There are two different mounting systems in photography based on 42mm threads. These are the M42 format from the Pentax era of old, which has many different old lenses out there using this mounting system. The other is the T2 system used in astronomy, which while still using 42mm threads, they differ from the M42 system in terms of the thread pitch i.e. the distance between two screw threads (M42 has 0.5mm pitch threads whereas T2 has 0.75mm pitch). This means that while it is possible to thread accessories/equipment of these two different mounts, they will not thread in fully. I have been using different ring adapters to make such an adapter (M42-52mm, 52mm-T2), but now these are readily available on eBay.
I bought mine for £3.25. There are T2-M42 adapter rings available to, but I do not think I will be needing this which is why I have not bought it currently. But should I need it in the future, I know exactly where to find one.
Now it is entirely possible to adapt securely M42 focusing helicoids to become adjustable T2 mounts, which opens up differentl possibilities for astrophotography – e.g. setting imaging camera and guider camera to be parfocal, adjusting camera sensor to be the exact distance from a coma corrector for it to work properly.
It is amazing that I have been after such an adapter ring for so long, and now it is available on eBay. Just goes to show that it is worth searching for things which never existed, as someone may actually come up with the exact solution sometime down the line. Great!