Name
fwhm - Full Width at Half Maximum computation
Synopsis
fwhm [options] image.fits
Description
fwhm computes the full width at half maximum around peaks
in a FITS data cube. For each plane in the input cube, it
computes the fwhm in x, y, and an average of both measure-
ments, around a single user-provided peak (see options -x
and -y) or an automatically detected list of peaks (see
options -d and -s).
Algorithm
For each peak in each image of the input cube, the follow-
ing algorithm is applied:
1. Check the validity of the pixel box window and resize
it if needed (and possible).
2. Find the position of the pixel of maximum intensity and
re-center the window on that pixel (resize the window if
needed and possible). Check out that the maximum intensity
still lies within the new box. If it does not, output a
warning and go on computing with the recentered box. The
warning looks like:
*** peak has moved with re-centering, new peak will be
used
*** window won't be shifted again
3. Around that peak, on both the vertical and horizontal
axis, find the nearest couple of points (on each side)
where the closest point is above half max and the other is
below.
4. Use a linear interpolation to find more precisely where
the half max is.
5. Print out the results in arcseconds if the plate scale
is known (through the FITS header or the -p option) or
print out the results in pixels.
Options
-x value -y value
to provide a single position in the image where to
compute the FWHM. The same position will be used
for all planes in the input cube. Default value is
the image center.
-h value or --halfsize value
to provide the half size of the computation box.
fwhm will center this pixel box on each peak, find
out the maximum pixel value in this box, and com-
pute an FWHM around this position. For a data cube
in which the peak is offseted a lot from image to
image, increase this value. Default is 15 pixels
(31x31 pixel box around peak value).
-p value or --plate_scale value
Plate scale in arc-seconds per pixel, for an output
in arc-seconds.
-t value or --threshold value
Often for average Strehl ratios the peak my be
superposed with a gaussian and the program may
incorrectly find the floor of the peak. Use this
option to force the floor to a given (graphically
determined) value.
-d or --detpeak
This option requests fwhm to find out the peaks in
each image by itself. The method which is used to
determine bright objects in an image is the same as
in detpeak.
-s factor or --sigma factor
to be used only with the -d option, this specifies
the sigma level to use for detection of bright
peaks in the image.
Examples
To determine the FWHM of an star that we know to be
located in a window of 21x21 pixels centered on pixel
56,81 of image star.fits with a very unlikely plate scale
of 2" per pixel and a floor you decided to set to 1300
ADU, use:
> fwhm -x 56 -y 81 -h 10 -t 1300 -p 2 star.fits
To determine the FWHM around all peaks in an image, using
the automatic detection, a plate scale of 0.7" and a
default pixel box size (31x31), use:
> fwhm -d -p 0.7 star.fits
Files
Files shall all comply with FITS format
See Also
detpeak, stcube, strehl
Bugs
The ability to provide a list of peaks through an ASCII
file would be much convenient. In a near future...