I am wondering if there is a telescope in or outside the ISS that has been used for astronomical imaging? I have seen here that there is already a Celestron telescope on board but it is only used for Earth observation.
The Celestron telescope is not stationary. It can be moved around inside the space station to look through any window.
It is however just used to observe the Earth, and the reason for that is the orbital velocity of the ISS. Moving at almost $8km/s$, the view change with an angular velocity of 3.88 arc-degrees per second because the station rotates to keep the same side facing the Earth. If the ISS had instead stayed fixed relative to the stars, like the Hubble telescope, you get the same problem with Earth observations. You can not simply do both.
That is what makes it hard to focus on one part of the sky. Given the limited view from one of the windows, you can only observe a region of the sky for about 10 minutes.
That does not mean that The ISS does not have equipment to observe the sky, it does for instance have the Alpha Magnetic Spectrometer.