William Optics Rotator

[900SL]

Hi 

I've added a few bits to my rig, in order:

WO rotator

6Aiii FFR

Neumann CTU

ZWO filter drawer.

I've set the backfocus as per WO recommended 64mm using the adjustable FF. 

Focus seems good, I'm racked out about 15mm, will check backfocus if I ever get clear skies 😂☁️

My question is relatively straightforward.. does the Rotator nominally increase the focal length of the scope ( it's around 20mm thickness)  given its location in front of the FFR? 

 

[Richard_]

Technically, it won't affect the focal length or where the image plane is, but it will affect the position of your focuser draw tube to achieve focus. 

Introduction of the rotator will effectively push the flattener and camera 20mm further away from the image plane, which means you need to reduce the length of the focus draw tube by 20mm to bring everything closer to the image plane. 

I couldn't find a similar schematic for your scope, but I have one for mine (WO FLT120). In this picture, you can see the focuser is set to 43.83mm to achieve focus with a specific flattener and 55mm back focus. If I inserted something between telescope and focuser with a thickness of 20mm, then I need to set the focuser to 23.83mm to achieve focus. 

You mentioned that you achieve focus with 15mm on the draw tube, was that before or after you introduced the rotator?

[900SL]

After, from memory it was around 30mm beforehand. That stacks up with your explanation. Many thanks! 

[Richard_]

Perfect! Sounds like you have enough space left over to get focus 🙂 Hope it works out for you.