Astronomical Spectroscopy - Physics - University of Cincinnati

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night. More about this is discussed below in § 3.3.3.
3.2. Further Details
The description of the reduction techniques above were intended as a short introduction
to the subject. There are some further issues that involve both observation and reduction
techniques that are worth discussing in some additional depth here; in addition, there are
some often-neglected topics that may provide the spectroscopist with some useful tools.
3.2.1. Differential Refraction
Differential refraction has two meanings to the spectroscopist, both of them important.
First, there is the issue of refraction as a function of wavelength. Light passing through
the atmosphere is refracted, so that an object will appear to be higher in the sky than it
really is. The amount of refraction is a function of the wavelength and zenith distance, with
blue light being refracted the most, and the effect being the greatest at low elevation. If
one looks at a star near the horizon with sufficient magnification one will notice that the
atmosphere itself has dispersed the starlight, with the red end of the spectrum nearest the
horizon, and the blue part of the spectrum further from the horizon. The second meaning
has to do with the fact that the amount of refraction (at any wavelength) depends upon the
object’s zenith distance, and hence the amount of refraction will differ across the field in the
direction towards or away from the zenith.
This wavelength dependence of refraction has important implications for the spectroscopist.
If one is observing a star at low elevation with the slit oriented parallel to the
horizon, the blue part of the star’s light will be above the slit, and the red part of the star’s
light below the slit if one has centered on the slit visually. Thus much of the light is lost.
Were one to instead rotate the slit so it was oriented perpendicular to the horizon then all
of the star’s light would enter the slit, albeit it at slightly different spatial locations along
the slit. So, the spectrum would appear to be tilted on the detector, but the light would not
have been selectively removed.
The position angle of the slit on the sky is called the parallactic angle, and so it is good
practice to set the slit to this orientation if one wishes to observe very far from the zenith.
How much does it matter Filippenko (1982) computed the amount of refraction expected
relative to 5000Å for a variety of airmasses making realistic assumptions. Even at a modest
airmass of 1.5, the image at 4000Å is displaced upwards (away from the horizon) by 0.71