When the observer elongates the light direction of a star the light seems to come from a place which seems in general higher above the horizon. The displacement depends on the height above horizon and the wavelength of the light.
So blue light will be shifted in direction of more height above horizon compared to the red light. This effect gets stronger when the star gets lower heights above horizon..
This effect is responsible for the shifting of the observed planetary disk in the different wavelengths. The planetary disk in the blue light is shifted to higher positions and the disk in the red light is shifted to lower positions compared to the other wavelength. So the observer would see a blue rim in the upper part of the planetary disk and a red rim in the lower part. Since in a Kepler telescope the image is inverted, the observer sees the upper part of the planet with a red rim and the lower part with a blue rim.
The graph on the right shows that this effect depends on the height above horizon or the distance to the zenith. Depending on the distance to the zenith the spread between blue and red starlight grows and exceed for zenith distances above 55° (35° above horizon) the diameter 1arcsec corresponding to the resolution of a 4” refractor.
You can imagine that for planetary observations with a 8” telescope (resolution 0.5”) this effect has a serious impact on the image quality.
Calculations show that the diffraction limit (Strehl > 0.9) can no longer be guaranteed for following conditions:
- objects with 77° height above horizon for a 24” telescope
- objects with 70° height above horizon for a 12” telescope
- objects with 60° height above horizon for a 8” telescope |