I know your question has already been answered, but I thought I'd share my thoughts on tricks to use with it.
Note, I haven't had ML long, and am waiting for the opportunity to try this myself, but I think it'll work.
So, Besides 1:1 crop, there are cases where something else could be useful. For instance since 640 by X is the smallest resolution you can choose. Using something like 5:1 will crop the image down to 640x128. For planetary images that only take a small portion of the frame, this will help keep the buffer from overflowing and in theory reduce overall file size by 5x.
To plan your framing. I use Stellarium, with it you can simulate the FOV your camera and telescope can produce. I can tell you that with my 1000mm F/5 scope, using 640x128 (5:1 crop) I can easily fit the bright Jupiter and the 4 dimmer Galilean moons in one frame. Coupled with HDR video, I think I can simultaneously capture properly exposed frames and compose them in post.
To config Stellarium the 600D sensor has these attributes.
Resolution: 5184*3456
Chip Width: 22.3
Chip Height: 14.9
Pixel width and height: 4.3
From there adjust resolution and chip size proportionally for custom crops.
You will also have to configure your telescope specs, eyepieces and barlows.
Enjoy.
