Wavelength flexure correction

If (as in this example), you’re using a baseline arc lamp observation, taken in twilight or daytime, it’s advisable to correct for any zero-point error(s) in your wavelength solution(s) using sky lines. There is often at least one strong line that can be used for this purpose, except at the bluest settings. [1]

  • One convenient way to measure a zero-point correction at usable S/N is to re-use the sky spectrum that gfskysub / gfreduce averages over the background IFU field and subtracts from your data, which is recorded in its own FITS extension of the output file. When using more than one sky line, wavelength offsets can be measured with respect to a line list using the IRAF task rvidlines:

    rvidlines steqbpxprgS20140919S0059.fits[sky]
    rvidlines steqbpxprgS20140919S0062.fits[sky]

    In the task’s plot window, press l to try to match the line list automatically and then q to exit (or ? for help).

    The output measurements are recorded in F110_rvid.log, from which the -Zobs value can be multiplied by the approximate wavelength of 6300A to obtain corrections to the values of the WCS keyword CRVAL1. In this case, the sub-Angstrom corrections are only a few times the measurement errors, but nevertheless improve the accuracy. One could also measure just the 6300A line (eg. with the p p option in implot).

    If you have a usable sky line but cannot identify its wavelength reliably, you can still correct any differences in its wavelength between exposures, permitting accurate registration when eventually mosaicking the datacubes.

  • Add your measured zero-point corrections (with the opposite sign from Zobs) to the world co-ordinate system coefficients in the image headers:

    hedit steqbpxprgS20140919S0059.fits[sci] CRVAL1 5600.058 update+ verify-
    hedit steqbpxprgS20140919S0062.fits[sci] CRVAL1 5598.171 update+ verify-
  • You can overplot spectra from both images using implot with its wcs=world option, to verify their alignment (eg. type l 300 340 to plot an average over a sky block, :o to overlay the next plot, :i filename[sci] to load the next image and l 300 340 again).

When in doubt, it’s a good idea to take arc exposures on the sky with your science data and flats, especially at blue wavelengths (despite adding several minutes to each observing cycle), allowing you to skip this section.

[1]At red settings on faint targets, it’s sometimes preferable to use sky lines directly for wavelength calibration, instead of the arc, but we won’t go into that here.