casa::SpectralIndex Class Reference

Models the spectral variation with a spectral index. More...

#include <SpectralIndex.h>

Inheritance diagram for casa::SpectralIndex:

Public Member Functions
	SpectralIndex ()
	The default SpectralIndex has a reference frequency of 1 GHz in the LSR frame and a spectral index of zero.
	SpectralIndex (const MFrequency &refFreq, Double exponent=0.0)
	Construct a SpectralIndex with specified reference frequency and exponent.
	SpectralIndex (const SpectralIndex &other)
	The copy constructor uses copy semantics.
virtual	~SpectralIndex ()
	The destructor does nothing special.
SpectralIndex &	operator= (const SpectralIndex &other)
	The assignment operator uses copy semantics.
virtual ComponentType::SpectralShape	type () const
	return the actual spectral type ie., ComponentType::SPECTRAL_INDEX
const Double &	index () const
	set/get the spectral index.
void	setIndex (const Double &newIndex)
const Vector< Double > &	stokesIndex () const
void	setStokesIndex (const Vector< Double > &newIndex)
virtual Double	sample (const MFrequency &centerFrequency) const
	Return the scaling factor that indicates what proportion of the flux is at the specified frequency.
virtual void	sampleStokes (const MFrequency &centerFrequency, Vector< Double > &iquv) const
	return full Stokes version especially for models which have different frequency dependence for the Stokes param (1 or 4 elements) So as allow for fractional pol change and angle change of linear pol w.r.t frequency A a four Vector of original IQUV should be passed in and it will hold the return values
virtual void	sample (Vector< Double > &scale, const Vector< MFrequency::MVType > &frequencies, const MFrequency::Ref &refFrame) const
	Same as the previous function except that many frequencies can be sampled at once.
virtual void	sampleStokes (Vector< Vector< Double > > &scale, const Vector< MFrequency::MVType > &frequencies, const MFrequency::Ref &refFrame) const
	So as allow for fractional pol change and angle change of linear pol w.r.t frequency Vector of four Vectors of original IQUV should be passed in and it will hold the return values Uitimately this math should really go in Flux and FluxRep to where a rotation of linear pol is allowed.
virtual SpectralModel *	clone () const
	Return a pointer to a copy of this object upcast to a SpectralModel object.
virtual uInt	nParameters () const
	return the number of parameters.
virtual void	setParameters (const Vector< Double > &newSpectralParms)
virtual Vector< Double >	parameters () const
virtual void	setErrors (const Vector< Double > &newSpectralErrs)
virtual Vector< Double >	errors () const
virtual Bool	fromRecord (String &errorMessage, const RecordInterface &record)
	These functions convert between a Record and a SpectralIndex.
virtual Bool	toRecord (String &errorMessage, RecordInterface &record) const
virtual Bool	convertUnit (String &errorMessage, const RecordInterface &record)
	Convert the parameters of the spectral index object to the specified units.
virtual Bool	ok () const
	Function which checks the internal data of this class for consistant values.
Private Attributes
Double	itsIndex
Vector< Double >	itsStokesIndex
Double	itsError

Detailed Description

Models the spectral variation with a spectral index.

Intended use:

Public interface

Review Status

Date Reviewed:: yyyy/mm/dd
Test programs:: tSpectralIndex
Demo programs:: dSpectralModel

Prerequisite

SpectralModel

Synopsis

This class models the spectral variation of a component with a spectral index.

This class like the other spectral models becomes more useful when used through the SkyComponent class, which incorporates the flux and spatial variation of the emission, or through the ComponentList class, which handles groups of SkyComponent objects.

A spectral index is the exponent in a power law model for the variation flux with frequency. It is mathematically is defined as:

 (nu / nu_0)^alpha

Where:

nu_0: is the reference frequency
alpha: is the spectral index
nu: is the user specified frequency

As with all classes derived from SpectralModel the basic operation of this class is to model the flux as a function of frequency. This class does not know what the flux is at the reference frequency. Instead the sample functions return factors that are used to scale the flux and calculate the amount of flux at a specified frequency.

Besides the reference frequency this class has one parameter; the spectral index. This parameter can be set & queried using the general purpose parameters functions or the class specific index functions.

