In order to do astrophotography prime focusing on a Newtonian telescope, often times a detector/DSLR will need to be moved so far into the focuser assembly that you need to move your mirrors closer together thus moving the focal point out to the imaging plane. This leaves the focusing assembly so close though that now if you want to do visual astronomy or eyepiece projection imaging (afocal astrophotography, like planetary imaging) you won't be able to move the eyepiece far enough away to get into focus.
This model adds an extra ~35mm to the 1.25 inch eyepiece adapter to compensate for adjusting the focal point in prime focusing.
The user re-uploaded this model. The user is not the original author of the model.