NAME

bwfilter -- Fourier filter a 1-D image or table column.

USAGE

bwfilter input output

DESCRIPTION

This task filters a 1-D array in the following way. After subtracting a background from the input, the forward Fourier transform is taken. The data in the transform domain are multiplied by a filter of the form: signal / (signal + noise). The inverse Fourier transform is then taken, the background is added back in, and the result is written to the output.

The input and output may be either an image or a table column, or one of each. For either the input or output parameter, if the value is a single word it is assumed to be an image name, but if it is two words separated by one or more spaces it is assumed to be a table name and a column name.

The background that is subtracted before taking the Fourier transform is obtained by fitting a straight line. The linear fit is done to the log of the data if all values are positive; otherwise, the fit is done to the data values directly.

The signal and noise are estimated as follows. The task fits a constant (the noise level) to the higher frequencies, and it fits a quadratic (the signal) to the log power spectrum at the lower frequencies. Then the filter in the Fourier domain is signal / (signal + noise).

History records are added to the output header to give the date and the cutoff frequency, and for an output table, the column name of the filtered data.

PARAMETERS

input [string]

The name of the input. This can be a single image name, or it can be a table name and column name separated by a space.

output [string]

The name of the output. This can be a single image name, or it can be a table name and column name separated by a space. The input and output may not be the same image; they may be the same table, however. If the output table already exists it will be written to in-place; if the column exists it will be overwritten.

(interactive = yes) [boolean]

The default interactive = yes means that the cutoff frequency is gotten interactivly from a plot. In this case the task displays a plot of the log power spectrum, you position the cursor near the boundary between signal and noise and hit return (or space or any lower case letter). The X cursor position is taken as the cutoff frequency. Then the data are filtered, the log power spectrum of the filtered data is overplotted, and the filtered data are written to output.

If interactive = no, the task gets the cutoff frequency from the freq parameter.

(freq) [real, min>0, max=0.5]

This is the frequency where signal and noise are approximately equal. Higher frequencies will be filtered out. The frequency is specified in cycles per pixel; in these units the Nyquist frequency is 0.5. Note that this parameter is only gotten if interactive = no.

(device = "stdgraph") [string]

The graphics device that is used if interactive = yes.

(cursor) [graphics cursor]

The graphics cursor that is gotten if interactive = yes.

(Version = "19August1994") [string]

This is the date of installation of the current version of the software.

EXAMPLES

1. Filter the image "fos.hhh" in the scidata directory, putting the output in column "flux" of table "fos.tab"

	pl> bwfilter scidata$fos.hhh "fos.tab flux"

2. Filter column "flux" in table "fos.tab", putting the output in column "f_flux" of the same table.

	pl> bwfilter "fos flux" "fos f_flux"

BUGS

REFERENCES

This task was written by Phil Hodge based on algorithms given by Brault and White, 1971, Astron. & Astrophys., 13, 169.

SEE ALSO