Quote:
Originally Posted by l3gendluk3
This might be a silly question but here it goes. Assuming that everything is perfect (mirror surface, eyepiece lenses, collimation etc), if I have a 25 mm eyepiece in perfect focus and change it to a 9 mm eyepiece, will I have to move the focuser 16 mm to achieve a perfect focus?
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No, not generally. The focal plane of the eyepiece (where the image is formed relative to the eyepiece components) and the eyepiece focal length are two different things.
Some manufacturers try to make their eyepieces parfocal - which in layman's terms means the focal plane falls in the same place for every eyepiece in the range - whereas others have widely varying focal points. It's a different focal plane that requires racking the focuser in/out, not the focal length.
Clear as mud?