gsextract and GMOS longslit: How to extract spectra with flux errors

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This topic contains 1 reply, has 2 voices, and was last updated by  zhoujian 1 month, 2 weeks ago.

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  • #3200

    alejocosmico
    Participant

    I want to obtain GMOS long-slit spectra with flux errors. In gsextract, I set fl_vardq+, weights=variance, clean=yes, and this results in an extracted spectrum with BANDID1=spectrum, BANDID2=raw, and BANDID3=sigma, but the latter is just all zeros.

    I am using gsextract after I already have reduced the raw images, transformed to wavelength, subtracted sky, and calibrated for flux. Throughout all these steps I am keeping fl_vardq+ to propagate DQ and VAR extensions.

    #3417

    zhoujian
    Participant

    I also have this issue before. Yes, based on the manual of gsextract (see below), we might not directly obtain the flux errors by setting fl_vardq+

    > If fl_vardq is enabled and weights is set equal to “variance”, the output variance plane is derived using APALL rather than from the input variance plane. If fl_vardq is enabled and weights is set equal to “none”, fl_vardq will be switched off.

    The solution I have is as follow. After using gsextract to obtain the ecstgs file (the SCI extension is the final spectral flux), then do the following
    > onedspec
    > scopy cstgs+xxx.fits[SCI] raw_flux.fits
    > apall raw_flux.fits out=flux_proxy.fits
    > scopy cstgs+xxx.fits[VAR] raw_variance.fits
    > apall raw_variance.fits out=variance_proxy.fits ref=raw_flux.fits

    The extracted flux is in flux_proxy.fits and error in variance_proxy.fits. I will then use these two to estimate a “S/N spectra” and apply it back to the ecstgs file.

    Hope these help,
    ZJ

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