Astronomical Spectroscopy - Physics - University of Cincinnati

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the flat-field division. Alternately, telluric standards can be obtained at the same slit
positions as science targets. Making a ratio of the object to telluric standard will also
correct for vignetting along the slit.
4. Make the bad pixel mask. A good bad pixel mask can be constructed from a set
of dark and lamps-on flat field images. A histogram from a dark image reveals high
values from “hot” pixels. Decide where the cut-off is. Copy the dark image, call it
‘hot’, set all pixels below this level to zero, then set all pixels above zero to 1. Display
a histogram of your flat and decide what low values are unacceptable. Copy the flat
image calling it dead and set all values below this low value to 1. Set everything else
to 0. Finally, take many identically observed flat (on) exposures, average them and
determine a sigma map (in IRAF, this is done using imcomb and entering an image
name for “sigma”). Display a histogram, select your upper limit for acceptable sigma,
and set everything below that to zero in the sigma map. Then set everything above
that limit to 1. Now average (no rejection) your three images: dead, hot and sigma.
All values above 0.25 get set to 1.0 and voila!, one has a good bad pixel mask. One
should then examine it to see if one was too harsh or too lax with your acceptable
limits.
5. Trace and extract spectrum. This step is nearly identical to what is done for the
optical: one has to identify the location of the stellar spectrum on the array and map
out its location as a function of position. Although the sky has already been subtracted
to first order (Step 1), one might want to do sky subtraction again during this stage for
a couple of reasons, namely if one needs to remove astrophysical background (nebular
emission or background stellar light), or if the previous sky subtraction left strong
residuals due to temporal changes, particularly in the sky lines. Be sure not to use
optimal extraction, as the previous sky subtraction has altered the noise characteristics.
If you do subtract sky at this stage, make sure it is the median of many values, else
one will add noise. (It may be worth reducing a sample spectrum with and without
sky subtraction turned on in the extraction process to see which is better.) Bad pixels
can be flagged at this stage using the bad pixel mask constructed in Step 4 and will
disappear (one hopes!) when all of the many extracted frames are averaged below. In
IRAF the relevant task is apall.
6. Determine the wavelength scale. One has a few choices for what to use for the
wavelength calibration, and the right choice depends upon the data and goals. One
can use the night-sky emission spectrum as a wavelength reference, and in fact at high
dispersion at some grating settings these may be the only choice. (One needs to be at
high dispersion though to do so as many of the OH emission lines are hyperfine doubles;