background + source (b + s)
II A: background + source (b + s)
B: background (b)
B
(s + b)B - bA
bB – (s + b)A
Merja Tornikoski
Metsähovi Radio Observatory
D = 2S
Not the same background!
Eliminates e.g. effects of a radome very well!
Point source observations...
• 1-point observation, problems:
One must rely on the pointing pattern &
offsets required earlier.
• Drift scans:
The moving source ”drifts” over the beam.
Gaussian fit  Smax.
• 5-point observation:
For relatively bright sources!
Default ”center” point + 4 other positions.
2-dimensional Gaussian fit: amplitude + offsets.
Merja Tornikoski
Metsähovi Radio Observatory
Pointing
• Radio telescope pointing can never be quite perfect:
telescope size, gravitaional deformation, heating
(more exotic problems: pedestial tilts, earth quakes, ...)
• Pointing pattern.
• Pointing checks.
• 5-point (9-point) measurement:
offsets
amplitude correction (2-dimensional gaussian fit).
Merja Tornikoski
Metsähovi Radio Observatory
Focusing
• Antenna deformation may be caused by gravitaional or
wind forces, or by differential thermal expansion.
 illumination pattern is controlled by changing
the position/shape of the secondary mirror
Very important in the submm region!
Merja Tornikoski
Metsähovi Radio Observatory
Calibration
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From the observed DV or DT into Sobserved.
Load calibration (noise diode, chopper wheel).
Opacity (tau) measurements (skydips).
Flux calibration.
(Pointing check: sometimes called ”pointing calibration”).
Merja Tornikoski
Metsähovi Radio Observatory
... calibration
• cm-frequencies: noise diode injection for receiver calibration: Gain
fluctuations (temperature, electronics, mechanical stress).
• mm-frequencies: absorbing blakcbody load (blocks the sky
emission, corrects for atmospheric attenuation), can be done
frequently
 ”corrected antenna temperature” TA*
• Skydips:
– Take up observing time.
– Assume homogeneous, plane-parallel atmosphere.
– Corrections usually done at data reduction stage.
• Primary flux calibrator:
a bright source with a known, constant flux.
• Secondary calibrator:
a bright ”relatively well-known, relatively constant flux”.
Merja Tornikoski
Metsähovi Radio Observatory
... calibration
• Microwave domain: primary: DR21, Jupiter, Mars;
secondary: e.g., 3C274, 3C84.
(”Baars scale”).
• Submillimetre domain: primary: Mars, Uranus;
secondary: planetary nebulae, giant stars,
ultracompact HII...
• Consult the support staff for advise on the
observatory calibration procedures
+ suitable calibrators & their flux information!
• Always make sure to make frequent
flux calibrator observations!!!
Merja Tornikoski
Metsähovi Radio Observatory
... calibration
• Tau measurements / skydips:
– Measure the noise temperature of the sky at various elevations.
– Effect on the flux calibration: exp(tau/sin(el)).
Merja Tornikoski
Metsähovi Radio Observatory
Telescope performance, sky noise
• Performance is represented by the Noise Equivalent Flux Density
(NEFD).
– Depends very much on the weather and varies with sky
transmission.
– Often the fundamental sensitivity limit is set by the ”sky noise”.
– Sky noise  spatial and temporal variations in the emissivity of
the atmosphere.
– Sky noise can degrade the NEFD by more than an order of
magnitude.
– Chopped beams travel through slightly different atmospheric
paths  Use narrow beam switching! Still some residual.
– With a bolometer array one can try to remove large-scale effects
to high order.
Merja Tornikoski
Metsähovi Radio Observatory
From an observation
into a data point
• Observing strategy:
– Is focusing needed?
– Is load calibration needed?
– Is skydip needed?
- Choose target source. Bright enough for a pointing source?
No: check pointing or use reliable offsets in the same direction.
- Observe source, keep an eye on S/N if possible;
if not, make a quick-look analysis to check if integration time
was optimal.
- Observe calibrator source.
Avoid unnesessary slewing, avoid blind pointing,
make sure to keep an eye on the weather changes,
observe primary and secondary calibrators often enough!
Merja Tornikoski
Metsähovi Radio Observatory
... From an observation
into a data point
• Data reduction:
– Enough flux calibration data?
– Enough tau information?
– Was the weather OK all through your run?
– Bolometer array data:
reduce the image following local instructions.
– Single channel bolometer or heterodyne data:
obtain the average result of the various ON/ON pairs
with their cumulative error.
– Make tau corrections + any other general corrections.
– Obtain flux conversion factors from the calibrators.
Are they consistent???
– Use the FCFs for your own sources.
– Error estimates from rms values + deviations in FCFs.
Merja Tornikoski
Metsähovi Radio Observatory
Bolometer data reduction
• E.g. SCUBA photometry:
Flatfielding; Extinction correction; Despiking;
Examine individual bolometer quality & change if
needed;
Sky removal;
Average or parabola fit over the 9-point map;
Catenation of individual measurements
Merja Tornikoski
Metsähovi Radio Observatory
How to write a good proposal
or: ” OK you think you’ve got a good science case...
but THEY don’t know it!
• Why this telescope?
• Why this/these observing band(s)?
• Read the manuals: observing manuals, data reduction,
show them that you understand it all.
• Any specific constraints? Sun, gain/elevation,
weather, pointing, ...
• Work out the realistic integration times including realistic
overheads, explain them all.
• If using heterodyne receiver, pay attention to:
tuning
sidebands
backend
data reduction
Merja Tornikoski
Metsähovi Radio Observatory
Observational radio astronomy
•
N. Bartel et al. 1987, ApJ 323. 507:
“No data were taken at station D during the period 0830 to 1630
GST due to the presence of a red racer snake (Coluber
constrictor) draped across the high-tension wires (33,000 V)
serving the station. However, even though this snake, or rather a
three-foot section of its remains, was caught in the act of causing
an arc between the transmission lines, we do not consider it
responsible for the loss of data. Rather we blame the
incompetence of a red tailed hawk (Buteo borealis) who had
apparently built a defective nest that fell off the top of the nearby
transmission tower, casting her nestlings to the ground, along
with their entire food reserve consisting of a pack rat, a kangaroo
rat, and several snakes, with the exception of the abovementioned snake who had a somewhat higher destiny.
No comparable loss of data occurred at the other antenna sites.”
Merja Tornikoski
Metsähovi Radio Observatory