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Originally Posted by ausastronomer
The sight tube really needs to be matched reasonably closely to the F-Ratio of the scope. It doesn't have to be exact ie An F5 sight tube on an F6 scope will be fine, an F7 sight tube on an F4.5 scope won't help much. My combination cheshire/sight tube is optimised for an F5 scope and the sight tube is about 5 inches long. The F-Ratio that Andrews device is optimised for is anyone's guess with the thing only being 3" long.
CS-John B
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NB: I edited some unnecessary parts out.
This came from
this thread. Hence you would need a Cheshire around 5" long, or there abouts.