What kind of filter would that be? If it is a wide angle / ultra wide angle lens and your filter is ND then have a look at gel filters. They will be mounted at the rear element. Do check mirror clearance if your camera is DSLR (again: what is your setup?)Trying to put a filter between the camera and lens, but the filter holder will push the camera back, such that the rig cannot reach focus at infinity, so i need a solution to increase backfocus distance.
well then it looks like your options are adding an optical element (negative diopter/diverging lens) or tweaking your lens' internal elements (only if you know what you're doing, and generally only for cheaper lenses, which yours are not).
I'd say about a -2.7 lens would do, but it might change your distance again, so you'd have to recalculate.
BTW, your system is essentially myopic
Why does the filter need to be between lens and body? Can it be mounted in front of the lens?
The T mount is a 42mm screw mount, right? Then you could add stepping rings to gain the few mm from 17+22 up to 44mm. Does it need 44mm for infinity focus or would 39mm already suffice?
Are there any good diopter lenses? I need good quality optics. This rig is for astrophotography, and stars at edge of the image must still be pinpoint, or at least round.