Good advice by Daniel and Jennifer here , avoid the Newtonions that have the built in Barlow in the focuser, these state a 900mm focal length but in reality are only about 450mm fl and use the built in Barlow to give a psudo 900mm,s and the spherical mirror figure these scopes have wont perform well at all , avoid !
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You can tell this type easily as the tube,s are only 500mm or so long , not the 900-1000mm long of a true 900mm fl scope.
But a true 900mm fl Newtonion will give great views of the moon and planets up to an easy 200x on good nights .
Can you post a link of what you are looking at ? This will help immensely as it will show the mount as well , some of these are also woefully under mounted and will kill your enthusiasm very fast
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Looking forward to your reply.
Brian