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for deep sky you will need equatorial mounting. Each of the scopes that you have looked at are all ALT/AZ mounts and won't be able to carry out long exposures due to field rotation.
If you can get a wedge for any of those three then it may then be possible to do deep sky.
Most imaging is done using Equatorial mounts although Cassegrain scopes can be fitted with Field de-rotators to compensate although they are quite expensive.
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