The Earth's atmosphere acts like a prism, refracting the light
which passes through it and thus causing the apparent position of
objects to deviate from their actual position. The amount of atmospheric
refraction depends on two factors:
Essentially, the atmosphere changes the light of a point source
into a spectrum which is aligned with the elevation axis. This
dispersion is not a significant problem when observing only a
narrow wavelength range, but when imaging with broadband filters
or acquiring spectra over a wide range, the effect of
differential atmospheric refraction -- the difference
between the apparent position of the bluest and reddest light
being observed -- can be significant. Images will suffer from
poor image quality due to the extended apparent shape of the
target, and spectra may lose a substantial amount of light
(varying with wavelength) if care is not taken to align the slit
with the elevation axis and thus capture all of the light.
- Elevation: the dispersion is zero at the zenith
but increases quickly at elevations close to the horizon.
- Wavelength: dispersion is small in the red but
pronounced in the blue and ultraviolet.
Figure 1: Dispersion at high and low elevations.
These two figures show the effects of differential atmospheric
refraction on spectroscopy. Both figures show a point source
observed at a slit angle which is not aligned with the dispersion
axis. In the left image (high elevation) the dispersion is
minimized and most of the light goes down the slit. In the right
image (low elevation) the atmosphere induces much more dispersion
in the light much of the light no longer enter the slit, leading
to loss of signal as a function of wavelength.
The Cassegrain Atmospheric Dispersion Compensator module (Cass
ADC) was designed to reduce the effects of differential
atopsheric refraction in both the imaging and spectroscopy modes
of the Low Resolution Imaging Spectrograph
(LRIS) on the Keck I telescope. For a small penalty in
throughput, the dual-prism module improves delivered image
quality over a wide range of elevations and wavelengths.
See the ADC Characteristics
for details regarding how well the ADC corrects for