Astronomical Spectroscopy - Physics - University of Cincinnati

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• The data frames themselves doubtless contain an overscan strip along the side, or
possibly along the top. As a CCD is read out, a “bias” is added to the data to assure
that no values go below zero. Typically this bias is several hundred or even several
thousand electrons (e − ) 13 . The exact value is likely to change slightly from exposure to
exposure, due to slight temperature variations of the CCD. The overscan value allows
one to remove this offset. In some cases the overscan should be used to remove a one
dimensional bias structure, as demonstrated below.
• Bias frames allow one to remove any residual bias structure. Most modern CCDs,
used with modern controllers, have very little (if any) bias structure, i.e., the bias
levels are spatially uniform across the chip. So, it’s not clear that one needs to use
bias frames. If one takes enough bias frames (9 or more) and averages them correctly,
one probably does very little damage to the data by using them. Still, in cases where
read-noise dominants your program spectrum, subtracting a bias could increase the
noise of the final spectrum.
• Bad pixel mask data allows one to interpolate over bad columns and other non-linear
pixels. These can be identified by comparing the average of a series of exposures of
high counts with the average of a series of exposures of low counts.
• Dark frames are exposures of comparable length to the program objects but obtained
with the shutter closed. In the olden days, some CCDs generated significant dark
current due to glowing amplifiers and the like. Dark frames obtained with modern
CCDs do little more than reveal light leaks if taken during daytime. Still, it is generally
harmless to obtain them.
• Featureless flats (usually “dome flats”) allow one to correct the pixel-to-pixel variations
within the detector. There have to be sufficient counts to not significantly degrade
the signal-to-noise ratio of the final spectrum. This will be discussed in more detail in
§ 3.2.6.
• Twilight flats are useful for removing any residual spatial illumination mismatch
between the featureless flat and the object exposure. This will be discussed in more
detail in § 3.2.6.
• Comparison arcs are needed to apply a wavelength scale to the data. These are
usually short exposures of a combination of discharge tubes containing helium, neon,
13 We use “counts” to mean what gets recorded in the image; sometimes these are also known as analog-todigital
units (ADUs). We use “electrons” (e − ) to mean the things that behave like Poisson statistics, with
the noise going as the square root of the number of electrons. The gain g is the number of e − per count.