Quote:
Originally Posted by tileys
OK - thanks for clarifying the point about the PRIMARY mirror. So that will shorten the focal length slightly ?
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As I understand it, the focal length doesn't change - that is dependent on the physical characteristics of your primary mirror. But it does move the point at which the rays from the primary mirror, bounced off the secondary and through the focusser, come into focus, to a point further away from the tube. This means that your eyepiece can now be placed further away from the tube to do its magnification job. So an eyepiece that didn't come into focus when your focusser draw tube reached its maximum in-travel, may now come into focus before that position is reached.