I had a big problem when imaging with my TAL-2M when I inserted the camera into the eye piece holder I could not reach focus at all: the distance between the CCD chip of the camera and the focal plane of my focuser when using the original eye piece holder simply was too much.

To find out where the focal plane of my scope was, I took take a short (about 10 cm) piece of plastic tube and taped one end off with (semi)transparent tape.
I aimed at the moon and inserted the tube into the empty focuser hole.
I looked through the tube and moved it up and down until I reached focus and yes: I saw the moon clearly projected on the tape! I had found the focal plane!
But it was INSIDE my eyepiece holder alas :-[

Many QCUIAG members have solved this problem by disassembling their cameras and remount the electronics in a small box, thereby moving the CCD closer to the telescope.
I was afraid to do so (also because electronics is not my speciality ... )
Others went for an even more drastic solution: they modified their telescope and moved the main mirror upwards, so that the focal plane inside the focuser was also shifted upwards. Brrr.
So I tried an alternative road and took advantage of a TAL specific feature. The TAL-2M has the facility to unscrew the original eye piece holder, which reveals an M42x1 threading, which is meant to be used as a mounting for a - rather dated - type of conventional camera.
The M42x1 thread is found on Pentax, Ricoh, Mamiya, Exa and Practica cameras of years gone by and is used by current production Zenit 35mm cameras.

I replaced the original eye piece holder with a home made experimental short one. When that worked I found a friend-with-a-lathe who made me a short eye piece holder: 31 mm versus the orginal 46 mm height.

This worked fine: now I could focus in Prime Focus and I made a lot of images this way.

I also found out that imaging with my Barlow was only possible with the original eye piece holder. I only discovered that after I had gained experience in prime focus imaging though.

After a while I wanted more (I hope you know the feeling ...): I wanted to go for the DEEP SKY and hunt galaxies, nebulae etc.
Because of their size I needed a Focal Reducer to increase my Field of view.
Another - already familiar - problem arose: I needed ANOTHER 30 mm inward focuser travel.
I again found a friend-with-a-lathe who understood my problem very well (he is an astronomer too) and he made me a Very Short eye piece holder: height 15 mm and I hoped and hoped and hoped that this would be just enough to focus with the Focal Reducer.
On June 24, 2002 I could make the first test: Yippie!!! It focused!!!! I had to put the focuser in absolute bottom position, but it was enough!

Peter Katreniak suggested to lift the main mirror just a little bit and his suggestion worked beautifully: I put some plastic strips of 2 mm thick under my main mirror with the result that I can focus much better, because the focal plane is now a bit outside my focuser and not just at the top. This enables me to 'overshoot' and then go back to tune the focus precisely. The following picture clarifies what I did (I hope).

Of course I wanted to know the Field-Of-View when using my Focal Reducer and also the new f/ratio. You can read how I figured that out here. All I have to do now is to wait for clear dark skies ....

Some technical details about the threading
The TAL scopes all provide the Pentax Universal Screw Mount camera threads which are also referred to as M42x1. The diameter is 42mm with a pitch of 1mm.

Note: when selling my TAL-2M I did not include the low focuser eye piece holders because they also fit on my MTO Maksutov scope