TELLURIC, flux calibration, and standard stars

Frank Valdes wrote on Jun 05, 1998

Q:  I have a question about the telluric package (which I didn't know
existed on my last observing run!). It requires a hot, nearly
featureless spectrum as a calibrator-- can I use my standard stars,
which are for the most part hot sdO stars? Or does that introduce a
circularity in the calibration? I'm observing out to 9700A, so would
really love to be able to use this package on my data.


A:  TELLURIC is a new task.  We are trying to advertise it a little such
as in the new IRAF newsletter.

As you say, standard stars are basically the same type of calibrator
you want for telluric line removal.  You can certainly use them for
this purpose.  The problem with combining telluric line removal and
flux calibration is that the division by the telluric calibration will
alter the continuum.  But if you can continuum normalize a standard
star (such as with CONTINUUM or SPLOT), retaining the telluric
features, then you can use it and still flux calibrate the data.  I'm
not sure which order to do flux calibration and telluric line removal.
Probably either order is fine.  Another tip is that you might use SPLOT
with the drawing mode on the continuum normalized standard star to put
the continuum exactly at one with no noise and remove absorption
features so that you create a telluric calibrator that only really
modifies regions the telluric lines.  The TELLURIC task will then
allow you to iteratively scale and tweak the wavelengths to best remove
the lines assuming the Beer's law airmass scaling for the lines (which
is only an approximation).

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Last post on Jun 05, 1998