As long as you don't go over 30s you can get some pretty reasonable shots with a short focal length.
I do agree with Suavi, it is MUCH better to use with narrowband imaging than with the RGB of your DSLR.
The main reason that triplet refractors are so expensive is because they are very well colour corrected, with a cheap 120x600 F/5 refractor, you will get some really strange star halos! It is not noticeable with narrowband (I had personally considered getting a 150/750 cheap refractor for that exact purpose
I have at one stage tested imaging through a diagonal, not a good idea. It adds some strange artefacts into your images.