- Form: SMOOTH source [FW=f] [FWC=f] [FWR=f]
[BOXCAR] [RUNMEAN] [WID=]
-
- source
- is the object being smoothed,
- FW=f
- sets the full width of the (Gaussian or boxcar)
smoothing function to be f pixels, and
- FWC= and FWR=
- set the full width for the (Gaussian or boxcar)
smoothing function to have a different width in rows or columns.
- BOXCAR
- convolve with a box function instead of a Gaussian.
- RUNMEAN
- convolve with a box function, but using a
running-mean algorithm that is faster than the BOXCAR option.
The results are identical for odd-sized boxes.
SMOOTH will smooth or convolve the 'source' object with a 2D or 1D Gaussian
or boxcar. This is useful for reducing noise, enhancing low surface
brightness features, or as the first step for looking for sharp features in
an image (by subtracting the smoothed image from the original). Specify the
full-width-half-maximum (FWHM) of the Gaussian (boxcar) in pixels with the
FW keyword. If desired, you can specify differing widths in the column or
row directions with the FWC or FWR keywords. To smooth the image in the
column direction only, (each image row is convolved along its extent
separately) just specify the width with the FWC keyword. To smooth in the
row direction only, (down the columns) just specify the FWR keyword. For
Gaussian, the kernel will extend to WID*FWHM, where WID=3 by default,
although this can be overridden with the WID= keyword.
To convolve with a box, use the 'BOXCAR' word. Better yet, use the
'RUNMEAN' word, which gives identical results much faster for odd-sized
windows, but slightly different results for even-sized smoothing windows.
BOXCAR may be removed in future versions, to be supplanted by RUNMEAN.
The convolutions are done in the image domain and may be slow for large
widths of the filter. Edges are handled properly. At this time, the
FWHMs of the filters are limited to 33 pixel for the Gaussian and 99
pixels for the boxcar.
Examples:
- SMOOTH 2 FW=8.5
- Smoothes image 2 with a Gaussian having
full width 8.5 pixels. The width is
the same in the row and column directions.
- SMOOTH 2 FWR=8.5 FWC=8.0
- Smoothes image 2 with a Gaussian
having full width = 8.5 rows and 8.0 columns.
- SMOOTH 5 FWR=6.2
- Smoothes each column individually with a
Gaussian having full width 6.2 rows.
- SMOOTH 2 FW=5 BOXCAR
- Convolve with a box of size 5 pixels.
Since 1-D spectra are treated the same as 2-D images with only one row, use
the FWC= keyword when smoothing spectra. Remember that the width of the
smoothing kernel (Gaussian or Boxcar) is in units of PIXELS not Angstroms.