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Focus range issue


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I am using a 6" f/8 dob 1.25" focuser with a D90.

When the focuser is most of the way in I get a very nice image in the camera of the secondary mirror. As I move the focuser back out the image gets closer to being in focus but run out of travel before focus happens. It seems that the camera needs to be mounted back a bit farther with an extension tube. Is there a name for that part? Or is the situation different from what I am perceiving? 

Thank you for your help, 


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You can move the primary mirror forward towards the focuser to make certain you can achieve focus within the travel distance of your focuser. I do this by first measuring the travel distance of the focuser and then remove it and on a night with a nice n bright moon or a very distant light source hold the camera up to the hole of the focuser and put it right within the middle of the focuser's travel measurment. I then move the primary mirror forward till i reach focus while using the camera's liveview and mark it's position inside the tube. Then redrill the holes for the primary's mounting screws. You will now have no trouble focusing using a camera at prime focus. You may have to put the mirror back to it's original position to just view using an eyepiece.

Edited by Leveye
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