My quote from an older post is a little bit out of context but most is relevant to your situation. Plain and simple though is you do not have sufficient focuser travel to use some accessories. This is not a factory fault with your telescope, just a design limitation for the short tube refractor that it is.
Adding a 2” Barlow simply chews up more inward travel. Keeping the 2” Star Diagonal but using a 1.25” Barlow may work but you won’t be certain until you try it on the ED72. If that doesn’t work then using a 1.25” SD with 1.25” Barlow will almost certainly work but again it depends on just how much travel you have to play with. In the latter case the 2”-1.25” adaptor goes in first to the end of the focus tube.
There are a lot of factors that will subtract from the available travel you have. You have thickness of 2”-1.25” adaptor, light travel path length of diagonal, design of Barlow, I.e. long tube, short tube and whether it is parfocal with the eyepiece. A Barlow that is parfocal with the eyepiece simply means that if you focus with a particular eyepiece, then add the Barlow to the sane eyepiece, the focus shift required is zero or very minimal.
You could easily spend a lot of money testing things and a 2”, 2x Powermate could cost as much as your telescope and you still may have the same problem. I wouldn’t go that path, main reason being ergonomics and weight. Powermates work very well on all types of eyepieces unlike Barlow’s which through the exit pupil out too far on the longer focal length eyepieces, but they are not necessary or ideal in all situations.
You have to ask yourself what magnification do you want and which shorter focal length eyepiece will give you that. No need for a Barlow at all then. If you want a Barlow I highly recommend the Tele Vue 2x Barlow (3x also available) as it is parfocal and of high quality. There may be others that also work for you. If this still doesn’t work then you will also have to get a 1.25” star diagonal and one with a short path length just to be sure. Once again the Tele Vue 1.25” mirror diagonal is a good solution but if over budget there will likely be others that work.
See
https://www.televue.com/engine/TV3b_...d=52&Tab=_back
and
https://www.televue.com/engine/TV3b_...d=53&Tab=_back
and
https://www.televue.com/engine/TV3b_...d=62&Tab=_90EB
I find the information on the Tele Vue site invaluable whether you have or use their solutions or not.
There will no doubt be lower cost solutions from other manufacturers but I can only comment on what I have used (except not on ED72, but ED80). Perhaps some others can chime in with worked (not theoretical) solutions.
Take your time and ask lots of questions as buying accessories gets expensive real quick especially if you are just experimenting. Hope this helps.
