Takahashi F8 Visual EXT Tube

F/8 Visual extension tube used to take up back focus for straight through viewing. . Just insert the Takahashi F/8 Visual Extension Tube into your f/8 telescope's focuser, use your focuser's set screws to hold it in place. Place the eyepiece or accessory into the extender and use its set screws to tighten it in place. If you prefer, you can also use a 1.25" adapter and "extend" the Takahashi F/8 Visual Extension Tube applications with your equipment.

M43x0.75 external thread on the telescope side 
M43x0.75 internal thread on the accessory side, light path 34mm. 
Can also be used in conjunction with the TCA-4 eyepiece projection adapter