.help ndprep Mar92 noao.onedspec
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NAME
ndprep -- Make a neutral density filter calibration image
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USAGE
ndprep filter_curve output
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PARAMETERS
.ls filter_curve
Neutral density filter curve.  The directory specified by the parameter
\fIdirectory\fR is prepended to this name so if a directory is specified
then it should not be given here.  If '?' a list of filter curves
in the specified directory is typed.
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.ls output
Output neutral density filter image.
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.ls w0   
Starting wavelength for the output image in Angstroms.
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.ls dw   
Wavelength increment for the output image in Angstroms.
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.ls nw   
Number of wavelength points for the output image (i.e. the size of the
output image).
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.ls nspace = 0
Number of spatial points for a two dimensional image.  If the value is
zero then a one dimensional image is created.
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.ls logarithm = no
Use logarithmic wavelengths and intervals?  If yes then the wavelengths
will have the same starting and ending points and number of pixels but
the wavelength intervals will be logarithmic.
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.ls flux = yes
Conserve flux when rebinning to logarithmic wavelength intervals?
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.ls dispaxis = 1
Dispersion axis for two dimensional images.  Dispersion along the lines
is 1 and dispersion along the columns is 2.
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.ls directory = "onedstds$ctio/"
Directory containing neutral density filter curves.  This directory is
prepended to the specified fiter curve file (and so must end with '/'
or '$').
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DESCRIPTION
A neutral density (ND) filter curve is converted to a calibration image
with the same size and wavelength range as the images to be calibrated.
A list of standard neutral density curves is typed if the filter
curve name is given as '?'.  The ND curves are text files containing
wavelength and filter transmission pairs.  Comments begin with '#'.
A plot of the ND curve can be obtained using \fBgraph\fR.

The ND curve is first interpolated to a one dimensional image of
\fInw\fR wavelength points with starting wavelength \fIwO\fR and
wavelength increment \fIdw\fR using the task \fBsinterp\fR.  The
wavelength parameters must be in the same units as the filter curves
(currently Angstroms) even if the final calibration image is to be in
logarithmic wavelength intervals.  If logarithmic wavelength format
is specified the image is rebinned over the same wavelength range with
the same number of points using the task \fBdispcor\fR.  The rebinning
may include flux conservation to account for the changing size of
pixels or simply interpolate.  Note that flux conservation will
change the apparent shape of the ND curve.

If the number of points across the dispersion, \fInspace\fR is zero then
the final calibration image is one dimensional.  If it is greater than
zero the one dimensional ND image is expanded to the specified number
of spatial points with the dispersion axis specified by the parameter
\fIdispaxis\fR (1 = dispersion along the lines, 2 = dispersion along
the columns).
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EXAMPLES
To get a list of standard ND filter curves:

	cl> ndprep ?

To graph the ND filter curve:

	cl> graph onedstds$ctio/nd1m.100mag.dat

Naturally, if a calibration image is made then the image plotting tasks
such as \fBgraph\fR, \fBimplot\fR, and \fBsplot\fR may also be used.

To make a one dimensional ND calibration spectrum:

.nf
	cl> ndprep w0=4000 dw=1.2 nw=512
	Input ND filter curve:  onedstds$ctio/nd1m.100mag.dat
	Output calibration image: NDimage
.fi

To make a two dimensional ND calibration spectrum in logarithmic wavelength:

.nf
	cl> ndprep w0=4000 dw=1.2 nw=512 nspace=200 log+
	Input ND filter curve:  onedstds$ctio/nd4m.u000mag.dat
	Output calibration image: NDimage
.fi
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REVISIONS
.ls NDPREP V2.10
This task was moved from the \fBproto\fR package.  It was originally
written at CTIO for CTIO data.  It's functionality is largely unchanged
though it has been updated for changes in the \fBonedspec\fR package.
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SEE ALSO
sinterp, dispcor
.endhelp
