Spare Focuser Travel Spare Focuser Travel: The extra amount of travel the focuser can move inward from the point where the image is in focus. This is the distance from the top of the focuser tube (when racked all the way in) to the focal plane (where the light from the primary mirror comes to a focus). Some eyepieces focus farther in than others. Shorter focal lengths usually need to be racked farther in than longer ones. Note: If you will be using the telescope for terrestrial viewing, you will need some extra "out" travel to focus on objects which are closer than the heavenly bodies. You will usually have more spare "out" travel than "in" travel. The amount of "Spare Focuser In Travel" should usually be about 1/2 inch. One way to set up a telescope right on the first try (and to avoid drilling focuser holes and mirror mount holes all over the place) is to design the scope with about 1/2 inch spare travel. Then, using the calculated measurements for component placement, mount the diagonal mirror and the focuser and place an eyepiece in the focuser. Mount the primary mirror in it's mirror cell, and get an assistant to slide the mirror into the calculated position in the tube. Try to focus on a VERY distant object. The assistant should slide the primary mirror in and out until the object comes into focus. Mark this position, remove the optics, and then drill the holes for the primary mirror mount. Using an object that is not far enough away to focus upon will give an improper result. An object at least 1/2 mile distant would be the minimum. Another way to get the proper Spare Travel is to measure where each eyepiece's focal plane lies in relation to it's seating position in the focuser. The field stop in an eyepiece is usually at the eyepiece's focal plane. This focal plane must coincide with the primary mirror's focal plane for an image to be in focus. Measure the distance from the bottom of the eyepiece tube (the part that slides into the focuser) to the field stop. Then measure the distance from the bottom of the eyepiece tube to where the eyepiece stops in the focuser (the top of the part that slides into the focuser). If the field stop sits down in the focuser tube, you will have to rack the focuser out to reach focus. If the field stop sits above the focuser tube, you MUST add some extra "in" travel to move the eyepiece in far enough to focus. The amount to add is the distance of the field stop above the focus tube, plus any safety margin desired.