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Extension Tube?


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Had a go at imaging Jupiter last night.  I decided to try using my Televue 2.5x powermate with my ZWOASI120, filterwheel and so on..

The problem that I had was that I could not get my C8 Edge HD to focus.  The image seemed to be starting to come into focus as I turned the focus knob clock-wise.  Then I ran into the focus stop.  

I tried various combinations of tubes, and bits, but could make no progress last night.  This morning (when I could see what I was doing) I played around a bit.  To achieve focus (and then only just), I had to have the following combination:  visual back; Televue (the entire unit); filter wheel; and then a bit of my Celestron T-Adapter; and finally the ZWO.  Even then I could not seat the ZWO fully home if I wanted focus.  All told, the above Heath-Robinson affair extended out maybe a foot or so from the back of the telescope.

I assume that I need some sort of extension tube.  But before buying one, I wanted to check that there was not something obvious that I was missing.  Also, are all these tubes pretty much of a muchness, or is there a specific make/model I should be considering?  

Thanks, as ever, in anticipation of your assistance.  

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