PPH78 But explain to my self what is T2- T2 adaptater ? If it is 42 M to 42 M it is not present in my boxes ( MC 533 & EFW 2' x5).
The ring that I mentioned is not T2-T2, it is the ZWO M54M-M42F-0.
I.e., the outside of the ring has a M54 male thread. The hole of the ring has a M42 female thread. The link that I gave should show a photo of it.
You first remove the black ring (11mm thick) from your camera. You then screw this M54M-M42F-0 tightly in place of the original black ring.
The result is that the camera now appears to have a M54 male flange. And because it does not use up any extra backfocus, while the original black ring uses up 11mm, the flange focal distance is now reduced by 11mm to 6.5mm.
This "M54 camera" now just screws into the back of the EFW, like the photo that ZWO's post shows.
The EFW should also come with some thin black M54 washers. Those are used as spacers between the EFW and the camera, to changs the angle of the EFW relative to the body of the camera, so that it presents the minimum "3rd axis balance" problem, when the camera and EFW is installed in your OTA. If you use these washers, you also have to factor in their thicknesses when you set backfocus.
The M54M-M42F-0 part should have come in the box with the EFW. If it isn't, it is a cheap common part that a dealer close to you should carry. Check your box again, though. It is a ring that is 54mm in diameter and 4mm thick, with a 42mm hole. The outside is threaded for M54x0.75 and the hole is threaded for M42x0.75.
Instead of the "zero thickness" ZWO M54M-M42F-0 ring, you can also use something like this:
https://www.highpointscientific.com/pegasus-astro-m54-male-to-m42-female-adapter-peg-adapt-m54m-m42f
Notice that this part is also M54M to M42F, but it is 2mm thick, and your sensor is now located 8.5mm from the back of the EFW, instead of being 6.5mm.
But first recheck the box that the EFW came in. It should include the ZWO M54M-M42F-0 part.
Finally, there is something else that a lot of people miss when they choose to buy an EFW. You did not mention what OTA you use. If your OTA ends up with a reducer or a flattener at the camera end (instead of being a pure Petzval design), and if your set of glass filters has different glass thickness, you may also need to change the length of the spacers from the OTA each time you change filters.
If one filter is 3mm thick, and another filter is 1.8mm thick, the difference is 1.2mm, and you will need to change the backfocus spacing by one third of the glass thickness (0.4mm) when you switch between these two filters.
The EFW from ZWO may be electronic and controllable from indoors, it still requires you to go outdoors to change spacers if you want sharp stars from corner to corner when your filters don't all have the same thickness.
The different glass thicknesses will affect a fast f/3.6 OTA more than a slow f/7 OTA. The slower OTA can probably get by without readjusting the backfocus to the order of 0.5mm, but the faster OTA's star images will suffer when the different glass thickness changes the backfocus by even just 0.25mm. You cannot just refocus the OTA -- you need to change the backfocus.
Chen
