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+.help deredden Feb94 noao.onedspec
+.ih
+NAME
+deredden -- Apply interstellar reddening correction
+.ih
+USAGE
+deredden input output [records] value
+.ih
+PARAMETERS
+.ls input
+List of input spectra to be dereddened.  When using record
+format extensions the root names are specified, otherwise full
+image names are used.
+.le
+.ls output
+List of derreddened spectra.  If no output list is specified then
+the input spectra are modified.  Also the output name may be
+the same as the input name to replace the input spectra by the
+calibrated spectra.  When using record format extensions the
+output names consist of root names to which the appropriate
+record number extension is added.  The record number extension
+will be the same as the input record number extension.
+.le
+.ls records (imred.irs and imred.iids only)
+The set of record number extensions to be applied to each input
+and output root name when using record number extension 
+format.  The syntax consists of comma separated numbers or
+ranges of numbers.  A range consists of two numbers separated
+by a hyphen.  This parameter is not queried when record number
+formats are not used.
+.le
+.ls value
+Extinction parameter value as selected by the type parameter.
+This value may be a visual extinction, A(V), the color excess between
+B and V, E(B-V), or the logarithmic H beta extinction.
+These quantities are discussed further below.
+.le
+.ls R = 3.1
+The ratio of extinction at V, A(V), to color excess between B and V, E(B-V).
+.le
+.ls type = "E(B-V)"
+The type of extinction parameter used.  The values may be:
+.ls A(V)
+The absolute extinction at the V band at 5550 Angstroms.
+.le       
+.ls E(B-V)
+The color excess between the B and V bands.
+.le
+.ls c     
+The logarithmic H beta extinction.
+.le
+.le
+.ls apertures = ""
+List of apertures to be selected from input one dimensional spectra
+to be calibrated.  If no list is specified then all apertures are
+corrected.  The syntax is the same as the record number
+extensions.  This parameter is ignored for N-dimensional spatial
+spectra such as calibrated long slit and Fabry-Perot data.
+.le
+.ls override = no, uncorrect = yes
+If a spectrum has been previously corrected it will contain the header
+parameter DEREDDEN.  If this parameter is present and the override
+parameter is no then a warning will be issued and no further correction
+will be applied.  The override parameter permits overriding this check.  If
+overriding a previous correction the \fIuncorrect\fR parameter determines
+whether the spectra are first uncorrected to the original values before
+applying the new correction.  If \fIuncorrect\fR is yes then the image
+header DEREDDEN parameter will refer to a correction from the original data
+while if it is no then the new correction is differential and the keyword
+will only reflect the last correction.  When correcting individual spectra
+separately in a multispectra image with different extinction parameters the
+uncorrect parameter should be no.
+.le
+.ih
+DESCRIPTION
+The input spectra are corrected for interstellar extinction, or
+reddening, using the empirical selective extinction function of
+Cardelli, Clayton, and Mathis, \fBApJ 345:245\fR, 1989, (CCM).
+The function is defined over the range 0.3-10 inverse microns
+or 100-3333 nanometers.  If the input data extend outside this
+range an error message will be produced.
+
+The extinction function requires two parameters, the absolute extinction at
+5550A, A(V), and the ratio, R(V), of this extinction to the color excess
+between 4350A and 5550A, E(B-V).
+
+One of the input task parameters is R(V).  If it is not known one
+may use the default value of 3.1 typical of the average 
+interstellar extinction.  The second input parameter is chosen by
+the parameter \fItype\fR which may take the values "A(V)", "E(B-V)", or
+"c".  The value of the parameter is specified by the parameter
+\fIvalue\fR.
+
+If A(V) is used then the CCM function can be directly evaluated.  If
+E(B-V) is used then A(V) is derived by:
+
+.nf
+(1)     A(V) = R(V) * E(B-V)
+.fi
+
+For planetary nebula studies the logarithmic extinction at H beta,
+denoted as c, is often determined instead of E(B-V).  If this type
+of input is chosen then A(V) is derived by:
+
+.nf
+(2)     A(V) = R(V) * c * (0.61 + 0.024 * c).
+.fi
+
+This relation is based on the relation betwen E(B-V) and c computed
+by Kaler and Lutz, \fBPASP 97:700\fR, 1985 to include corrections between
+the monochromatic parameter c and the broadband parameter E(B-V).
+In particular the function is a least squares fit to the values of
+c and E(B-V) in Table III of the form:
+
+.nf
+(3)     E(B-V) = c * (A + B * c)
+.fi
+
+The input spectra are specified by a list of root names (when using record
+extension format) or full image names.  They are required to be dispersion
+corrected (DC-FLAG >= 0) and not previously corrected (DEREDDEN absent).
+Spectra not satisfying these requirements are skipped with a warning.  The
+DEREDDEN flag may be overridden with the \fIoverride\fR parameter.  This
+may be done if different extinction parameters are required for different
+spectra in the same multiple spectrum image or if a new correction is
+to be applied.  The \fIuncorrect\fR parameter determines whether the
+previous correction is removed so that the final correction is relative
+to the original data or if the new correction is differential on the
+previous correction.  Note that if applying separate corrections to
+different spectra in a single multispectral image then override should
+be yes and uncorrect should be no.
+
+A subset of apertures to be corrected may be selected from one dimensional
+spectra with the \fIapertures\fR parameter.  Long slit or other higher
+dimensional spatially sampled spectra are treated as a unit.  The output
+calibrated spectra may replace the input spectra if no output spectra list
+is specified or if the output name is the same as the input name.  When
+using record number extensions the output spectra will have the same
+extensions applied to the root names as those used for the input spectra.
+
+Note that by specifying a negative extinction parameter this task may
+be used to add interstellar extinction.
+.ih
+EXAMPLES
+1.  To deredden a spectrum with an extinction of 1.2 magnitudes at V:
+    
+.nf
+	cl> deredden obj1.ms drobj1.ms 1.2 type=A
+.fi
+
+2.  To deredden a spectrum in place with a color excess of 0.65 and
+and R(V) value of 4.5:
+
+.nf
+	cl> deredden obj2.ms obj2.ms R=4.5
+	E(B-V): .65
+.fi
+
+3.  To deredden a series of IRS planetary nebula spectra using the
+H beta extinction in the irs package:
+
+.nf
+	cl> deredden pn12 drpn12 1-5,12-14 type=c
+	c: 1.05
+.fi
+
+4.  To redden a spectrum:
+
+.nf
+	cl> deredden artspec artspec -1.2 type=A
+.fi
+
+5. To deredden a long slit or Fabry-Perot spectrum either DISPAXIS
+must be in the image header or be specified in the package parameters.
+The summing parameters are ignored.
+    
+.nf
+	cl> deredden obj1 drobj1 1.2 type=A
+.fi
+.ih
+REVISIONS
+.ls DEREDDEN V2.10.3
+Extended to operate on two and three dimensional spatial spectra such as
+calibrated long slit and Fabry-Perot data.
+
+An option was added to allow a previous correction to be undone in order
+to keep the DEREDDEN information accurate relative to the original
+data.
+.le
+.ls DEREDDEN V2.10
+This task is new.
+.le
+.ih
+NOTES
+Since there can be only one deredding flag in multispectral images
+one needs to override the flag if different spectra require different
+corrections and then only the last correction will be recorded.
+.ih
+SEE ALSO
+calibrate
+.endhelp