5x powermate can be useful, but you do have to watch your focal length on nights with less than optimal seeing... I have a 2x ED Barlow, a 3x TeleVue and the 5x TV PM, and since buying the powermate, I've not used the other barlows. However my scope only hits 6m focal length with the 5x, yours would be 10m plus whatever the extension of the filter wheel adds... Thats a LONG focal length.. It would only be useful on THE best nights of the year.....
I'd say get a 2.5x Powermate or a 3x TV barlow. If you have the money to throw around, get a 5x aswell, otherwise you'll get that night of brilliant seeing and be kicking yourself that you dont have more focal lenght.
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