Last modified: December 2023

AHELP for CIAO 4.16 Sherpa


Context: models


Galactic extinction: the Seaton model from Synphot.




The interstellar extinction is calculated using the formula from [1] as implemented in STSCI's Synphot program [2] . The supported wavelength range is 1000 to 10000 Angstroms, and the Notes section describes the changes from [1] . This model is intended to be used to modify another model (e.g. by multiplying the two together). It is for use when the independent axis is in Angstrom.


>>> create_model_component("seaton", "mdl")
>>> print(mdl)

Create a component of the seaton model and display its default parameters. The output is:

   Param        Type          Value          Min          Max      Units
   -----        ----          -----          ---          ---      -----
   mdl.ebv      thawed          0.5 -3.40282e+38  3.40282e+38           


The attribute for this object is:

Attribute Definition
ebv E(B-V)


The formulae are based on an adopted value of R=3.20.

For wavelengths above 3704 Angstrom, the function interpolates linearly in 1/lambda in table 3 [1] . For wavelengths below this, the formulae from table 2 [1] are used (see also [3] , corrected to R=3.2). The formulae match at the endpoints of their respective intervals. There is a mismatch of 0.009 mag/ebmv at nu=2.7 (lambda=3704 Angstrom). Seaton's tabulated value of 1.44 mag at 1/lambda = 1.1 may be in error; 1.64 seems more consistent with his other values.

For wavelengths below 1000 Angstrom, a constant value equal to the value at 1000 Angstrom is used. For wavelengths above 10000 Angstroms a linear extrapolation (in 1/lambda) is used.

When evaluated on a binned grid, the lower-edges of the bins are used for the calculation.



See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.

See Also

ccm, fm, lmc, sm, smc, xgal