Backspace 26c camera

[Feock]

I am a little unclear on where the 17.5 mm backspace on the cooled cameras starts. I have a filter draw screwed straight to the threads on the camera. If I use my micrometer and measure from the bottom of these threads down the side of the camera body that would suggest the sensor is deep inside the housing near the desiccant tube. Or does the 17.5 mm go from the top of the m42:m42 ring that is commonly screwed to the camera flange?

ZWO cameras calculate their 17.5 mm from the top of the m42:m42 extension tube while I don't seem to have a similar ring on my Altair camera. The ring on top of the ZWO is 11mm thick, if there is a ring on the Altair how thick is it?

 

Thanks

Ross