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Hi I see your in Sydney - but you don't say where.
I've found over time that the most important things are the level of light pollution you have and the surrounding objects on the ground that disturb your 'seeing' because they give off heat at night.
If your willing to go out to dark sites with good seeing or your lucky enough to live somewhere devoid of streetlights and heat sources (neighbours roofs etc) then upping the mag with a barlow isn't an issue because there is lots of light and contrast anyway. If you intend to only observe from your back yard and you have seeing and light pollution issues then increased mag just shows you how bad the seeing really is.
The Jet stream which also finds itself over Sydney on occasions can destroy your seeing as it churns up the higher level air.
My suggestion - see if you can hook up with a local Astro Club and ask the local members what you can expect from the seeing conditions - which will over time influence your choice of EPs in terms of Mag and how much light you are going to get will determine whether you can stack a big barlow in front of the EP
HTH
Mike
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