Excuse me, is there any requirement for the focal length of a SLR telescope? I mean, zoom in behind the eyepiece, not focus directly.

The focal length of the telescope should not be too long in the case of direct focus, otherwise the imaging focus can not fall on the focal plane of the camera. However, if it is behind the eyepiece, the requirement for the focal length of the objective lens is not particularly strict, but the exit pupil size of the eyepiece should be appropriate. If it is too big, it will be wasted; if it is too small, the image will be too dark. And the eyepiece will also affect the imaging quality. Personally, I strongly recommend direct focus photography.