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deredden noao.onedspec



deredden -- Apply interstellar reddening correction


deredden input output [records] value


List of input spectra to be dereddened. When using record format extensions the root names are specified, otherwise full image names are used.
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.
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.
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.
R = 3.1
The ratio of extinction at V, A(V), to color excess between B and V, E(B-V).
type = "E(B-V)"
The type of extinction parameter used. The values may be:
The absolute extinction at the V band at 5550 Angstroms.
The color excess between the B and V bands.
The logarithmic H beta extinction.
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.
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 uncorrect parameter determines whether the spectra are first uncorrected to the original values before applying the new correction. If uncorrect 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.


The input spectra are corrected for interstellar extinction, or reddening, using the empirical selective extinction function of Cardelli, Clayton, and Mathis, ApJ 345:245 , 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 type which may take the values "A(V)", "E(B-V)", or "c". The value of the parameter is specified by the parameter value .

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:

(1)     A(V) = R(V) * E(B-V)

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:

(2)     A(V) = R(V) * c * (0.61 + 0.024 * c).

This relation is based on the relation betwen E(B-V) and c computed by Kaler and Lutz, PASP 97:700 , 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:

(3)     E(B-V) = c * (A + B * c)

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 override 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 uncorrect 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 apertures 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.


1. To deredden a spectrum with an extinction of 1.2 magnitudes at V:

	cl> deredden 1.2 type=A

2. To deredden a spectrum in place with a color excess of 0.65 and and R(V) value of 4.5:

	cl> deredden R=4.5
	E(B-V): .65

3. To deredden a series of IRS planetary nebula spectra using the H beta extinction in the irs package:

	cl> deredden pn12 drpn12 1-5,12-14 type=c
	c: 1.05

4. To redden a spectrum:

	cl> deredden artspec artspec -1.2 type=A

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.

	cl> deredden obj1 drobj1 1.2 type=A


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.

This task is new.


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.



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