[floyd_2 (Dean)]

You might want to use a puffer brush before you wipe lenses / filters just to make sure that there are no particles that could scratch the surface. In my case I use puffer brush, then set aside 3 x small tear drop shaped pieces of pure cotton wool. I put a drop of pure isopropyl alcohol on the first cotton wool, and gently clean the lens / filter. I then switch to cotton #2 (dry) and clean the IPA off the lens / filter before it evaporates and leaves anything behind on the surface. I then switch to cotton #3 and breathe a mist onto the lens / filter and give it a final clean. All cotton gets used once and binned.

With your MPCC setup, my assumption would be that the MPCC is the first thing into your focuser. And then you measure 55mm from the rear most lens of the MPCC to the surface of the camera chip as a starting point.

I did this with my refractor and ASI533MC-PRO and it wasn't even close lol. I ended up putting in extensions from that point until it was right. I guess it might need some playing around, but 55mm from the last lens element as a starting point might be ok as Kuz said. Your camera looks to have 17.5mm of back focus including the mounting ring, so that would be deducted from the 55mm. Looking at your photo, you're about right apart from perhaps where the surface of the final element of the MPCC is. If it's not at the top edge of the MPCC, there may be some extra mm of back focus being added into the equation. If you have some extra spacers, it might be worth doing a quick experiment next time you get dark skies.