Without ADC Loss of Performance already at 30° Zenith Distance or (60° above Horizon) ! | |||||||||||||||||
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The atmospheric dispersion shifts the images of the different colors. This results not only in a chromatic bleeding but also in a worse point spread function reducing the strehl value. This loss of contrast is about a factor 3 for objects with 30° above horizon (60° zenith distance) observed with a 20cm refractor ! The contrast improvement by using an ADC system is also given for observing objects with small zenith distances. The diffraction limited performance is defined by Huygens with a Strehl value of 0.8. But experience shows that a telescope with a Strehl value > 0.9 has well distinguished performance advantages. |
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