2000
Dec 29: Feb 2001 S&T contains a Letter from Alan Holmes
of SBIG commenting on
1] the colour fidelity of CMY colour reconstruction to RGB and
2] the magenta filter passing both red/ blue light making it effectively impossible to match images due to atmospheric refraction i.e. the blue image rises to a slightly higher altitude in the sky than the red image.
These slitless spectra of Venus, via the Starlight Xpress MX5c camera using CMY filter technology, specially taken in a 3 hour session through daylight and dusk to check validity of this claim - the planet's altitude reducing from 23o to 13o. Whilst clearly atmospheric refraction [acting as a cross disperser] bends the blue end of the spectrum progressively upwards there is no mismatch of the red/blue images as claimed. This effect should be pronounced for the colourless grey IR part of the spectrum as each filter passes IR unimpeded. The blue end of the spectrum is progressively absorbed whilst setting - cutting off shorter wavelengths.
The images were processed to show the full spectrum from blue through to the colourless grey IR end. The real [zero order] images of Venus to the left are therefore overexposed with an orange cast due to the transmission characteristic of the grating.
2001 Jan 7: This sequence included to show stellar [point] sources with Fomalhaut at very low altitude and the blue end of its spectrum bent upwards through atmospheric refraction - remaining stars over 45o altitude don't show this effect. Fomalhaut's prime image [zero order spectrum] is also blurred due atmospheric turbulence and the sequence of exposures show slight evidence of twinkling in colour through scintillation.
These spectra of Saturn and Jupiter included the planetary images sharpened.
Effects of atmospheric refraction are negligible.
