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Although you probably know this already, an important characteristic of a telescope is its Focal Ratio (e.g. F10, F6.3, etc), where, FR=Focal Length/Aperture.
So, for a 180mm diameter telescope with an FR of F12, we have a focal length of 2160mm, and, for the ubiquitous 200mm F10 SCT, we have a focal length of 2000mm.
For imaging purposes, the above 2 scopes are “slow”. Modern camera lenses have maximum FR’s of say, F1.8 to F4, which means with their diaphragm (iris) wide open, they can “suck in” a lot of light quickly.
Generally, telescopes are slower, having FR’s from say, F7 to F10 or F12, so they require much longer exposures to record sufficient photons to produce a noise free image.
So, an appropriate Reducer/Corrector or a Reducer/Flattener can be fitted to a ‘scope which effectively reduces the FR of the scope as well as corrects any field curvature.
If you fit a Celestron F6.3 Reducer/Corrector to a Celestron C9.25 F10 scope, then the effective focal length changes from the native 2350mm (at F10) to around 1400mm (at F6.3) giving a wider field of view as well as the capability to record images “faster” due to the “faster” FR of F6.3.
Hope that makes sense!
Cheers
Dennis
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