IRAF Newsletter -- Number 14 -- April 1998

[ Previous ] [ Next ] [ Table of Contents ] [ Search this issue ]


Update on the NOAO Spectroscopy Packages

The changes in the NOAO spectroscopy packages for IRAF V2.11 are largely evolutionary. Though there are no new packages or major new tasks there are some useful new small tasks and features which are highlighted in this article.

The three new spectroscopy tasks are autoidentify , skytweak , and telluric . The latter two are described in a separate article. The task autoidentify is the result of algorithm development for automatically identifying lines in dispersion calibration spectra. The goal is to simply give the algorithm a calibration spectrum, hopefully with some approximate information about the wavelength coverage, and have it recognize the line identifications from a list of possible lines. The automatic line identification works very well in many cases but also fails in some cases where there is no information about the wavelength coverage or the dispersion relation is significantly non-linear. Additional work for future releases is being done to address the problems of non-linear dispersion functions.

The autoidentify task is a variant of identify which attempts to find a dispersion solution in the input spectrum before entering the usual interactive line identification phase. This routine is used in the various integrated image reduction tasks such as doslit and dofibers . These tasks also have new parameters for the automatic line identification algorithm such as guesses for the central wavelength and dispersion. The automatic line identification algorithm is also available as a new keystroke, `b', in the standard interactive identify.

The dispersion units for spectra are set during the dispersion calibration process using identify / reidentify . Improvements were made to allow one to specify the dispersion units as task parameters or in a comment in the line list. Because line lists now have clearly defined units this also means one can use a line list in one set of units but do the calibrations in another set of units. The unit information is then carried along in the identify databases and in the image headers and used appropriately by all the spectroscopy tasks including the radial velocity tasks in the RV package. The end result is that you can now use the spectroscopy packages to work with data that is naturally or desired in nanometers, wavenumbers, or other units.

The other significant new feature in the one dimensional spectroscopy area is the addition of Lorentzian and Voigt profiles to the existing Gaussian profile fitting. These new profile fitting functions are found in splot and fitprofs . [Note that synthetic Lorentzian and Voigt profiles now may also be created by artdata.mk1dspec .]

The aperture spectrum extraction package, apextract , also has just a few improvements. The new version allows selecting apertures for various operations such as extraction. In other words, one may select a subset of the apertures from an aperture database to resize, recenter, or extract. The application that suggested this change was to allow sets of apertures to be extracted with different parameters.

The output extraction formats were improved for the case where each defined aperture is subdivided into a number of equal width subapertures using the "nsubaps" parameter. If the output format is "echelle" then the first subaperture from each order is written to one output Echelle file, the second to another, and so forth. The multiple output Echelle files can then be handled easily with the Echelle dispersion calibration tasks ecidentify / ecreidentify . After dispersion calibration the subapertures can then be summed back together. This technique is useful when lines of constant wavelength in each order are curved or not aligned with the image lines or columns. An output format of "onedspec" will write each subaperture to a separate spectrum file.

The last change to note in the apextract package is that the aperture identification information can now be specified within the image header in addition to using a separate text file. In the future multifiber or multiobject spectrographs will be able to pass this information on to the reduction software entirely within the image. For instance, the WIYN/Hydra raw data contains the information for dohydra to use without needing the ".iraf" file.

Frank Valdes

[ Previous ] [Next ] [ Table of Contents ] [ Search this issue ]

IRAF Group, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726, Phone: (520) 318-8160, FAX: (520) 318-8360, Email iraf@noao.edu

Posted: 07May1998