Yet another OVL flattener back focus question

[IDM]

I know there have been several threads on back focus but I am completely stumped. I am trying to get an OVL flattener to work with my QHY 183C camera. I think (please correct if I am wrong) the back-focus should be 55mm (though possibly 56mm). The sensor in the camera is 17.5mm recessed so the spacer needs to be 55-17.5= 37.5mm. 

The flattener has a 55mm female thread to 48mm male thread adaptor on the back. This adaptor is about 8mm thick.  So does the 37.5mm include the 8mm, therefore the spacer would be 37.5-8= 29.5mm or should the spacer be the full 37.5mm? I have searched everywhere for a manual for the flattener but I cannot find one. I hope someone can explain which spacer is needed.

Thanks,

Ian

[Clarkey]

The spacing is normally from the back of the flattener excluding the actual thread - so you will need about 37.5mm of spacers.

[IDM]

21 minutes ago, Clarkey said:

The spacing is normally from the back of the flattener excluding the actual thread - so you will need about 37.5mm of spacers.

So, the 55mm does not include the adaptor. Effectively the 55 to 48mm adapter are part of the flattener and everything gets measured after that. Is that correct?

thanks

Ian

[Clarkey]

Normally yes. I have the Stella Mira version with the 48 and 42mm reducers and the measurement is from the back of that.