I've just taken delivery of the 72ED and OVL FF which @Rustang listed in the classifieds last week. I want to use it with my Altair 183c Pro Tec, which has 17.5mm front flange to sensor, and an Altair filter holder which is 17mm thick with M48 front & T2 back threads. I was planning on adding a 15.5mm M48 spacer, to give the desired 55mm back focus. I've discovered the FF has a M/T?42 thread at the back , luckily I have a suitable adapter which is only a couple of mm deep, so I'll be able to connect my 15.5mm tube (indirectly) to the back of the FF.
Plan will be FF > M/T42 > M48 15.5mm > M48 Filter Holder T42 > Hypercam
Is the 55 mm distance critical? I've seen a few discussions that ~60mm might give better results. With having to convert the rear side of the FF back to M48, I'm gaining the thickness of the conversion ring, I do have the M48 variable ring I could use instead of the 15.5, it's got a minimum distance of 13.85mm, so would give me 1.65mm back.
Thanks
John