Drawtube at nax position. Need more space for ZWO EAF to do a focus routine

[Expanding-universe]

Hi All:

 

my scope achieves focus when the drawtube is all the way in. Unfortunately I need more space for the ZWO EAF to do a focus routine as it needs to go past the point of focus to create the V curve. I do have the required 55mm of back focus consisting of the following:

zwo cooled camera, 11mm spacer, ZWO filter drawer, ZWO OAG and the William optics 6Aii flattner set at 7.9mm. My scope is a GT81 from William Optics. 
 

my questions are this: do I need to add or subtract some space between the camera and the flatner? If yes, how much and where in the image train should I add or subtract?
 

many thanks, in advance.

[Vroobel]

It looks like your optical train is too long. Try to replace something like the filter drawer or the OAG, if you have any guiding scope, with something a few mm shorter, e.g. 15mm M48 spacer. 

From my experience: be careful with cheap spacers from popular shopping platforms, they may stuck and you will have a big problem to unscrew them. 

Edit. 

My advice probably will affect your back focus. 

 

Edited Monday at 14:32 by Vroobel