GNIRS reduction – Standard star subtraction and telluric lines

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This topic contains 3 replies, has 3 voices, and was last updated by  Catherine Huitson 3 years, 4 months ago.

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    Dear All,

    I am a beginner when it comes to IRAF data reduction, but I was able to follow the example script for the GNIRS longslit reduction in the Gemini IRAF package and get a my spectra out the other end. However, this script does not go into any detail as to removing the telluric standard star, leaving just the atmospheric lines, so I still have an A0V star imprinted on all of my spectra. So I was wondering which are the preferred models for these types of stars as well as and if anyone has any guidance/tips/scripts for removing modeled A0V stars from my standards so that I can remove atmospheric lines from my data set.

    I have also had a little trouble with controlling the shift of the telluric spectrum, causing some of my spectra to have emission and absorption atmospheric features, rather than canceling them out.

    Any help in these areas would be greatly appreciated.


    Catherine Huitson

    That’s a great question and I would also be interested in hearing answers and thoughts on this. I am an optical observer myself but I do have some infrared spectra that I have not yet reduced.



    hi marshabr89

    At this point you probably have your science and telluric standard in 1D format and wavelength calibrated. In order to remove the telluric features of your science spectrum you have to use the telluric standard spectrum subtracted from the hydrogen lines as well from the continuum.

    There is a couple ways to procede to remove the hydrogen lines. You can either use the vega star spectrum as a template for your star or subtract the lines manually by fitting some profile (Lorentzian for example). If you are using IRAF you can open your spectrum and type “k” and “l” in wings of the lines to fit a Lorentzian profile to the lines. With “-” and “-” at the same point you will subtract it. I tip on this is be careful to do remove the line completely. typing “i” you can save the new spectrum.

    After that you have to subtract the continuum shape of the telluric star (without the H lines). This can be done with the “continuum” task in IRAF. The result will be your template for the telluric absorption for one specfic object. Note that you should have (ideally) one telluric for each object with different airmasses.

    Use this template in the “telluric” task. In the telluric task you can choose the regions where optimize the subtractions. Type “s” and “s” to delimite these regions (type “t” to clean up all the regions). Some times selecte this regions will give you a best result that try to optmize for all spectra. Is up to you choose between a global vs local optmization in the task parameters.

    Hope this answer your question and help you with your data reduction.
    ps.: I don’t know if you are in the cross-dispersed mode, but I strongly recommend to check the Rachel Manson script for GNIRS data reduction. It is posted in the forum and works wonderfully.


    Catherine Huitson

    Thanks for the advice – that’s very helpful!

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