<Dec-2013> Have added a full stokes spectral variation ..\. This is done via setting setStokesIndex. If setIndex is used then only the first element of 4 parameters is set The 4 elements are 0) alpha for stokes I ..such that 1) alpha for linear pol fraction i.e make sure that Q and U is such that sqrt(Q_0^2+U_0^2)/I_0 * (nu/nu_0)^alpha(1)) is obeyed. 2) Rot measure (RM=alpha(2)) value to rotate linear pol by angle RM*(lambda^2- lambda_0^2) 3) alpha for circular pol fraction i.e to make V such that V/I=V_0/I_0 *(nu/nu_0)^alpha(3) </Dec-2013> This class also contains functions (toRecord & fromRecord) which perform the conversion between Records and SpectralIndex objects. These functions define how a SpectralIndex object is represented in glish. The format of the record that is generated and accepted by these functions is:

 c := [type = 'spectral index',
 frequency = [type = 'frequency',
 refer = 'lsr',
 m0 = [value = 1, unit = 'GHz']
 ],
 index = 0.7
 ]

The frequency field contains a record representation of a frequency measure and its format is defined in the Measures module. Its refer field defines the reference frame for the direction and the m0 field defines the value of the reference frequency. The parsing of the type field is case insensitive. The index field contains the spectral index.

Example

These examples are coded in the tSpectralModel.h file.

Example 1:

In this example a SpectralIndex object is created and used to calculate the flux at a number of frequencies.

<Dec-2013> The  example below was NEVER implemented..\.
 Could well have written the following and call it documentation:
 Sous un arbre, vos laitues naissent-elles ?
 Si vos laitues naissent, vos navets aussi naissent !
 Leaving it as is for archeological purposes
 </Dec-2013>
 SpectralIndex siModel;
 siModel.setRefFrequency(MFrequency(Quantity(1.0, "GHz")));
 siModel.setIndex(1.0, Stokes::I);  
 siModel.setIndex(0.5, Stokes::Q);  
 siModel.setIndex(0.5, Stokes::U);  
 siModel.setIndex(-1.0, Stokes::V);
 const Flux<Double> LBandFlux(1.0, 1.0, 1.0, 1.0);
 const MVFrequency step(Quantity(100.0, "MHz"));
 MVFrequency sampleFreq = siModel.refFrequency().getValue();
 Flux<Double> sampleFlux;
 cout << "Frequency\t I-Flux\t Q-Flux\t U-Flux\t V-Flux\n";
 for (uInt i = 0; i < 11; i++) {
 sampleFlux = LBandFlux.copy();
 sampleFlux.convertPol(ComponentType::LINEAR);
 sampleFlux.convertUnit(Unit("WU"));
 siModel.sample(sampleFlux,
 MFrequency(sampleFreq, siModel.refFrequency().getRef()));
 cout << setprecision(3) << sampleFreq.get("GHz")
 << "\t\t " << sampleFlux.value(0u).re
 << "\t " << sampleFlux.value(1u).re
 << "\t " << sampleFlux.value(2u).re
 << "\t " << sampleFlux.value(3u).re
 << " " << sampleFlux.unit().getName() << endl;
 sampleFreq += step;
 }
<Dec-2013>
 Now for an example
 const MFrequency f1(Quantity(1.0, "GHz"), MFrequency::LSRK);
 const MFrequency f2(Quantity(2.0, "GHz"), MFrequency::LSRK);
 SpectralIndex siModel;
 siModel.setIndex(1.0);
 cout << "scale value at 1 GHz for setIndex 1.0 " << siModel.sample(f1) << endl;
 Vector<Double> indices(4);
 indices(0)=1.0; indices(1)=0.2; indices(2)=0.0005; indices(3)=0.1;     
 siModel.setStokesIndex(indices);
 Vector<Double> iquv(4);
 iquv(0)=10.0; iquv(1)=0.2; iquv(2)=0.4; iquv(3)=0.1;
 cerr << "iquv in " << iquv << "  indices " << indices << endl;
 siModel.sampleStokes(f1, iquv);
 cerr << "scale value of I at 1.0 GHz " << siModel.sample(f1) << " iquv out " << iquv << endl;
 siModel.sampleStokes(f2, iquv);
 cerr << "scale value of I at 2.0 GHz " << siModel.sample(f2) << " iquv out " << iquv << endl;
</Dec-2013>

Motivation

A Spectral Index frequency variation is the most widely used model in radio astronomy. In particular the NFRA package 'newstar' uses it extensively.

To Do

Nothing I hope

Definition at line 190 of file SpectralIndex.h.

Constructor & Destructor Documentation

casa::SpectralIndex::SpectralIndex ( )

The default SpectralIndex has a reference frequency of 1 GHz in the LSR frame and a spectral index of zero.

