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RE: Focal Length

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Posted by Kip Crawford on October 4, 2000 00:05:26 UTC

When using prime focus, you must remember that the focus plane (point of focus) is a little lower without an eyepiece. Your cameras focus plane is deep into the camera and you must crank in the focuser to achieve focus at that point. This is common with fast focus scopes. Refractors are a straight shot and achieving focus is much simpler. On a Newtonian, the focus is "turned" on a diagonal and going up through the tube where the focus plane is. If you cannot achieve prime focus then there are a couple of ways to fix that. 1. You can move the mirror up an inch to raise the focus plane an inch or, 2. Move the secondary and focuser back 1" to get the same results. Number 1 option would be perferable and less of a hassle. Eyepeice projection will work a little better because the focal lengths of the EP`s. On newts with a low-profile focuser, getting prime focus can be difficult. I myself had to move my mirror(8"f/6 dobs) 1.25" up for me to achieve prime focus. I am using a helical focuser (low-profile). Refractors usually have enough travel to do this, only because of the straight focal. Sounds to me you`ll have to do some modifacations. Good luck!!

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