The apparent position (elevation) of an object is dependent on wavelength, due
to the atmospheric dispersion. When the guiding of the telescope is done for a
given spectral band (e.g., the V-band), light of different wavelengths
may not effectively be introduced to the slit. In Figure the
atmospheric differential refraction between a given wavelength and 5500Å
is shown as a function of the zenith distance at the summit of Mauna Kea
(T = 0 degree celcius and
P = 625 hPa). Users should note that the effect of atmospheric
dispersion is quite significant in the UV region.
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Compensation for the effects of atmospheric differential dispersion is accomplished with the ADC. Though the ADC cuts-off wavelengths shorter than ~360 nm so is not suitable for the UV.
Another method to avoid the effect of atmospheric dispersion is to align the slit image with the zenith direction. By use of this setting, the light from different wavelength regions can be introduced to the slit regardless of the atmospheric differential dispersion. Automatic control of the image rotator to set the slit image to the zenith direction is implemented at present.
Observation with a fixed position angle of the slit image is possible. A FORTRAN program to calculate the position angle of the slit image which is aligned to the zenith direction for a target object as a function of time is available (CALCPA.f or CALCPAwopg.f) on the following Web page. See the Readme file in the same page for details.