As such it is no different from the ConstantSpectrum class (except slower).

casa::SpectralIndex::SpectralIndex	(	const MFrequency &	refFreq,
		Double	exponent = `0.0`
	)

Construct a SpectralIndex with specified reference frequency and exponent.

casa::SpectralIndex::SpectralIndex ( const SpectralIndex & other )

The copy constructor uses copy semantics.

virtual casa::SpectralIndex::~SpectralIndex ( ) [virtual]

The destructor does nothing special.

Member Function Documentation

virtual SpectralModel* casa::SpectralIndex::clone ( ) const [virtual]

Return a pointer to a copy of this object upcast to a SpectralModel object.

The class that uses this function is responsible for deleting the pointer. This is used to implement a virtual copy constructor.

Implements casa::SpectralModel.

virtual Bool casa::SpectralIndex::convertUnit	(	String &	errorMessage,
		const RecordInterface &	record
	)			`[virtual]`

Convert the parameters of the spectral index object to the specified units.

Only one field of the supplied record is used, namely 'index'. This field is optional as the spectral index is a unitless quantity. If the index field is specified it must have the empty string as its value. This function always returns True unless the index field is specified and does not contain an empty string.

Implements casa::SpectralModel.

virtual Vector<Double> casa::SpectralIndex::errors ( ) const [virtual]

Implements casa::SpectralModel.

virtual Bool casa::SpectralIndex::fromRecord	(	String &	errorMessage,
		const RecordInterface &	record
	)			`[virtual]`

These functions convert between a Record and a SpectralIndex.

These functions define how a SpectralIndex object is represented in glish and this is detailed in the synopsis above. These functions return False if the record is malformed and append an error message to the supplied string giving the reason.

Implements casa::SpectralModel.

const Double& casa::SpectralIndex::index ( ) const

set/get the spectral index.

virtual uInt casa::SpectralIndex::nParameters ( ) const [virtual]

return the number of parameters.

There is one parameter or 4 for this spectral model, namely the spectral index for I or I,Q,U,V. So you supply a unit length vector or one 4 element long when using these functions. Otherwise an exception (AipsError) may be thrown.

Implements casa::SpectralModel.

virtual Bool casa::SpectralIndex::ok ( ) const [virtual]

Function which checks the internal data of this class for consistant values.

Returns True if everything is fine otherwise returns False.

Reimplemented from casa::SpectralModel.

SpectralIndex& casa::SpectralIndex::operator= ( const SpectralIndex & other )

The assignment operator uses copy semantics.

Reimplemented from casa::SpectralModel.

virtual Vector<Double> casa::SpectralIndex::parameters ( ) const [virtual]

Implements casa::SpectralModel.

virtual void casa::SpectralIndex::sample	(	Vector< Double > &	scale,
		const Vector< MFrequency::MVType > &	frequencies,
		const MFrequency::Ref &	refFrame
	)			const `[virtual]`

Same as the previous function except that many frequencies can be sampled at once.

The reference frame must be the same for all the specified frequencies. Uses a customised implementation for improved speed.

Implements casa::SpectralModel.

virtual Double casa::SpectralIndex::sample ( const MFrequency & centerFrequency ) const [virtual]

Return the scaling factor that indicates what proportion of the flux is at the specified frequency.

ie. if the centreFrequency argument is the reference frequency then this function will always return one. At other frequencies it will return a non-negative number.

Implements casa::SpectralModel.

virtual void casa::SpectralIndex::sampleStokes	(	Vector< Vector< Double > > &	stokesval,
		const Vector< MFrequency::MVType > &	frequencies,
		const MFrequency::Ref &	refFrame
	)			const `[virtual]`

So as allow for fractional pol change and angle change of linear pol w.r.t frequency Vector of four Vectors of original IQUV should be passed in and it will hold the return values Uitimately this math should really go in Flux and FluxRep to where a rotation of linear pol is allowed.

Implements casa::SpectralModel.

virtual void casa::SpectralIndex::sampleStokes	(	const MFrequency &	centerFrequency,
		Vector< Double > &	stokesval
	)			const `[virtual]`

return full Stokes version especially for models which have different frequency dependence for the Stokes param (1 or 4 elements) So as allow for fractional pol change and angle change of linear pol w.r.t frequency A a four Vector of original IQUV should be passed in and it will hold the return values

Implements casa::SpectralModel.

virtual void casa::SpectralIndex::setErrors ( const Vector< Double > & newSpectralErrs ) [virtual]

Implements casa::SpectralModel.

void casa::SpectralIndex::setIndex ( const Double & newIndex )