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It's for planetary imaging, Ian. A barlow is a must to get the focal length and image scale we desire.
Fahim I guess the $50 ones would be ok, you're imaging in mono so you won't get the colour spread that cheap barlows would probably give.
The 2.5x powermate is probably best, but with the filterwheel etc, you'll actually get less than 2.5x so you may not get the image scale you want.
Ultimately you may even want a 4x to get more image scale on those nights of really good seeing.
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