This is a very nicely finished piece of metalwork. I am using the Samyang with an ASI 1600 and EFW. The official ZWO EOS adapter was ok but there was significant distortion on one side of all my test images, not helped by the weight of the camera I suspect . Being F2 I imagined this was caused by a very small tilt that would be difficult to resolve. In searching for a practical solution I found this adapter mentioned on the SGL forums.

Its very quick to fit and aligns very well with the lens body. Everything is now nice and rigid. I have the lens in ring clamps and it supports my camera just like a normal OTA does. The tilt has gone, the central stars are all perfectly round. I now have to tweak the final spacing between lens and sensor to improve the stars at the edge. The recommended starting point for spacing is 45mm if you have typical filters or a more accurate rule of thumb spacing is 44 + (filter thickness )*0.3. For ZWO filters that works out to be 44.6mm. With that spacing my edge star shapes are still not perfect and 44.5mm looks to be better.

To get the that spacing I'm using a Baader 0.8mm aluminium M48 spacer, 16.5mm ZWO M48 to M42 adapter, Baader 0.6mm Delrin spacer all purchased from FLO. Combined with 20mm for the EFW, 0.1 for the ZWO dew heater and 6.5 mm for the camera sensor I get to 44.5 mm. ( I like having spacers at each joint, it reduces the chance of parts cold welding together.)

Removing the EOS plate and fitting this adapter takes very little time. If I ever decide to use the lens with my DSLR or to sell it on , refitting the EOS plate takes moments.

Just got it today March 10 2021, I will try it as soon as the weather permits. The adapter seems to be of good quality.

The adapter also works on the Minolta/Sony A Mount version, probably also on all other versions? I was able to realise an OAG setup on the Samyang.

A well-made adapter, easy to fit and excellent low profile. You can also use it to connect directly to an EFW2 ATIK filter wheel via the ZWO adapter https://www.firstlightoptics.com/adapters/zwo-m54-to-m48-adapter-short.html. This adds no back focus and is ideal for the job. If only FLO offered a T2-threaded version...

This is perfect for removing tilt when connecting a Samyang 135mm canon fit lens to my qhy183c