Help setting up Fader to control prizm rotation speed in ML fixture.

I'm a self-taught volunteer. Trying to setup a fader to control rotation speed. The fader I setup acts like a switch 0 to 20 with nothing in between. For ref the fixtures are Clay Paky Mythos 2 & I'm wanting to control the Prizm rotation speed with a fader that controls rotation as a variable speed 0 to 20 an in between.

Looking for help with how to set this up correctly. 

Thanks 

Jeff

Sanford PAC Maine

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