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Hi Markus, A basic barlow is suitable for both visual and imaging, coming
in usual sizes of 2 to 5x. Many people with short focal length scopes use a 5x
to give them a decent focal length when viewing or imaging small targets. A 2x doubles the focal length of the scope, but at a cost; you need exposures four times as long to get the same effective exposure. The increase in magnification is linear, but the increase in exposure time is a square function, so a 3x barlow requires 9 times the exposure time.
Some more expensive barlows do have more than one function, such as
correcting various aberrations. Some have ED glass, which serves little purpose, as a barlow is a divergent lens, not a convergent one.
You won't do much better with the dob, because at the high powers needed for planetary work, and without tracking, your videos will be very short,
giving not a lot of frames to work with.
raymo
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