Description

Task sdfixscan is used to remove a striping pattern generated along the scan direction in raster scan data. By default, the scanning noise is removed using the FFT-based 'Basket-Weaving' method [1] that requires multiple images observed over exactly the same area with a different scanning direction. When doing 'Basket-Weaving', you can mask small structures and protect them using the maskwidth parameter. If only one image is available, the 'Pressed-out' method [2] can be used to remove the scanning effect.

For 'Basket-Weaving', scans must have been conducted in at least two different directions. Normally, the scanning direction should be specified for each input image. As direction parameters, you should specify the angles perpendicular to the scan directions in units of degrees. The angle is defined in the counterclockwise direction from the x-axis. It removes scan noise by masking wave components perpendicular to the scan direction in the Fourier domain. Scan noise and striation structures appear as band-like artifacts in the Fourier transformed data. The parameter maskwidth controls the width of the mask in Fourier space and is defined as a percentage in the Fourier domain (maskwidth = 10 means masking a region of 10% width in Fourier space). In the perfect case, where scan noise is an ideal striation without structure in the direction perpendicular to scan direction, the scan noise can be removed by specifying a small value for maskwidth (ex. maskwidth = 1). In the actual case, maskwidth should be set to some larger number, since scan noise also has some structure in the direction perpendicular to the scanning direction. Larger maskwidth values may also remove the emission from the scientific target at the same time, so careful adjustment and characterization by the user is necessary.

NOTE: Such masks are applied for each input scan image and the combined image is returned. If the size of the mask is large, the improvements to the noise level by combining images becomes small, since a smaller amount of data is subject to the processing. And note that 'Basket-Weaving' mode supports combining orthogonal scan noise images only (It is possible, but, at least, this function has not been tested enough).

 For 'Pressed-out', the scanning must be in a single direction. There are two ways to specify the size of the smoothing beam used for this process. One is to specify smoothing size directly. To do this, smoothsize should be specified as a string of a numerical value and a unit (e.g. '10.0arcsec'). The value of beamsize will be ignored in this case. The other way is to set smoothing size as a scale factor of the observed beam size. In this case, beamsize is interpreted as the observed beam size, and smoothsize is the scale factor. If the beamsize is provided as a float value, its unit is assumed to 'arcsec'. It is also possible to set the beamsize as a string consisting of the numerical value and the unit. The smoothsize must be a float value.

The infiles must be an image (CASA or FITS), and does not work with MS or Scantable. The direction is an angle with respect to horizontal, in units of degrees. Preferred entries should range from 0.0 to 180.0 degrees. The tmax and the tmin parameters are used to specify a threshold that defines a range of spectral values used for processing. Data values larger than tmax or smaller than tmin will be excluded from the processing. The default (0.0) is to apply no threshold. The outfile specifies an output CASA image name. If outfile is empty, the default name ('sdfixscan.out.im') will be used.

Citation Number 1
Citation Text  Emerson & Grave 1988 (PDF)
Citation Number 2
Citation Text Sofue & Reich 1979 (PDF)