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Just for clarification: The FS-60 CB becomes an FS-60 Q once the Q1.7 adapter is attached (see the last picture). Takahashi also sells the FS-60 Q as a unit with the Q1.7 adapter already screwed in. Of course, the Q.17 extender can be removed and then it becomes an FS-60 CB. In the FS-60 CB configuration, it is a doublet-lens system and in the FS-60 Q configuration, it is a quadruplet-lens system. The Q1.7 acts as an 1.7x extender that also flattens the field. But not only that, it further improves the correction of the system and thus, is good for both, visual observation and photography (at F10).
The Sky-90 reducer/flattener reduces the F ratio from the native F5.9 to F4.7 and flattens the field. Together with the Q1.7 extender that gives two options for photography F10 (600mm) or F4.7 (280mm). AEC has a flattener F5.9 = 355mm) and another reducer (F4.2 = 255mm) available giving 2 more photographic options. Very few telescopes can provide that many photographic options and are also excellent for visual observation.
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