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SKY-90 optical features

Optical design and compared chromatic correction

The SKY-90, in spite of its short focal distance, is adapted to all the fields allowed by its aperture, thanks to its very wide chromatic correction and thanks to some subtle compromises : a light under-correction of the spherical aberration at prime focus, the Extender-Q/corrector 1.6x optional addition and an objective assembled on adjustable barrel. For more details, see Optical section.

Compared chromatic correction between SKY-90 and FS-90 simulation

The chromatic correction curves show that the SKY-90 could not be built according to the old FS optical design. Indeed, the FS refractor loses its performances notably as soon as focal ratio goes below 8. Takahashi thus uses a less current optical design but much better corrected for imaging and planetary observation, in order to offer a transportable and general-purpose apochromatic telescope, an optics offering at the same time wide field and high-definition.

Spot diagrams

We clearly note on the spots diagrams the utility of the specific F/4.5 flattener/reducer for deep-sky prime focus imaging. Even if a KAF-400 CCD sensor can still be used without this flattener, it will not work any more when using a larger sensor or film. However, for visual use, this correction is not essential.