NAME

filtpars -- edit the filter function parameters

USAGE

filtpars

PARAMETERS

f_type = "ramp"

Type of filter to be used. Possible choices are

ramp

A ramp function which begins to rise at the cuton wavenumber and reaches full value (i.e. passes the full value of the component) at the fullon wavenumber. It begin to decline at the cutoff wavenumber and returns to zero at the fulloff wavenumber.

Hanning

A Hanning function is used to attenuate the fourier components over the range specified by the cuton and cutoff parameters.

Welch

A Welch function is used to attenuate the fourier components over the range specified by the cuton and cutoff parameters.

Square

A standard step function which is zero outside the cuton and cutoff component numbers and one within those numbers.

cuton = 0

The fourier wavenumber at which the filter begins to pass the filtered fft component.

cutoff = 0

The fourier wavenumber at which the filter ceases to pass fft components.

fullon = 0

Used only for a ramp filter. The fourier wavenumber at which the filter reaches full value and passes all of the data.

fulloff = 0

Used only for a ramp filter. The fourier wavenumber at which the filter reaches zero value and passes none of the data.

DESCRIPTION

The filtering parameters control the type of filter to be used on the Fourier transformed data as well as the range in wavenumbers over which it will operate. Filtering of the data may be necessary to remove high frequency noise or low-frequency tends not removed by continuum subtraction. If the filtering is enabled, then once the data have been transformed, a bandpass filter of the type chosen by the f_type parameter is applied to the Fourier components of the spectra. Wavenumbers lower than that specified by the cuton parameter are set to zero and wavenumbers up to that specified by the cutoff parameter (or the fulloff parameter in the case of a ramp filter) are attenuated or passed in full according to the filter chosen. Since the data are assumed to be linearized in log-wavelength space, applying a filter to the data in Fourier space introduces no phase shift and has the same effect as smoothing the data in real space. The data are centered and zero padded in an array of length 2**N such that the number of elements is greater than or equal to the number of actual data points. This array in then Fourier transformed, and the resulting fft is then filtered prior to correlation.

Filtering is enabled by turning on the fxcor.filter parameter and setting it to something other than "none". Filtering may be done on only one of the two spectra or both prior to correlation.

The filter choices behave as follows:

Square Filter

The fourier components at wavenumbers between the cuton and cutoff wavenumbers are passed without change. Those wavenumbers outside this region are set to zero.

Ramp Filter

Fourier components below the cuton and above the fulloff wavenumbers are set to zero. At the cuton wavenumber the filter function begins to rise until the fullon wavenumber is reached. Data in this region is weighted by the slope of the filter until at the fullon wavenumber data are passed through without change. Similarly, the filter begins to fall at the cutoff wavenumber until it completely blocks (i.e. zeros) the fourier components at the fulloff wavenumber.

Welch Filter

Fourier components below the cuton and above the cutoff wavenumbers are set to zero. Components between these regions are weighted according to the equation for a Welch window. Namely,

						     2      
	w(j)  = 1. - [ (j - 1/2(N-1)) / (1/2(N+1)) ] 
		        
		where j =  (wavenumber - cuton_wavenumer) 
	      	      N =  (cutoff - cuton) + 1

Hanning Filter

Fourier components below the cuton and above the cutoff wavenumbers are set to zero. Components between these regions are weighted according to the equation for a Hanning window. Namely,

	w(j)  =  1/2 [ 1. - cos( (TWOPI*j) / (N-1) ) ]

		where j =  (wavenumber - cuton_wavenumer) 
	              N =  (cutoff - cuton) + 1

TASK COLON COMMANDS

The values of the filtpars pset may be changed, displayed, or updated from within the Fourier mode of the fxcor task. Simply typing the parameter name will have the default action of printing the current value of that parameter. An optional value may be added to change the named parameter.

:update filtpars

Update the pset with the current values of the filter parameters. The argument "filtpars" must be present or else the command will default to the task parameters.

:unlearn filtpars

Reset the parameter values to their defaults. The argument "filtpars" must be present or else the command will default to the task parameters.

:show filtpars

Clear the screen and display all values in the filtpars pset. The argument "filtpars" must be present or else the command will default to the task default.

:filttype [ramp|welch|hanning|square|none]

Set or show the current value of the filter type to use

:cuton [int_value]

Set or show the current value of the cuton fourier component

:cutoff [int_value]

Set or show the current value of the cutoff fourier component

:fullon [int_value]

Set or show the current value of the fullon fourier component

:fulloff [int_value]

Set or show the current value of the fulloff fourier component

EXAMPLES

1. List the filtering parameters.

	rv> lpar filtpars

2. Edit the filtering parameters

	rv> filtpars

SEE ALSO

fxcor