Astronomical Spectroscopy - Physics - University of Cincinnati

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bumps and wiggles are present in both the program data as in the flat, and will divide
out. If instead some of the bumps and wiggles are due to filters in front of the flatfield
lamp, or the extreme difference in color temperature between the lamps and the
object of interest, or due to wavelength-dependent variations in the reflectivity of the
paint used for the flat-field screen, then one is better off fitting the bumps and wiggles
by using a very high order function, and using the flat only to remove pixel-to-pixel
variations. It usually isn’t obvious which will be better a priori, and really the only
thing to do is to select an object whose spectrum is expected to be smooth (such as a
spectrophotometric standard) and reduce it through the entire process using each kind
of flat. It will be easy to tell in the end. The IRAF task for handing the normalization
of the flat is response, and the flat field division is handled by ccdproc.
An example is shown in Figure 12. The flat-field has a very strong gradient in the
wavelength direction, and a bit of a bend around pixel 825 (about 4020Å). Will one do
better by just normalizing this by a constant value, ascribing the effects to the grating
and spectrograph Or should one normalize the flat with a higher order function
based on the assumption that these features are due to the flat-field system and very
red lamps (Blue is on the right and red is on the left.) The only way to answer the
question is to reduce some data both ways and compare the results, as in Figure 12.
The flat fit by a constant value does a better job removing both the gradient and the
bump. (The spectrum needs to be magnified to see that the bump at 4020Å has been
removed.) Thus the flat was a good reflection of what was going on in the spectrograph,
and is not due to issues with the calibration system.
Once one determines the correct normalized flat, it needs to be divided into all of the
data. The IRAF task for handling this is again ccdproc.
7. Construct and Use Illumination Function Correction. The non-uniformity in
the spatial direction is referred to as the “slit illumination function” and is a combination
of any unevenness in the slit jaws, the vignetting within the focal plane or within
the spectrograph, etc. Most of these issues should have been removed by dividing by
the flat field in step 6. Is this good enough If the remaining non-uniformity (relative
to the night sky) is just a few percent, and has a linear gradient, this is probably fine
for sky subtraction from a point source as one can linearly interpolate from the sky
values on either side of the object. However, if the remaining non-uniformity is peaked
or more complex, or if the goal is to measure the surface brightness of an extended
source, then getting the sky really and truly flat is crucial, and worth some effort.
The cheapest solution (in terms of telescope time) is to use exposures of the bright
twilight sky. With the telescope tracking, one obtains 3-5 unsaturated exposures,
moving the telescope slightly (perpendicular to the slit) between exposures. To use