[mmaimcal] Re: [alma-sw-ssr] Calibration Outline
Robert Lucas
lucas at iram.fr
Wed Apr 4 03:25:08 EDT 2001
Hi Steve:
I include a few `first look' comments to your outline of calibration
operations (which is a quite useful task to do ...)
Best regards
Robert
--
Robert LUCAS, Institut de Radioastronomie Millimetrique
300 rue de la Piscine, F-38406 St Martin d'Heres Cedex (FRANCE)
Tel +33 (0)4 76 82 49 42 Fax +33 (0)4 76 51 59 38
E-mail: mailto:lucas at iram.fr http://iram.fr/~lucas/
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ALMA Calibration Outline
Draft 3-Apr-2001 v0.1
S. Myers
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Before setting down the offline (and pipeline) requirements for calibration,
it would be a good idea to come up with an outline of the ALMA calibration
process. It is the goal of this outline to list the various ALMA calibration
operations, and to delineate the possible calibration procedures with either
a set of flow charts or sequences of operations and states.
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Online Calibration:
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These occur in the on-line system and are applied to control parameters
of the system such that the output of the correlator is a correct
representation of the correlation coefficient. We will not be concerned
with these here, but list a subset of them for completeness. To some extent
getting these wrong online will be corrected for in later operations.
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Delay
Pointing
Focus
Level Control
Quantization Corrections
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Real Time Calibrations:
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These are based on environmental information, monitor data, noise tube
measurements, calibration vane measurements, WVR measurements and are
meant to be applied to the data stream (though possibly well after the
fact).
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Applies To: (C=continuum S=spectrum Inf=Interferometer TP=total power)
Inf TP
Code C S C S What Comes From Produces
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TA x x x x Temperature Scale Noise tubes/vanes CC -> Ta
WV x x Atmospheric Phase WVR Phase-stable
OP x x x x Atmospheric Opacity WVR? FTS? Tip Ta -> Ta* (@)
RP x x Polarization Cor. Cal signal? Orthogonal (@@)
(@ Probably using parallel tip curve observations taken in a subarray.
Timescale would be longer than most of these Real Time corrections)
RL>> In the mm and sumbb, it is not wise to separate TA and OP. They
are nearly always integrated as e.g in the traditional `chopper
wheel' method. We nearly never do antenna tippings at Plateau de
Bure (only used a few times a year to check the antenna forward
efficiencies, not the atmosphere, which varies too fast, and is
not uniform enough to be monitored by antenna
tippings). Atmospheric opacity is implicitly corrected for by
monitoring the atmospheric emission as compared to ambient load
and cold load, or ambient load and receiver temperature. WVR
should give a reliable monitoring of the dry water content which
is the main variable component.
RL>> You should explicitly mention here the interferometric
measurement of side band gain ratio.
(@@ This is if a cal signal is used to correct polarization products in
real time)
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A Priori Calibrations:
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These require some previously known calibration information (eg. from
baseline determinations, gain curves) usually not part of
the observations themselves.
--------------------------------------------------------------------------------
Applies To:
Inf TP
Code C S C S What Comes From Produces
--------------------------------------------------------------------------------
GC x x x x Antenna Gain Eff. vs. Elev. Normalized gain
IB x x Baseline Correction Calibration run Phase-stable
PB x x x x Primary Beam Holography PB Correct (+)
(+ incorporated into imaging, includes polarization primary beam)
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A Posteriori Calibrations:
--------------------------------------------------------------------------------
These are usually determined from the data itself or calibrations taken
along with the data, and usually require a-priori calibration of the
data before determination. Some of these (bandpass, leakage) could be
done as a-priori if they were sufficiently stable.
--------------------------------------------------------------------------------
Applies To:
Inf TP
Code C S C S What Comes From Produces
--------------------------------------------------------------------------------
FL x x x x Flux Scale Source Calibration(&) Flux Density
GA x x Interferometer Gain Phase&Amp Referencing Phase Coherence
BP x x Bandpass Bright Source Flat bandpass
PD x x Pol. Phase Difference Pol. Cal. Source Orthogonal (*)
PL x x x x Polarization Leakage Pol. Cal. Source Stokes (**)
(& or conversion from Tant to Flux using aperture efficiency, as is
standard in VLBI)
(* eg. R/L phase diff for circular. In single dish case applied online.)
(** after conversion from polarization products to Stokes parameters)
RL> With the ALMA system, BP has to be done even for continuum:
continuum will be only obtained from many correlator channels to
which the bandpass will have to be applied for optimum continuum
sensitivity. Also the different basebands and sidebands will have
to be averaged coherently (we may regard this as a sort of
generalization of bandpass calibration).
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Calibration States:
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The data can be considered to be in one of a number of "calibration states" at
a given time, depending on what has been applied to that point. The order in
which some of these are applies in not critical (eg. RW can be applied after
RT, you might be able to skip temperature and go directly to flux if the
system were stable enough). The following is a possible flow of states
during the calibration procedure:
State Units Input Apply Notes
--------------------------------------------------------------------------------
Raw Correlation coefficient raw only online short integ
RawGain Correlation coefficient Raw WV longer integ
RawTemp Antenna Temperature (Ta) RawGain TA
NorTemp Antenna Temperature (Ta*) RawTemp OP
RawFlux Flux density (PolProd) NorTemp FL
NorFlux Flux density RawFlux GA
NorGain Flux density NorFlux GC
NorPol Flux density (Stokes) NorGain PD,PL for poln data
NorSpec Flux density NorPol BP for spectral line
or NorFlux
RL> Again Ta (RawTemp) is not a very meaningful intermediate state.
RL> The state names are not very well chosen : e.g. RawGain vs Raw
looks like you applied a kind of gain correction ... which is not
the case. May be refer to it only by the list of operators applied?
We can write these in operator notation, starting from Raw data or an
intermediate state:
NorTemp = TA*WV
NorFlux = GA*FL*NorTemp = GA*FL*TA*WV
Some alternative formulations:
Spectra in K.km/s: TemSpec = BP*GC*GA*NorTemp
One could make a set of flow charts, but I think the operator notation works
pretty well. These in some sense correspond to matrices in the measurement
equation, for example. If you think of the input streams of LTA output
(frequency channels x bands x polarization products), these are preserved
by the operations (scaling) until application of PD/PL, which mix the products
into Stokes (a rotation).
RL> Things missing here: the `interferometric gain' may rely on the
more complicated transfer of phase from a low frequency to the
high frequency, which in turn needs bandpass calibration at the
low frequency, calibration of the relative phases of the two
frequency bands ...
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