With the ETX - the mount quality versus the high focal length will get you in trouble. I would suggest (as Doug did) an ED80 and an EQ5 as a good minimum for imaging work. You'll have no lack of new friends with exactly that popular combination of gear.
If you go down the ETX route you will probably get some OK shots with lots of effort but your ratio of good to bad shots will be so low you will probably lose enthusiasm quickly. It's not impossible just not a good beginner's option.
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