When using as a telephoto lens, I found internal reflection if there is bright light source around. With careful inspection, it was due to a silver ring inside the adapter chain. Therefore, I've blackened that ring, and now the problem was solved.
Tonight, I tried to use it again on the moon, and the result is very good.
Then, I placed the EOS-1.25" eyepiece adapter in, and I will need to insert a diagonal between the lens and the diagonal, the internal reflection issue comes back! I soon realized why, it's because the silver barrel of the barlows! On shielding it with black cover, it's all solved!
It's a good astrograph, it's a good day time telephoto lens. However, it's not so good as a telescope for:
1. a diagonal has to be inserted, and thus, it has effective focal length of more than 900mm... so it's no longer a wide field instrument.
2. for high power observation, it's not wide to use a f2.8 objective
Therefore, I am going to sell it.