|
Basically the 47 3/4" is the focal length of the mirror. This is how far you want your eyepiece from the mirror.
You will want to figure out what diameter tubing you will be using. If you use a tube 9" in diameter then the light has to travel 4.5 inches from the secondary mirror to the edge of the tube. So there mirrors should be about 43 1/4" apart. However you will need to take your focuser into account as well. I would guess there is another 4 inches or so there. So you would want to to reduce the distance to something like 39 1/4". Measure up your focuser assembly and decide in what tube size you are using and do the maths.
Just remember that you will want the focus point pretty close to the focuser with the draw tube fully retracted so you can use a range of eyepieces, say 4mm through 40mm.
L8r
TT
|