Tareq, honestly there is nothing wrong with an 8" f5 Newt for imaging. Learn the craft on that before you go adding to the collection. For DSOs you will need a good coma corrector, like a Baader MPCC mk3, and I assume you have an adequate mount. For planetary you will need to look at a Parracor etc to get you the necessary focal length multiplier.
If your heart is set on a good triplet APO, don't compromise with cheaper colour handiacaped Doublet refractors, ED or not all Doublet are compromised.Your Newt has no colour issues, keep that in mind.
Good luck.
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