afox I've always assumed from the specification diagrams that the ASI294MC Pro sensor recess depth is mechanically 6.5mm and have been adding 1mm to allow for the AR window and sensor cover slip when calculating the optical length for my focal reducer spacing.
They have always claimed the flange focal distance to be the optical distance.
However, if you dig around, I have also seen ZWO specified the tolerance of distance to be +/- 0.5 mm for various of their cameras.
Moreover, ZWO's D21 AR window is only about 1 mm thick, so it would at most alter the optical path from the mechanical path by 0.33 mm, even less than the flange focus tolerance of +/- 0.5 mm.
All in all, you will need to measure it yourself using the flatness of your image as the gauge. With a reducer, Tak's backfocus is typically 56.2 mm (sometimes rounded to 56mm). No, they don't use the 55 mm ad-hoc standard that was started by Celestron eons ago.
See here if you are not familiar with how to tune the backfocus spacing (scroll down to "How to tell if your backfocus is off")
https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus
If you are using M42 interface, the Baader VariLock is a good way to finely measure backfocal distances (although your image rotates as you adjust).
If you are using M54 interfaces, you have fewer options (than using spacing washers and fumbling in the dark).
I have started collecting O-rings to experiment with using the new version of the "ZWO M54 Tilt Plate" as a backfocus adjuster (i.e., move all tilt adjustment screws by the same amount, and add o-rings to prevent light leak).
ZWO also has one for M42, if you want to experiment with it -- just don't make the mistake and buy the older M42 tilter -- that one leaks light like a sieve and belongs to the city dump. The new ones came out recently (unknown yet how much light it leaks when used as a tilter).
https://cloudbreakoptics.com/products/m54-sensor-tilt-plate
https://cloudbreakoptics.com/products/m42-sensor-tilt-plate
Be sure to find ways to block light when you use these as backfocus adjustments -- that is why I am waiting for some O-rings before trying the idea out -- the O-ring in the tilt plate is too skinny to allow larger backfocus adjustment. The adjustment screws are fine threaded M3 (0.5mm pitch), so one full rotation of the adjustment screws only changes the backfocus by 0.5 mm. If I can prevent light leak with the o-rings, this is good enough to adjust backfocus with.
There is enough thread to get at least +/- 1 mm backfocus adjustment from the ZWO plates.
Once measured, you can then get the proper spacer from PreciseParts.com if you don't find the three M3 screws to be secure enough. The advantage of using it as a permanent backfocus spacer is that you can still use it to tilt by applying different tilt screws distances.
Chen
