As an amateur astronomer, I have several Canon EF adapters lying around. I found this one to be perfect for such purpose, since there are three fixing screws so that you can turn the M42 thread again the EF mount!
So the orientation of the lens could be fixed easily! Note the pencil mark I made above in order to find out the best orientation.
Tonight, I have conducted photographic and visual test.
Visually, it's sharp at low magnifications. I have used a diagonal by placing a 2x barlows lens in front of it, serving as a relay lens. It could reach focus without problem. Pushing the power will result in softening, unacceptable I would say, and it looks like internal reflection due to the extra lens elements on the rear part. My primary impression is that, this is not a good visual telescope, and it is not going to replace my C5 therefore.
Photographically, it's great. It's very sharp and free of any color fringes. On pushing it with a Kenko 2x, I found that it will produce error, but of course, if I use a M42-EOS adapter with built-in chip, the error will go away, and I will have focus confirm signal too. So, I will need to buy myself a rotatable M42-EOS adapter with built-in chip, and then the project will probably close.
Conclusion: this is a very nice camera lens, but not exactly a great telescope. I found that my 200mm f2.8L suffers from the same problem, while the Canon 100-400mm is a great telescope, however. Faster lens is not really good candidate for visual purpose at high power.